# rtld-audit

## NAME



           rtld-audit - auditing API for the dynamic linker
    


## SYNOPSIS



           **#define _GNU_SOURCE** /* See feature_test_macros(7) */
           **#include <link.h>**
    


## DESCRIPTION



           The GNU dynamic linker (run-time linker) provides an auditing API
           that allows an application to be notified when various dynamic
           linking events occur.  This API is very similar to the auditing
           interface provided by the Solaris run-time linker.  The necessary
           constants and prototypes are defined by including _< link.h>_.
    
           To use this interface, the programmer creates a shared library
           that implements a standard set of function names.  Not all of the
           functions need to be implemented: in most cases, if the programmer
           is not interested in a particular class of auditing event, then no
           implementation needs to be provided for the corresponding auditing
           function.
    
           To employ the auditing interface, the environment variable
           **LD_AUDIT** must be defined to contain a colon-separated list of
           shared libraries, each of which can implement (parts of) the
           auditing API.  When an auditable event occurs, the corresponding
           function is invoked in each library, in the order that the
           libraries are listed.
    
       **la_version()**
    
           **unsigned int la_version(unsigned int** _version_**);**
    
           This is the only function that _must_ be defined by an auditing
           library: it performs the initial handshake between the dynamic
           linker and the auditing library.  When invoking this function, the
           dynamic linker passes, in _version_ , the highest version of the
           auditing interface that the linker supports.
    
           A typical implementation of this function simply returns the
           constant **LAV_CURRENT** , which indicates the version of _< link.h>_ that
           was used to build the audit module.  If the dynamic linker does
           not support this version of the audit interface, it will refuse to
           activate this audit module.  If the function returns zero, the
           dynamic linker also does not activate this audit module.
    
           In order to enable backwards compatibility with older dynamic
           linkers, an audit module can examine the _version_ argument and
           return an earlier version than **LAV_CURRENT** , assuming the module
           can adjust its implementation to match the requirements of the
           previous version of the audit interface.  The **la_version** function
           should not return the value of _version_ without further checks
           because it could correspond to an interface that does not match
           the _< link.h>_ definitions used to build the audit module.
    
       **la_objsearch()**
    
           **char *la_objsearch(const char ***_name_**, uintptr_t ***_cookie_**,**
                              **unsigned int** _flag_**);**
    
           The dynamic linker invokes this function to inform the auditing
           library that it is about to search for a shared object.  The _name_
           argument is the filename or pathname that is to be searched for.
           _cookie_ identifies the shared object that initiated the search.
           _flag_ is set to one of the following values:
    
           **LA_SER_ORIG**
                  This is the original name that is being searched for.
                  Typically, this name comes from an ELF **DT_NEEDED** entry, or
                  is the _filename_ argument given to dlopen(3).
    
           **LA_SER_LIBPATH**
                  _name_ was created using a directory specified in
                  **LD_LIBRARY_PATH**.
    
           **LA_SER_RUNPATH**
                  _name_ was created using a directory specified in an ELF
                  **DT_RPATH** or **DT_RUNPATH** list.
    
           **LA_SER_CONFIG**
                  _name_ was found via the ldconfig(8) cache
                  (_/etc/ld.so.cache_).
    
           **LA_SER_DEFAULT**
                  _name_ was found via a search of one of the default
                  directories.
    
           **LA_SER_SECURE**
                  _name_ is specific to a secure object (unused on Linux).
    
           As its function result, **la_objsearch**() returns the pathname that
           the dynamic linker should use for further processing.  If NULL is
           returned, then this pathname is ignored for further processing.
           If this audit library simply intends to monitor search paths, then
           _name_ should be returned.
    
       **la_activity()**
    
           **void la_activity( uintptr_t ***_cookie_**, unsigned int** _flag_**);**
    
           The dynamic linker calls this function to inform the auditing
           library that link-map activity is occurring.  _cookie_ identifies
           the object at the head of the link map.  When the dynamic linker
           invokes this function, _flag_ is set to one of the following values:
    
           **LA_ACT_ADD**
                  New objects are being added to the link map.
    
           **LA_ACT_DELETE**
                  Objects are being removed from the link map.
    
           **LA_ACT_CONSISTENT**
                  Link-map activity has been completed: the map is once again
                  consistent.
    
       **la_objopen()**
    
           **unsigned int la_objopen(struct link_map ***_map_**, Lmid_t** _lmid_**,**
                                   **uintptr_t ***_cookie_**);**
    
           The dynamic linker calls this function when a new shared object is
           loaded.  The _map_ argument is a pointer to a link-map structure
           that describes the object.  The _lmid_ field has one of the
           following values
    
           **LM_ID_BASE**
                  Link map is part of the initial namespace.
    
           **LM_ID_NEWLM**
                  Link map is part of a new namespace requested via
                  dlmopen(3).
    
           _cookie_ is a pointer to an identifier for this object.  The
           identifier is provided to later calls to functions in the auditing
           library in order to identify this object.  This identifier is
           initialized to point to object's link map, but the audit library
           can change the identifier to some other value that it may prefer
           to use to identify the object.
    
           As its return value, **la_objopen**() returns a bit mask created by
           ORing zero or more of the following constants, which allow the
           auditing library to select the objects to be monitored by
           **la_symbind***():
    
           **LA_FLG_BINDTO**
                  Audit symbol bindings to this object.
    
           **LA_FLG_BINDFROM**
                  Audit symbol bindings from this object.
    
           A return value of 0 from **la_objopen**() indicates that no symbol
           bindings should be audited for this object.
    
       **la_objclose()**
    
           **unsigned int la_objclose(uintptr_t ***_cookie_**);**
    
           The dynamic linker invokes this function after any finalization
           code for the object has been executed, before the object is
           unloaded.  The _cookie_ argument is the identifier obtained from a
           previous invocation of **la_objopen**().
    
           In the current implementation, the value returned by **la_objclose**()
           is ignored.
    
       **la_preinit()**
    
           **void la_preinit(uintptr_t ***_cookie_**);**
    
           The dynamic linker invokes this function after all shared objects
           have been loaded, before control is passed to the application
           (i.e., before calling _main_()).  Note that _main_() may still later
           dynamically load objects using dlopen(3).
    
       **la_symbind*()**
    
           **uintptr_t la_symbind32(Elf32_Sym ***_sym_**, unsigned int** _ndx_**,**
                                  **uintptr_t ***_refcook_**, uintptr_t ***_defcook_**,**
                                  **unsigned int ***_flags_**, const char ***_symname_**);**
           **uintptr_t la_symbind64(Elf64_Sym ***_sym_**, unsigned int** _ndx_**,**
                                  **uintptr_t ***_refcook_**, uintptr_t ***_defcook_**,**
                                  **unsigned int ***_flags_**, const char ***_symname_**);**
    
           The dynamic linker invokes one of these functions when a symbol
           binding occurs between two shared objects that have been marked
           for auditing notification by **la_objopen**().  The **la_symbind32**()
           function is employed on 32-bit platforms; the **la_symbind64**()
           function is employed on 64-bit platforms.
    
           The _sym_ argument is a pointer to a structure that provides
           information about the symbol being bound.  The structure
           definition is shown in _< elf.h>_.  Among the fields of this
           structure, _st_value_ indicates the address to which the symbol is
           bound.
    
           The _ndx_ argument gives the index of the symbol in the symbol table
           of the bound shared object.
    
           The _refcook_ argument identifies the shared object that is making
           the symbol reference; this is the same identifier that is provided
           to the **la_objopen**() function that returned **LA_FLG_BINDFROM**.  The
           _defcook_ argument identifies the shared object that defines the
           referenced symbol; this is the same identifier that is provided to
           the **la_objopen**() function that returned **LA_FLG_BINDTO**.
    
           The _symname_ argument points a string containing the name of the
           symbol.
    
           The _flags_ argument is a bit mask that both provides information
           about the symbol and can be used to modify further auditing of
           this PLT (Procedure Linkage Table) entry.  The dynamic linker may
           supply the following bit values in this argument:
    
           **LA_SYMB_DLSYM**
                  The binding resulted from a call to dlsym(3).
    
           **LA_SYMB_ALTVALUE**
                  A previous **la_symbind***() call returned an alternate value
                  for this symbol.
    
           By default, if the auditing library implements **la_pltenter**() and
           **la_pltexit**() functions (see below), then these functions are
           invoked, after **la_symbind**(), for PLT entries, each time the symbol
           is referenced.  The following flags can be ORed into _*flags_ to
           change this default behavior:
    
           **LA_SYMB_NOPLTENTER**
                  Don't call **la_pltenter**() for this symbol.
    
           **LA_SYMB_NOPLTEXIT**
                  Don't call **la_pltexit**() for this symbol.
    
           The return value of **la_symbind32**() and **la_symbind64**() is the
           address to which control should be passed after the function
           returns.  If the auditing library is simply monitoring symbol
           bindings, then it should return _sym- >st_value_.  A different value
           may be returned if the library wishes to direct control to an
           alternate location.
    
       **la_pltenter()**
           The precise name and argument types for this function depend on
           the hardware platform.  (The appropriate definition is supplied by
           _< link.h>_.)  Here is the definition for x86-32:
    
           **Elf32_Addr la_i86_gnu_pltenter(Elf32_Sym ***_sym_**, unsigned int** _ndx_**,**
                            **uintptr_t ***_refcook_**, uintptr_t ***_defcook_**,**
                            **La_i86_regs ***_regs_**, unsigned int ***_flags_**,**
                            **const char ***_symname_**, long ***_framesizep_**);**
    
           This function is invoked just before a PLT entry is called,
           between two shared objects that have been marked for binding
           notification.
    
           The _sym_ , _ndx_ , _refcook_ , _defcook_ , and _symname_ are as for
           **la_symbind***().
    
           The _regs_ argument points to a structure (defined in _< link.h>_)
           containing the values of registers to be used for the call to this
           PLT entry.
    
           The _flags_ argument points to a bit mask that conveys information
           about, and can be used to modify subsequent auditing of, this PLT
           entry, as for **la_symbind***().
    
           The _framesizep_ argument points to a _long int_ buffer that can be
           used to explicitly set the frame size used for the call to this
           PLT entry.  If different **la_pltenter**() invocations for this symbol
           return different values, then the maximum returned value is used.
           The **la_pltexit**() function is called only if this buffer is
           explicitly set to a suitable value.
    
           The return value of **la_pltenter**() is as for **la_symbind***().
    
       **la_pltexit()**
           The precise name and argument types for this function depend on
           the hardware platform.  (The appropriate definition is supplied by
           _< link.h>_.)  Here is the definition for x86-32:
    
           **unsigned int la_i86_gnu_pltexit(Elf32_Sym ***_sym_**, unsigned int** _ndx_**,**
                            **uintptr_t ***_refcook_**, uintptr_t ***_defcook_**,**
                            **const La_i86_regs ***_inregs_**, La_i86_retval ***_outregs_**,**
                            **const char ***_symname_**);**
    
           This function is called when a PLT entry, made between two shared
           objects that have been marked for binding notification, returns.
           The function is called just before control returns to the caller
           of the PLT entry.
    
           The _sym_ , _ndx_ , _refcook_ , _defcook_ , and _symname_ are as for
           **la_symbind***().
    
           The _inregs_ argument points to a structure (defined in _< link.h>_)
           containing the values of registers used for the call to this PLT
           entry.  The _outregs_ argument points to a structure (defined in
           _< link.h>_) containing return values for the call to this PLT entry.
           These values can be modified by the caller, and the changes will
           be visible to the caller of the PLT entry.
    
           In the current GNU implementation, the return value of
           **la_pltexit**() is ignored.
    


## VERSIONS



           This API is very similar to the Solaris API described in the
           Solaris _Linker and Libraries Guide_ , in the chapter _Runtime Linker_
           _Auditing Interface_.
    


## STANDARDS



           None.
    


## NOTES



           Note the following differences from the Solaris dynamic linker
           auditing API:
    
           •  The Solaris **la_objfilter**() interface is not supported by the
              GNU implementation.
    
           •  The Solaris **la_symbind32**() and **la_pltexit**() functions do not
              provide a _symname_ argument.
    
           •  The Solaris **la_pltexit**() function does not provide _inregs_ and
              _outregs_ arguments (but does provide a _retval_ argument with the
              function return value).
    


## BUGS



           In glibc versions up to and include 2.9, specifying more than one
           audit library in **LD_AUDIT** results in a run-time crash.  This is
           reportedly fixed in glibc 2.10.
    


## EXAMPLES



           #include <link.h>
           #include <stdio.h>
    
           unsigned int
           la_version(unsigned int version)
           {
               printf("la_version(): version = %u; LAV_CURRENT = %u\n",
                       version, LAV_CURRENT);
    
               return LAV_CURRENT;
           }
    
           char *
           la_objsearch(const char *name, uintptr_t *cookie, unsigned int flag)
           {
               printf("la_objsearch(): name = %s; cookie = %p", name, cookie);
               printf("; flag = %s\n",
                       (flag == LA_SER_ORIG) ?    "LA_SER_ORIG" :
                       (flag == LA_SER_LIBPATH) ? "LA_SER_LIBPATH" :
                       (flag == LA_SER_RUNPATH) ? "LA_SER_RUNPATH" :
                       (flag == LA_SER_DEFAULT) ? "LA_SER_DEFAULT" :
                       (flag == LA_SER_CONFIG) ?  "LA_SER_CONFIG" :
                       (flag == LA_SER_SECURE) ?  "LA_SER_SECURE" :
                       "???");
    
               return name;
           }
    
           void
           la_activity (uintptr_t *cookie, unsigned int flag)
           {
               printf("la_activity(): cookie = %p; flag = %s\n", cookie,
                       (flag == LA_ACT_CONSISTENT) ? "LA_ACT_CONSISTENT" :
                       (flag == LA_ACT_ADD) ?        "LA_ACT_ADD" :
                       (flag == LA_ACT_DELETE) ?     "LA_ACT_DELETE" :
                       "???");
           }
    
           unsigned int
           la_objopen(struct link_map *map, Lmid_t lmid, uintptr_t *cookie)
           {
               printf("la_objopen(): loading \"%s\"; lmid = %s; cookie=%p\n",
                       map->l_name,
                       (lmid == LM_ID_BASE) ?  "LM_ID_BASE" :
                       (lmid == LM_ID_NEWLM) ? "LM_ID_NEWLM" :
                       "???",
                       cookie);
    
               return LA_FLG_BINDTO | LA_FLG_BINDFROM;
           }
    
           unsigned int
           la_objclose (uintptr_t *cookie)
           {
               printf("la_objclose(): %p\n", cookie);
    
               return 0;
           }
    
           void
           la_preinit(uintptr_t *cookie)
           {
               printf("la_preinit(): %p\n", cookie);
           }
    
           uintptr_t
           la_symbind32(Elf32_Sym *sym, unsigned int ndx, uintptr_t *refcook,
                   uintptr_t *defcook, unsigned int *flags, const char *symname)
           {
               printf("la_symbind32(): symname = %s; sym->st_value = %p\n",
                       symname, sym->st_value);
               printf("        ndx = %u; flags = %#x", ndx, *flags);
               printf("; refcook = %p; defcook = %p\n", refcook, defcook);
    
               return sym->st_value;
           }
    
           uintptr_t
           la_symbind64(Elf64_Sym *sym, unsigned int ndx, uintptr_t *refcook,
                   uintptr_t *defcook, unsigned int *flags, const char *symname)
           {
               printf("la_symbind64(): symname = %s; sym->st_value = %p\n",
                       symname, sym->st_value);
               printf("        ndx = %u; flags = %#x", ndx, *flags);
               printf("; refcook = %p; defcook = %p\n", refcook, defcook);
    
               return sym->st_value;
           }
    
           Elf32_Addr
           la_i86_gnu_pltenter(Elf32_Sym *sym, unsigned int ndx,
                   uintptr_t *refcook, uintptr_t *defcook, La_i86_regs *regs,
                   unsigned int *flags, const char *symname, long *framesizep)
           {
               printf("la_i86_gnu_pltenter(): %s (%p)\n", symname, sym->st_value);
    
               return sym->st_value;
           }
    


* * *