Abstract:
One embodiment provides a system that facilitates the execution of a web application. During operation, the system allocates a storage space on one or more storage devices for use by the web application. Next, the system creates, for the web application, a private filesystem comprising a private root directory within the storage space. Finally, the system enables access to the private filesystem for the web application through the private root directory in a manner that does not allow access to a host filesystem associated with the one or more storage devices from the web application.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/427,208, filed 21 Mar. 2009, titled “Filesystem Access For Web Applications and Native Code Modules,” which claims priority under 35 U.S.C. §119 to U.S. Provisional Application No. 61/113,062, entitled “Client-Side System for Web Applications,” by Eric Uhrhane and Matthew Papakipos, filed on 10 Nov. 2008. The entire contents of the prior applications are hereby incorporated by reference. The subject matter of this application is also related to the subject matter in a co-pending non-provisional application by inventors J. Bradley Chen, Matthew T. Harren, Matthew Papakipos, David C. Sehr, and Bennet S. Yee, entitled “Method for Validating an Untrusted Native Code Module,” having Ser. No. 12/117,634, and filing date 8 May 2008. The subject matter of this application is additionally related to the subject matter in a co-pending non-provisional application by inventors J. Bradley Chen, Matthew T. Harren, Matthew Papakipos, David C. Sehr, Bennet S. Yee, and Gregory Dardyk, entitled “Method for Safely Executing an Untrusted Native Code Module on a Computing Device,” having Ser. No. 12/117,650, and filing date 8 May 2008. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The present embodiments relate to techniques for executing web applications. More specifically, the present embodiments relate to a method and system for providing filesystem access to web applications and native code modules. 
         [0004]    2. Related Art 
         [0005]    Computer systems may include a number of native applications that perform a variety of tasks for users. Such native applications may include word processors, email clients, media players, image or signal processing modules, and/or web browsers. To run efficiently, native applications are often written in a combination of general purpose programming languages such as C or C++ and low-level languages such as assembly language. Furthermore, native applications may have access to system resources such as graphics hardware, sound cards, filesystems, and/or network resources. Consequently, native applications may provide computationally intensive and/or resource-specific features to users of computer systems on which the native applications are installed. However, the low-level nature of native applications may restrict the applications to specific platforms, incur a lengthy installation process, and/or pose a security risk. 
         [0006]    On the other hand, web applications, which have become more prevalent in recent years, are typically written in scripting languages that are more secure and require no installation. However, web applications may lack the performance capabilities of native applications. In particular, a web application may run one or two orders of magnitude more slowly than a native application. Furthermore, web applications are often unable to access low-level system resources that are available to native applications. As a result, web applications may be unable to implement certain features that are available in native applications. 
         [0007]    Hence, what is needed is a technique for enabling native performance and functionality in web applications without compromising the security of the web applications. 
       SUMMARY 
       [0008]    One embodiment provides a system that facilitates the execution of a web application. During operation, the system allocates a storage space on one or more storage devices for use by the web application. Next, the system creates, for the web application, a private filesystem comprising a private root directory within the storage space. Finally, the system enables access to the private filesystem for the web application through the private root directory in a manner that does not allow access to a host filesystem associated with the one or more storage devices from the web application. 
         [0009]    In some embodiments, the system also restricts access to the private filesystem from a source which is external to the web application. 
         [0010]    In some embodiments, restricting access to the private filesystem from the source involves preventing access to the private filesystem outside of a domain associated with the web application and managing access to the private filesystem from one or more native code modules within the domain. 
         [0011]    In some embodiments, managing access to the private filesystem from the one or more native code modules involves enforcing a set of access permissions to the private filesystem for each of the native code modules. 
         [0012]    In some embodiments, one or more of the access permissions are passed between the native code modules. 
         [0013]    In some embodiments, the system also enforces a size limit for the private filesystem. 
         [0014]    In some embodiments, enforcing the size limit for the private filesystem involves periodically monitoring a size of the storage space during access to the private filesystem by the web application and performing a remedial action if the size of the storage space exceeds the size limit. 
         [0015]    In some embodiments, performing the remedial action involves at least one of updating the size limit, restricting write privileges to the private filesystem by the web application, notifying a user of the web application, and terminating the web application. 
         [0016]    In some embodiments, the private filesystem is implemented using a relational database associated with the web application. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0017]      FIG. 1  shows a schematic of a system. 
           [0018]      FIG. 2  shows an exemplary private filesystem. 
           [0019]      FIG. 3  shows a flowchart illustrating the process of facilitating the execution of a web application. 
       
    
    
       [0020]    In the figures, like reference numerals refer to the same figure elements. 
       DETAILED DESCRIPTION 
       [0021]    The following description is presented to enable any person skilled in the art to make and use the embodiments, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present disclosure. Thus, the present embodiments are not limited to the embodiments shown, but are to be accorded the widest scope consistent with the principles and features disclosed herein. 
         [0022]    The data structures and code described in this detailed description are typically stored on a computer-readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. The computer-readable storage medium includes, but is not limited to, volatile memory, non-volatile memory, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs), DVDs (digital versatile discs or digital video discs), or other media capable of storing computer-readable media now known or later developed. 
         [0023]    The methods and processes described in the detailed description section can be embodied as code and/or data, which can be stored in a computer-readable storage medium as described above. When a computer system reads and executes the code and/or data stored on the computer-readable storage medium, the computer system performs the methods and processes embodied as data structures and code and stored within the computer-readable storage medium. 
         [0024]    Furthermore, methods and processes described herein can be included in hardware modules or apparatus. These modules or apparatus may include, but are not limited to, an application-specific integrated circuit (ASIC) chip, a field-programmable gate array (FPGA), a dedicated or shared processor that executes a particular software module or a piece of code at a particular time, and/or other programmable-logic devices now known or later developed. When the hardware modules or apparatus are activated, they perform the methods and processes included within them. 
         [0025]    Embodiments provide a method and system for facilitating the execution of a web application. The web application may be loaded in a web browser and executed on a computing system such as a personal computer (PC), a mobile phone, a personal digital assistant (PDA), a graphing calculator, a portable media player, a global positioning system (GPS) receiver, and/or another electronic computing device. The web application may be obtained by the computing system from a server using a network connection with the server. For example, the web application may be downloaded over the Internet from a website. 
         [0026]    More specifically, embodiments provide a method and system for providing filesystem services to the web application. The filesystem services may be provided by allocating a storage space on one or more storage devices for use by the web application and creating a private filesystem within the storage space. Access to the private filesystem for the web application may then be enabled through a private root directory in the private filesystem without allowing access to a host filesystem associated with the one or more storage devices from the web application. In addition, a size limit may be enforced for the private filesystem, while access to the private filesystem may be restricted for sources that are external to the web application. 
         [0027]      FIG. 1  shows a schematic of a system. The system includes a computing system  102  and a set of servers (e.g., server  1   104 , server x  106 ). Computing system  102  includes a web application  116  running within a web browser  110 , a plugin  108 , a filesystem manager  120 , a set of storage devices  124 - 126 , and a host filesystem  128 . Each of these components is described in further detail below. 
         [0028]    Computing system  102  may correspond to an electronic device that provides one or more services or functions to a user. For example, computing system  102  may operate as a mobile phone, personal computer (PC), global positioning system (GPS) receiver, portable media player, personal digital assistant (PDA), and/or graphing calculator. In addition, computing system  102  may include an operating system (not shown) that coordinates the use of hardware and software resources on computing system  102 , as well as one or more applications (e.g., web browser  110 , web application  116 ) that perform specialized tasks for the user. For example, computing system  102  may include applications such as an email client, address book, document editor, web browser  110 , and/or media player. To perform tasks for the user, applications may obtain the use of hardware resources (e.g., processor, memory, I/O components, network interface card, graphics-processing unit (GPU), storage devices  124 - 126 , etc.) on computing system  102  from the operating system, as well as interact with the user through a hardware and/or software framework provided by the operating system, as described below. 
         [0029]    In one or more embodiments, computing system  102  includes functionality to obtain and/or execute applications using a network connection. In particular, computing system  102  may obtain web application  116  from one or more servers (e.g., server  1   104 , server x  106 ) using a network connection with the server(s) and load web application  116  in web browser  110 . For example, web application  116  may be downloaded from an application server over the Internet by web browser  110 . 
         [0030]    Once loaded, web application  116  may provide features and user interactivity comparable to that of native applications on computing system  102 . For example, web application  116  may function as an email client, document editor, media player, computer-aided design (CAD) system, and/or computer game. Web application  116  may also include dynamic user interface elements such as menus, buttons, windows, sub-windows, icons, animations, and/or other graphical objects that emulate analogous user interface elements in native applications. In other words, web application  116  may correspond to a rich Internet application (RIA). 
         [0031]    Furthermore, web application  116  may execute on computing system  102  regardless of the type of platform (e.g., operating system, drivers, etc.) associated with computing system  102 . Though platform-independent applications such as web application  116  may be more portable and secure than native applications, such cross-platform applications may lack certain performance capabilities of native applications. 
         [0032]    More specifically, non-native applications such as web application  116  may be written using scripting languages that are interpreted rather than compiled, such as JavaScript (JavaScript™ is a registered trademark of Sun Microsystems, Inc.). The interpreted nature of web application  116  and/or other non-native applications may result in significantly slower execution times for the non-native applications than those of compiled native applications. Non-native applications may also be unable to utilize low-level libraries and/or application programming interfaces (API) that are available for use by native applications. Consequently, non-native applications may provide limited functionality in certain tasks. 
         [0033]    To enable native performance for web applications, computing system  102  may obtain and execute a native code module  118  within plugin  108 . Like web application  116 , native code module  118  may be obtained from one or more servers (e.g., server  1   104 , server x  106 ) by web browser  110 . For example, web application  116  may provide a hyperlink to native code module  118  on the Internet. Web browser  110  may then download native code module  118  from the Uniform Resource Locator (URL) specified in the hyperlink. Alternatively, native code module  118  may be specified by the user or by an external source, such as another web application and/or a native application. 
         [0034]    More specifically, native code module  118  may correspond to a software module containing native code that runs directly on hardware provided by computing system  102 , such as a CPU. As a result, native code module  118  may be used to perform tasks that require substantial access to CPU resources on computing system  102 . For example, native code module  118  may be used by web application  116  to provide computationally intensive features associated with physics simulation, signal processing, artificial intelligence, modeling, and/or analysis. 
         [0035]    In one or more embodiments, plugin  108  includes a variety of mechanisms to ensure the safe execution of native code module  118 . In particular, native code module  118  may be validated by a validator  112  provided by plugin  108  prior to execution. Native code module validation is described in a co-pending non-provisional application by inventors J. Bradley Chen, Matthew T. Harren, Matthew Papakipos, David C. Sehr, and Bennet S. Yee, entitled, “Method for Validating an Untrusted Native Code Module,” having Ser. No. 12/117,634, and filing date 8 May 2008, which is incorporated herein by reference. 
         [0036]    Once native code module  118  is validated, native code module  118  may be loaded into a secure runtime environment  114  provided by plugin  108 . Native code execution in a secure runtime environment is described in a co-pending non-provisional application by inventors J. Bradley Chen, Matthew T. Harren, Matthew Papakipos, David C. Sehr, Bennet S. Yee, and Gregory Dardyk, entitled, “Method for Safely Executing an Untrusted Native Code Module on a Computing Device,” having Ser. No. 12/117,650, and filing date 8 May 2008, which is incorporated herein by reference. Secure runtime environment  114  may also be provided by a web browser extension to web browser  110 , and/or secure runtime environment  114  may be built into web browser  110 . 
         [0037]    Furthermore, because native code module  118  may include binary code that runs directly on hardware, native code module  118  may be platform independent with respect to the operating system of computing system  102 , web browser  110 , and/or other software components on computing system  102 . As described in the above-referenced applications, plugin  108  and/or native code module  118  may also include mechanisms for executing on a variety of instruction set architectures, including the use of “fat binaries” and binary translators. 
         [0038]    Those skilled in the art will appreciate that security mechanisms used to execute web application  116  and native code module  118  may preclude web application  116  and native code module  118  from accessing system services on computing system  102 . In particular, the restricted execution of web application  116  and native code module  118  may prohibit access to filesystem services from web application  116  or native code module  118 . For example, security restrictions may prevent web application  116  and native code module  118  from accessing host filesystem  128  on computing system  102 . As a result, web application  116  and/or native code module  118  may have trouble storing, reading from, writing to, and transferring large amounts of binary data. For example, web application  116  and/or native code module  118  may have limited capacity in storing and handling files such as large images, media (e.g., audio, video, etc.), and/or game assets. 
         [0039]    In one or more embodiments, filesystem services for web application  116  and native code module  118  are enabled by filesystem manager  120 . Filesystem manager  120  may be implemented as a plugin to web browser  110 , a web browser extension to web browser  110 , and/or as a part of web browser  110 . As shown in  FIG. 1 , filesystem manager  120  may create a private filesystem  122  for use by web application  116  and native code module  118 . Private filesystem  122  may reside on one or more storage devices  124 - 126  within computing system  102  or attached to computing system  102 , such as one or more hard disk drives, optical disks, volatile memory (e.g., random access memory (RAM)), and/or flash memory devices. Moreover, as described below, filesystem manager  120  may facilitate the secure execution of web application  116  and native code module  118  by isolating private filesystem  122  from host filesystem  128  and restricting access to private filesystem  122  from outside the domain of web application  116 . 
         [0040]    To provide filesystem services to web application  116 , filesystem manager  120  may allocate a storage space on one or more storage devices  124 - 126  for use by web application  116 . Storage devices  124 - 126  may include storage devices within computing system  102 , attached to computing system  102 , and/or accessible from computing system  102  through a network connection. For example, some of the storage space may be allocated in a hard disk drive on a server (e.g., server  1   104 , server x  106 ). Filesystem manager  120  may then create private filesystem  122  within the storage space. Filesystem manager  120  may also enable access to private filesystem  122  from web application  116  through a private root directory in private filesystem  122 . For example, web application  116  may execute filesystem commands such as “open,” “seek,” “close,” “read,” “write,” “truncate,” “mkdir,” “unlink,” “rmdir,” “opendir,” “readdir,” “closedir,” “mv,” and “mmap” using the private root directory and a plugin interface with filesystem manager  120 . 
         [0041]    Furthermore, filesystem manager  120  may enable access to private filesystem  122  for web application  116  without allowing access to host filesystem  128  from web application  116 . In other words, filesystem manager  120  may implement filesystem security by preventing web application  116  from accessing files and directories outside of private filesystem  122 , including files and directories accessible to native applications in host filesystem  128  and/or private filesystems for other web applications in computing system  102 . 
         [0042]    Filesystem manager  120  may also restrict access to private filesystem  122  from sources which are external to web application  116 . More specifically, filesystem manager  120  may prevent access to private filesystem  122  outside of a domain associated with web application  116 . For example, filesystem manager  120  may define the domain of web application  116  as a set of webpages and/or modules (e.g., native code module  118 ) used by web application  116 . Webpages, modules, and/or applications outside of the domain may thus be unable to access private filesystem  122  and may instead have access to their own private filesystems as provided by filesystem manager  120  and/or other filesystem mechanisms on computing system  102 . 
         [0043]    Furthermore, filesystem manager  120  may manage access to private filesystem  122  from native code module  118  and/or other modules (e.g., native code modules, plugins, etc.) within the domain of web application  116 . In particular, module-based access to private filesystem  122  may be managed by enforcing a set of access permissions to private filesystem  122  for each of the modules. For example, filesystem manager  120  may separate private filesystem  122  into three regions, with each region accessible to only one of three modules within the domain of web application  116 . To separate private filesystem  122  into regions, filesystem manager  120  may define each region using a separate directory within the private root directory. To enforce access permissions to the regions by the modules, filesystem manager  120  may implement “chroot” functionality by setting each of the directories as the apparent root directory for one of the modules. As a result, each module may only have access to the apparent root directory assigned to the module within private filesystem  122 , as well as any subdirectories and files within the apparent root directory. 
         [0044]    Filesystem manager  120  may further allow access permissions to be passed between modules. For example, one module may grant access to a file and/or directory from another module by passing a file descriptor for the file and/or directory to the other module. Alternatively, filesystem manager  120  may allow each module to explicitly specify access permissions to files and directories within the module&#39;s apparent root directory from other modules. For example, a module may enable read and/or write access to a directory for one or more other modules through filesystem manager  120 . Apparent root directories, directories, files, and permissions in private filesystem  122  are described in further detail below with respect to  FIG. 2 . 
         [0045]    Filesystem manager  120  may also provide quota management for private filesystem  122 . In other words, filesystem manager  120  may enforce a size limit for private filesystem  122 . The size limit may be requested by web application  116 , set by the user, and/or automatically set by filesystem manager  120  based on a default size limit value for web applications. Furthermore, the size limit may be enforced by periodically monitoring a size of the storage space occupied by private filesystem  122  during access to private filesystem  122  by web application  116 . If the size of the storage space exceeds the size limit, a remedial action may be performed by filesystem manager  120 . For example, filesystem manager  120  may update (e.g., increase) the size limit, restrict write privileges to private filesystem  122  by web application  116 , notify a user of web application  116 , and/or terminate web application  116  and remove data stored in private filesystem  122 . In other words, filesystem manager  120  may utilize an eventual correctness model in enforcing the size limit for private filesystem  122  to save overhead associated with checking or altering the size of the storage space. 
         [0046]    However, filesystem manager  120  may also implement a real-time quota management system that continually monitors the size of the storage space and/or validates relevant filesystem calls (e.g., quota allocation calls) for strict conformity to the size limit if such strict conformity is warranted. For example, real-time quota management may be enabled for private filesystem  122  if a security policy for web application  116  and/or private filesystem  122  requires strict conformity to a size limit for private filesystem  122 . 
         [0047]    In one or more embodiments, filesystem manager  120  includes a coordinating mechanism for managing concurrent access to private filesystem  122  by multiple modules (e.g., web application  116 , native code module  118 , other modules, etc.) executing in separate processes. The coordinating mechanism may also execute as a separate process and perform tasks such as serialization of filesystem commands from multiple processes, file locking, quota allocation, and atomic file creation. In other words, the coordinating mechanism may manage interaction between the modules and private filesystem  122  if such interaction affects or is affected by the actions of other modules. For example, a module wishing to write to a new file may first obtain space for the new file from the coordinating mechanism, and then stream the data directly to private filesystem  122 . Furthermore, communication between the modules and the coordinating mechanism may be facilitated by an inter-module communication (IMC) runtime that establishes IMC channels between the modules and the coordinating mechanism. 
         [0048]    Those skilled in the art will appreciate that private filesystem  122  may be created by filesystem manager  120  in a number of ways. For example, private filesystem  122  may be created as an isolated subset of host filesystem  128 . Filesystem manager  120  may thus provide filesystem commands to web application  116  and associated modules as translated, guarded pass-through versions of analogous filesystem commands used to access host filesystem  128 . On the other hand, filesystem manager  120  may implement private filesystem  122  using a relational database associated with web application  116 . For example, filesystem manager  120  may implement filesystem commands on top of relational database queries for accessing a relational database on computing system  102  that provides local relational data storage for web application  116 . 
         [0049]      FIG. 2  shows an exemplary private filesystem. As described above, the private filesystem may be created by a filesystem manager (e.g., filesystem manager  120  of  FIG. 1 ) for use by a web application (e.g., web application  116  of  FIG. 1 ) and/or other modules within the domain of the web application, such as native code modules or plugins. As shown in  FIG. 2 , the private filesystem includes a private root directory  200 , a set of apparent root directories  202 - 206 , a set of directories  208 - 216 , and a set of files  218 - 224  organized in a tree structure. 
         [0050]    Private root directory  200  may serve as the root of the tree structure. As a result, apparent root directories  202 - 206 , directories  208 - 216 , and files  218 - 224  may all be located underneath private root directory  200 . Furthermore, private root directory  200  may serve as the access point to the private filesystem for the web application. In other words, the web application may make calls into the private filesystem by specifying a path from private root directory  200 . For example, the web application may open file  218  by specifying a path such as “/mydirectory/file 218 .txt” as a parameter to an “open” filesystem command provided by the filesystem manager. Within the path, private root directory  200  may be represented by the first forward slash (e.g., “/”), directory  208  may be specified using “mydirectory/,” and file  218  may be specified using “file 218 .txt.” The web application may then read from or write to file  218  using a file descriptor returned by the “open” command. 
         [0051]    As mentioned previously, the private filesystem may also be accessible to modules within the domain of the web application. Each module may further be associated with a set of access permissions to the private filesystem. In particular, each module may only have permission to access a particular region of the private filesystem as specified by the web application. In addition, the region accessible to a particular module may be represented by the apparent root directory  202 - 206  assigned to the module. For example, apparent root directory  204  may be assigned to a native code module associated with the web application. The native code module may access directories  212 - 214  and file  222  with apparent root directory  204  as the topmost directory (e.g., “/”) in paths to directories  212 - 214  and file  222 . Furthermore, the native code module may be unable to access directories and files outside of apparent root directory  204 . As a result, the use of multiple apparent root directories  202 - 206  in the private filesystem may allow for functionality similar to the “chroot” operation for modules associated with the web application. 
         [0052]    Similarly, access permissions for one module may be contained entirely within the accessible region for another module. For example, apparent root directory  202  may be assigned to a plugin associated with the web application. The plugin may thus have access to directory  210 , apparent root directory  206 , directory  216 , and file  224 . The plugin may also create and remove directories and files within apparent root directory  202  through filesystem commands provided by the filesystem manager. However, apparent root directory  206  may be assigned to a native code module associated with the web application. As a result, the native code module may only have access to directories and files at or below apparent root directory  206 , such as directory  216  and file  224 . Furthermore, changes to directory  210  and/or apparent root directory  206  may affect the native code module&#39;s access to the private filesystem. For example, the native code module may be unable to access the private filesystem if directory  210  or apparent root directory  206  were removed from the private filesystem. 
         [0053]    Access permissions for the modules may also be updated by passing permissions between the modules. As discussed above, a module may share access to a file or directory by passing the file descriptor for the file or directory to another module. For example, a native code module to which apparent root directory  204  is assigned may share file  222  by opening file  222  and passing the returned file descriptor to a native code module to which apparent root directory  202  is assigned. Changes to access permissions may also be explicitly made by the web application and/or modules. For example, the web application may change the apparent root directory of a module to allow the module to access a different region of the private filesystem. 
         [0054]    Modules may also have access to multiple apparent root directories within the private filesystem. For example, a module may have simultaneous access to two apparent root directories  204 - 206  through changes to the module&#39;s access permissions as specified by the web application and/or other modules. Upon gaining access to both root directories  204 - 206 , the module may be required to distinguish between apparent root directory  204  and apparent root directory  206  when specifying paths within the private filesystem. For example, the module may use different names for apparent root directory  204  and apparent root directory  206  instead of the forward slash (e.g., “/”) for one apparent root directory. 
         [0055]      FIG. 3  shows a flowchart illustrating the process of facilitating the execution of a web application. In one or more embodiments, one or more of the steps may be omitted, repeated, and/or performed in a different order. Accordingly, the specific arrangement of steps shown in  FIG. 3  should not be construed as limiting the scope of the technique. 
         [0056]    Initially, a storage space is allocated for use by the web application (operation  302 ). The storage space may be allocated on one or more storage devices, such as hard disk drives, optical disks, and/or flash memory. The storage devices may additionally be distributed across a number of computing systems. For example, one or more storage devices may be accessed as a network drive via a network connection. 
         [0057]    Next, a private filesystem is created within the storage space (operation  304 ) for the web application. The private filesystem may include a private root directory that is accessible from the web application. In other words, access to the private file system for the web application may be enabled through the private root directory (operation  306 ). Furthermore, because the web application may only be permitted to make filesystem calls through the private root directory, the web application may be unable to access a host filesystem associated with the storage device(s) on which the private filesystem resides. Such isolation of the private filesystem from the host filesystem may mitigate the occurrence of security exploits for both the host filesystem and the private filesystem. 
         [0058]    Filesystem-based security may additionally be implemented by preventing access to the filesystem outside of a domain associated with the web application (operation  308 ). As described above, the domain of the web application may correspond to a set of webpages and modules used by the web application. For example, the domain of the web application may be defined using a tuple that includes a host, a port, and a protocol associated with the web application. Webpages included in the host, port, and protocol, as well as modules loaded by the webpages, may thus be within the domain of the web application. 
         [0059]    Access to the private filesystem from one or more native code modules within the domain of the web application may also be managed (operation  310 ). In particular, module-based access to the private filesystem may be managed by enforcing access permissions to the private filesystem for each of the native code modules. The access permissions may be specified by providing functionality similar to “chroot” and assigning each native code module an apparent root directory within the private filesystem. The native code module may thus be allowed to access all subdirectories and files within the corresponding apparent root directory but may not have access to other areas of the private filesystem. However, access permissions may be passed between the native code modules by, for example, passing file descriptors, adding or removing apparent root directories for a native code module, and/or explicitly changing access permissions for a specific directory or file. 
         [0060]    Use of the private filesystem by the web application may also be subject to a size limit for the private filesystem. In other words, quota management may be provided for the private filesystem to ensure that the size limit for the private filesystem is not exceeded (operation  312 ). The size limit may be requested by the web application, set by a user, and/or based on a default size limit for web applications. To enforce the size limit, the size of the storage space may periodically be monitored during access to the private filesystem by the web application. If the size limit is not exceeded, no action is required. 
         [0061]    However, if the size of the storage space exceeds the size limit, a remedial action is performed to enforce the size limit (operation  314 ). The remedial action may involve updating the size limit, restricting write privileges to the private filesystem by the web application and associated native code modules, notifying a user of the web application, and/or terminating the web application. For example, if the size limit is exceeded, a user of the web application may be notified and prompted for the remedial action to be taken (e.g., terminating the web application, adjusting the size limit on a temporary or permanent basis, pausing the web application, etc.). As a result, the size limit may be enforced based on an eventual correctness model. Alternatively, real-time quota management may be implemented by continuously monitoring the size of the storage space for conformity to the size limit, by checking the size of the storage space before and after relevant filesystem calls (e.g., quota allocation calls), and/or by validating the arguments of relevant filesystem calls. 
         [0062]    Filesystem services may continue to be provided to the web application (operation  316 ). For example, the private filesystem may be available to the web application as long as the private filesystem stays within the size limit and security constraints associated with the private filesystem are met. The private filesystem may also persist between invocations of the web application. If filesystem services are to be provided, access to the private filesystem is enabled for the web application (operation  306 ) while access to the private filesystem is restricted from sources that are external to the web application (operations  308 - 310 ). The size limit for the private filesystem may also be enforced (operations  312 - 314 ) during use of the private filesystem by the web application. Filesystem services may continue to be provided until the private filesystem is no longer used by the web application and/or the web application has violated the size limit or a security constraint associated with the private filesystem. For example, access to the private filesystem from the web application may be terminated if the web application attempts to undermine the host filesystem, operating system, and/or underlying hardware using spurious filesystem calls to the private filesystem. 
         [0063]    The foregoing descriptions of various embodiments have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the present embodiments to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present embodiments.