The SFrame Format
Version 2

Indu Bhagat

Copyright c 2021-2024 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the termsof the GNU General Public License, Version 3 or any later version published by the Free
Software Foundation. A copy of the license is included in the section entitled “GNU General
Public License”.

iTable of Contents
1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1
1.2

2

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Changes from Version 1 to Version 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

SFrame Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1

SFrame Preamble . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1.1 SFrame Magic Number and Endianness . . . . . . . . . . . . . . . . . . . . 2
2.1.2 SFrame Version . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1.3 SFrame Flags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 SFrame Header . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2.1 SFrame ABI/arch Identifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3 SFrame FDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3.1 The SFrame FDE Info Word . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3.2 The SFrame FDE Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3.3 The SFrame FRE Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.4 SFrame FRE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4.1 The SFrame FRE Info Word . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3

ABI/arch-specific Definition . . . . . . . . . . . . . . . . . . 11
3.1
3.2

AMD64 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
AArch64 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Appendix A Generating Stack
Traces using SFrame . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1

1 Introduction
1.1 Overview
The SFrame stack trace information is provided in a loaded section, known as the .sframe
section. When available, the .sframe section appears in a new segment of its own,
PT GNU SFRAME.
The SFrame format is currently supported only for select ABIs, namely, AMD64 and
AAPCS64.
A portion of the SFrame format follows an unaligned on-disk representation. Some data
structures, however, (namely the SFrame header and the SFrame function descriptor entry)
have elements at their natural boundaries. All data structures are packed, unless otherwise
stated.
The contents of the SFrame section are stored in the target endianness, i.e., in theendianness of the system on which the section is targeted to be used. An SFrame section
reader may use the magic number in the SFrame header to identify the endianness of theSFrame section.
Addresses in this specification are expressed in bytes.
The rest of this specification describes the current version of the format, SFRAME_VERSION_
2, in detail. Additional sections outline the major changes made to each previously publishedversion of the SFrame stack trace format.
The associated API to decode, probe and encode the SFrame section, provided via
libsframe, is not accompanied here at this time. This will be added later.
This document is intended to be in sync with the C code in sframe.h. Please report
discrepancies between the two, if any.

1.2 Changes from Version 1 to Version 2
The following is a list of the changes made to the SFrame stack trace format since Version 1
was published.
• Add an unsigned 8-bit integral field to the SFrame function descriptor entry to encode
the size of the repetitive code blocks. Such code blocks, e.g, pltN entries, use an SFrame
function descriptor entry of type SFRAME FDE TYPE PCMASK.
• Add an unsigned 16-bit integral field to the SFrame function descriptor entry to serveas padding. This helps ensure natural alignment for the members of the data structure.
• The above two imply that each SFrame function descriptor entry has a fixed size of 20
bytes instead of its size of 17 bytes in SFrame format version 1.
SFrame version 1 is now obsolete and should not be used.

2

2 SFrame Section
The SFrame section consists of an SFrame header, starting with a preamble, and two othersub-sections, namely the SFrame function descriptor entry (SFrame FDE) sub-section, andthe SFrame frame row entry (SFrame FRE) sub-section.

2.1 SFrame Preamble
The preamble is a 32-bit packed structure; the only part of the SFrame section whose format
cannot vary between versions.
typedef struct sframe_preamble{
uint16_t sfp_magic;
uint8_t sfp_version;
uint8_t sfp_flags;} ATTRIBUTE_PACKED sframe_preamble;
Every element of the SFrame preamble is naturally aligned.
All values are stored in the endianness of the target system for which the SFrame section
is intended. Further details:
Offset0x00

Type
uint16_t

Name
sfp_magic

Description
The magic number for SFrame section: 0xdee2.
Defined as a macro SFRAME_MAGIC.

0x02

uint8_tsfp_version

The version number of this SFrame section. See
Section 2.1.2 [SFrame Version], page 2, for theset of valid values. Current version is SFRAME_
VERSION_2.

0x03

uint8_tsfp_flags

Flags (section-wide) for this SFrame section. See
Section 2.1.3 [SFrame Flags], page 3, for the setof valid values.

2.1.1 SFrame Magic Number and Endianness
SFrame sections are stored in the target endianness of the system that consumes them. A
consumer library reading or writing SFrame sections should detect foreign-endianness byinspecting the SFrame magic number in the sfp_magic field in the SFrame header. It may
then provide means to endian-flip the SFrame section as necessary.

2.1.2 SFrame Version
The version of the SFrame format can be determined by inspecting sfp_version. The
following versions are currently valid:
Version NameSFRAME_VERSION_1
SFRAME_VERSION_2

Number1
2

Description
First version, obsolete.
Current version, under development.

This document describes SFRAME_VERSION_2.

Chapter 2: SFrame Section3

2.1.3 SFrame Flags
The preamble contains bitflags in its sfp_flags field that describe various section-wide
properties.
The following flags are currently defined.
Flag
SFRAME_F_FDE_SORTED

Versions
All

Value
0x1

SFRAME_F_FRAME_POINTER

All

0x2

MeaningFunction Descriptor Entries
are sorted on PC.
All functions in the object file
preserve frame pointer.

The purpose of SFRAME F FRAME POINTER flag is to facilitate stack tracers toreliably fallback on the frame pointer based stack tracing method, if SFrame information is
not present for some function in the SFrame section.
Further flags may be added in future.

2.2 SFrame Header
The SFrame header is the first part of an SFrame section. It begins with the SFrame
preamble. All parts of it other than the preamble (see Section 2.1 [SFrame Preamble],
page 2) can vary between SFrame file versions. It contains things that apply to the section
as a whole, and offsets to the various other sub-sections defined in the format. As with therest of the SFrame section, all values are stored in the endianness of the target system.
The two sub-sections tile the SFrame section: each section runs from the offset given
until the start of the next section. An explicit length is given for the last sub-section, theSFrame Frame Row Entry (SFrame FRE) sub-section.
typedef struct sframe_header{
sframe_preamble sfh_preamble;
uint8_t sfh_abi_arch;
int8_t sfh_cfa_fixed_fp_offset;
int8_t sfh_cfa_fixed_ra_offset;
uint8_t sfh_auxhdr_len;
uint32_t sfh_num_fdes;
uint32_t sfh_num_fres;uint32_t sfh_fre_len;
uint32_t sfh_fdeoff;
uint32_t sfh_freoff;} ATTRIBUTE_PACKED sframe_header;
Every element of the SFrame header is naturally aligned.
The sub-section offsets, namely sfh_fdeoff and sfh_freoff, in the SFrame header are
relative to the end of the SFrame header; they are each an offset in bytes into the SFrame
section where the SFrame FDE sub-section and the SFrame FRE sub-section respectively
start.
The SFrame section contains sfh_num_fdes number of fixed-length array elements in the
SFrame FDE sub-section. Each array element is of type SFrame function descriptor entry;

Chapter 2: SFrame Section

4

each providing a high-level function description for the purpose of stack tracing. More details
in a subsequent section. See Section 2.3 [SFrame Function Descriptor Entries], page 5.
Next, the SFrame FRE sub-section, starting at offset sfh_fre_off, describes the stack
trace information for each function, using a total of sfh_num_fres number of variable-length
array elements. Each array element is of type SFrame frame row entry. See Section 2.4[SFrame Frame Row Entries], page 8.
SFrame header allows specifying explicitly the fixed offsets from CFA, if any, from which
FP or RA may be recovered. For example, in AMD64, the stack offset of the return address
is CFA - 8. Since these offsets are expected to be in close vicinity to the CFA in most
ABIs, sfh_cfa_fixed_fp_offset and sfh_cfa_fixed_ra_offset are limited to signed
8-bit integers.
The SFrame format has made some provisions for supporting more ABIs/architecturesin the future. One of them is the concept of the auxiliary SFrame header. Bytes in theauxiliary SFrame header may be used to convey further ABI-specific information. The
sframe_header structure provides an unsigned 8-bit integral field to denote the size (inbytes) of an auxiliary SFrame header. The auxiliary SFrame header follows right after
the sframe_header structure. As for the calculation of the sub-section offsets, namely
sfh_fdeoff and sfh_freoff, the end of SFrame header must be the end of the auxiliary
SFrame header, if the latter is present.
Putting it all together:
Offset0x00

Typesframe_preambleNamesfh_preamble

Description
The SFrame preamble. See Section 2.1[SFrame Preamble], page 2.

0x04

uint8_tsfh_abi_arch

The ABI/arch identifier.
See
Section 2.2.1 [SFrame ABI/arch Identifier], page 5.

0x05

int8_tsfh_cfa_fixed_fp_offset

The CFA fixed FP offset, if any.

0x06

int8_tsfh_cfa_fixed_ra_offset

The CFA fixed RA offset, if any.

0x07

uint8_tsfh_auxhdr_len

Size in bytes of the auxiliary header
that follows the sframe_header structure.

0x08

uint32_tsfh_num_fdes

The number of SFrame FDEs in thesection.

0x0c

uint32_tsfh_num_fres

The number of SFrame FREs in thesection.

0x10

uint32_t

sfh_fre_len

The length in bytes of the SFrame
FRE sub-section.

Chapter 2: SFrame Section

5

0x14

uint32_t

sfh_fdeoff

The offset in bytes to the SFrame FDE
sub-section.

0x18

uint32_t

sfh_freoff

The offset in bytes to the SFrame FRE
sub-section.

2.2.1 SFrame ABI/arch Identifier
SFrame header identifies the ABI/arch of the target system for which the executable andhence, the stack trace information contained in the SFrame section, is intended. There are
currently three identifiable ABI/arch values in the format.
ABI/arch Identifier
Value Description
SFRAME_ABI_AARCH64_ENDIAN_BIG

1

AARCH64 big-endian

SFRAME_ABI_AARCH64_ENDIAN_LITTLE

2AARCH64 little-endian

SFRAME_ABI_AMD64_ENDIAN_LITTLE

3

AMD64 little-endian

The presence of an explicit identification of ABI/arch in SFrame may allow stack trace
generators to make certain ABI/arch-specific decisions.

2.3 SFrame FDE
The SFrame function descriptor entry sub-section is an array of the fixed-length SFrame
function descriptor entries (SFrame FDEs). Each SFrame FDE is a packed structure whichcontains information to describe a function’s stack trace information at a high-level.
The array of SFrame FDEs is sorted on the sfde_func_start_address if the SFrame
section header flag sfp_flags has SFRAME_F_FDE_SORTED set. Typically (as is the case withGNU ld) a linked object or executable will have the SFRAME_F_FDE_SORTED set. This makes
the job of a stack tracer easier as it may then employ binary search schemes to look for thepertinent SFrame FDE.
typedef struct sframe_func_desc_entry{
int32_t sfde_func_start_address;
uint32_t sfde_func_size;
uint32_t sfde_func_start_fre_off;
uint32_t sfde_func_num_fres;uint8_t sfde_func_info;
uint8_t sfde_func_rep_size;
uint16_t sfde_func_padding2;} ATTRIBUTE_PACKED sframe_func_desc_entry;
Every element of the SFrame function descriptor entry is naturally aligned.
sfde_func_start_fre_off is the offset to the first SFrame FRE for the function. This
offset is relative to the end of the SFrame FDE sub-section (unlike the sub-section offsets in
the SFrame header, which are relative to the end of the SFrame header).

Chapter 2: SFrame Section6

sfde_func_info is the SFrame FDE "info word", containing information on the FRE
type and the FDE type for the function See Section 2.3.1 [The SFrame FDE Info Word],
page 6.
Apart from the sfde_func_padding2, the SFrame FDE has some currently unused bitsin the SFrame FDE info word, See Section 2.3.1 [The SFrame FDE Info Word], page 6, that
may be used for the purpose of extending the SFrame file format specification for future
ABIs.
Following table describes each component of the SFrame FDE structure:
Offset0x00

Typeint32_tNamesfde_func_start_address

Description
Signed 32-bit integral field denoting thevirtual memory address of the describedfunction.

0x04

uint32_t

sfde_func_size

Unsigned 32-bit integral field specifying
the size of the function in bytes.

0x08

uint32_t

sfde_func_start_fre_off

Unsigned 32-bit integral field specifying
the offset in bytes of the function’s first
SFrame FRE in the SFrame section.

0x0c

uint32_t

sfde_func_num_fres

Unsigned 32-bit integral field specifying
the total number of SFrame FREs used
for the function.

0x10

uint8_tsfde_func_info

Unsigned 8-bit integral field specifying theSFrame FDE info word. See Section 2.3.1[The SFrame FDE Info Word], page 6.

0x11

uint8_tsfde_func_rep_size

Unsigned 8-bit integral field specifying the size of the repetitive code
block for which an SFrame FDE of
type SFRAME FDE TYPE PCMASK isused. For example, in AMD64, the sizeof a pltN entry is 16 bytes.

0x12

uint16_t

sfde_func_padding2

Padding of 2 bytes. Currently unused
bytes.

2.3.1 The SFrame FDE Info Word
The info word is a bitfield split into three parts. From MSB to LSB:
Bit offset
7–6

Name
unusedDescription
Unused bits.

Chapter 2: SFrame Section

7

5

pauth_key(For AARCH64) Specify which key is used for signing thereturn addresses in the SFrame FDE. Two possible values:
SFRAME AARCH64 PAUTH KEY A (0), or
SFRAME AARCH64 PAUTH KEY B (1).
Ununsed in AMD64.

4

fdetype

Specify the SFrame FDE type. Two possible values:
SFRAME FDE TYPE PCMASK (1), or
SFRAME FDE TYPE PCINC (0).
See Section 2.3.2 [The SFrame FDE Types], page 7.

0–3

fretype

Choice of three SFrame FRE types. See Section 2.3.3 [TheSFrame FRE Types], page 7.

2.3.2 The SFrame FDE Types
The SFrame format defines two types of FDE entries. The choice of which SFrame FDE
type to use is made based on the instruction patterns in the relevant program stub.
An SFrame FDE of type SFRAME_FDE_TYPE_PCINC is an indication that the PCs in theFREs should be treated as increments in bytes. This is used fo the the bulk of the executablecode of a program, which contains instructions with no specific pattern.
In contrast, an SFrame FDE of type SFRAME_FDE_TYPE_PCMASK is an indication that thePCs in the FREs should be treated as masks. This type is useful for the cases where asmall pattern of instructions in a program stub is used repeatedly for a specific functionality.
Typical usecases are pltN entries and trampolines.
Name of SFrame FDE type

Value

Description

SFRAME FDE TYPE PCINC

0

Stacktracers perform a(PC >= FRE START ADDR) to look up amatching FRE.

SFRAME FDE TYPE PCMASK

1

Stacktracers perform a(PC % REP BLOCK SIZE>= FRE START ADDR) to look up a matching FRE. REP BLOCK SIZE is the size inbytes of the repeating block of program instructions and is encoded via sfde_func_
rep_size in the SFrame FDE.

2.3.3 The SFrame FRE Types
A real world application can have functions of size big and small. SFrame format defines
three types of SFrame FRE entries to effeciently encode the stack trace information forsuch a variety of function sizes. These representations vary in the number of bits needed to
encode the start address offset in the SFrame FRE.
The following constants are defined and used to identify the SFrame FRE types:

Chapter 2: SFrame Section

8

Name

Value

Description

SFRAME_FRE_TYPE_ADDR1

0

The start address offset (in bytes) of theSFrame FRE is an unsigned 8-bit value.

SFRAME_FRE_TYPE_ADDR2

1

The start address offset (in bytes) of theSFrame FRE is an unsigned 16-bit value.

The start address offset (in bytes) of theSFrame FRE is an unsigned 32-bit value.
A single function must use the same type of SFrame FRE throughout. The identifier toreflect the chosen SFrame FRE type is stored in the fretype bits in the SFrame FDE info
word, See Section 2.3.1 [The SFrame FDE Info Word], page 6.
SFRAME_FRE_TYPE_ADDR42

2.4 SFrame FRE
The SFrame frame row entry sub-section contains the core of the stack trace information.
An SFrame frame row entry (FRE) is a self-sufficient record containing SFrame stack trace
information for a range of contiguous (instruction) addresses, starting at the specified offset
from the start of the function.
Each SFrame FRE encodes the stack offsets to recover the CFA, FP and RA (whereapplicable) for the respective instruction addresses. To encode this information, each SFrame
FRE is followed by S*N bytes, where:− S is the size of a stack offset for the FRE, and− N is the number of stack offsets in the FRE
The entities S, N are encoded in the SFrame FRE info word, via the fre_offset_size
and the fre_offset_count respectively. More information about the precise encoding andrange of values for S and N is provided later in the See Section 2.4.1 [The SFrame FRE InfoWord], page 9.
It is important to underline here that although the canonical interpretation of thesebytes is as stack offsets (to recover CFA, FP and RA), these bytes may be used by future
ABIs/architectures to convey other information on a per SFrame FRE basis.
In summary, SFrame file format, by design, supports a variable number of stack offsets
at the tail end of each SFrame FRE. To keep the SFrame file format specification flexible
yet extensible, the interpretation of the stack offsets is ABI/arch-specific. The precise
interpretation of the FRE stack offsets in the currently supported ABIs/architectures
is covered in the ABI/arch-specific definition of the SFrame file format, See Chapter 3[ABI/arch-specific Definition], page 11.
Next, the definitions of the three SFrame FRE types are as follows:
typedef struct sframe_frame_row_entry_addr1{
uint8_t sfre_start_address;
sframe_fre_info sfre_info;} ATTRIBUTE_PACKED sframe_frame_row_entry_addr1;
typedef struct sframe_frame_row_entry_addr2

Chapter 2: SFrame Section

9{
uint16_t sfre_start_address;
sframe_fre_info sfre_info;} ATTRIBUTE_PACKED sframe_frame_row_entry_addr2;
typedef struct sframe_frame_row_entry_addr4{
uint32_t sfre_start_address;
sframe_fre_info sfre_info;} ATTRIBUTE_PACKED sframe_frame_row_entry_addr4;
For ensuring compactness, SFrame frame row entries are stored unaligned on disk. Appropriate mechanisms need to be employed, as necessary, by the serializing and deserializing
entities, if unaligned accesses need to be avoided.
sfre_start_address is an unsigned 8-bit/16-bit/32-bit integral field identifies the start
address of the range of program counters, for which the SFrame FRE applies. The value
encoded in the sfre_start_address field is the offset in bytes of the start address of theSFrame FRE, from the start address of the function.
Further SFrame FRE types may be added in future.

2.4.1 The SFrame FRE Info Word
The SFrame FRE info word is a bitfield split into four parts. From MSB to LSB:
Bit offset
7

Name
fre_mangled_ra_p

Description
Indicate whether the return address is mangled with any authorization bits (signed
RA).

5-6

fre_offset_size

Size of stack offsets in bytes. Valid values
are:
SFRAME FRE OFFSET 1B,
SFRAME FRE OFFSET 2B, andSFRAME FRE OFFSET 4B.

1-4

fre_offset_count

A max value of 15 is allowed. Typically, avalue of upto 3 is sufficient for most ABIs
to track all three of CFA, FP and RA.

0

fre_cfa_base_reg_id

Distinguish between SP or FP based CFA
recovery.

Name

Value

Description

SFRAME_FRE_OFFSET_1B

0

All stack offsets following the fixed-length
FRE structure are 1 byte long.

Chapter 2: SFrame Section

10

SFRAME_FRE_OFFSET_2B

1

All stack offsets following the fixed-length
FRE structure are 2 bytes long.

SFRAME_FRE_OFFSET_4B

2

All stack offsets following the fixed-length
FRE structure are 4 bytes long.

11

3 ABI/arch-specific Definition
This section covers the ABI/arch-specific definition of the SFrame file format.
Currently, the only part of the SFrame file format definition that is ABI/arch-specificis the interpretation of the variable number of bytes at the tail end of each SFrame FRE.
Currently, these bytes are only used for representing stack offsets (for all the currently
supported ABIs). It is recommended to peruse this section along with See Section 2.4[SFrame Frame Row Entries], page 8, for clarity of context.
Future ABIs must specify the algorithm for identifying the appropriate SFrame FRE
stack offsets in this chapter. This should inevitably include the blueprint for interpretingthe variable number of bytes at the tail end of the SFrame FRE for the specific ABI/arch.
Any further provisions, e.g., using the auxiliary SFrame header, etc., if used, must also beoutlined here.

3.1 AMD64
Irrespective of the ABI, the first stack offset is always used to locate the CFA, by interpretingit as: CFA = BASE_REG + offset1. The identification of the BASE_REG is done by using the
fre_cfa_base_reg_id field in the SFrame FRE info word.
In AMD64, the return address (RA) is always saved on stack when a function call is
executed. Further, AMD64 ABI mandates that the RA be saved at a fixed offset fromthe CFA when entering a new function. This means that the RA does not need to be
tracked per SFrame FRE. The fixed offset is encoded in the SFrame file format in the field
sfh_cfa_fixed_ra_offset in the SFrame header. See Section 2.2 [SFrame Header], page 3.
Hence, the second stack offset (in the SFrame FRE), when present, will be used to locate
the FP, by interpreting it as: FP = CFA + offset2.
Hence, in summary:
Offset ID
1
2

Interpretation in AMD64
CFA = BASE_REG + offset1
FP = CFA + offset2

3.2 AArch64
Irrespective of the ABI, the first stack offset is always used to locate the CFA, by interpretingit as: CFA = BASE_REG + offset1. The identification of the BASE_REG is done by using the
fre_cfa_base_reg_id field in the SFrame FRE info word.
In AARCH64, the AAPCS64 standard specifies that the Frame Record saves both FP
and LR (a.k.a the RA). However, the standard does not mandate the precise location in
the function where the frame record is created, if at all. Hence the need to track RA in the
SFrame stack trace format. As RA is being tracked in this ABI, the second stack offset isalways used to locate the RA, by interpreting it as: RA = CFA + offset2. The third stack
offset will be used to locate the FP, by interpreting it as: FP = CFA + offset3.
Given the nature of things, the number of stack offsets seen on AARCH64 per SFrame
FRE is either 1 or 3.
Hence, in summary:Chapter 3: ABI/arch-specific Definition

Offset ID
1
2
3

Interpretation in AArch64
CFA = BASE_REG + offset1
RA = CFA + offset2
FP = CFA + offset3

12

13

Appendix A Generating Stack Traces using
SFrameUsing some C-like pseudocode, this section highlights how SFrame provides a simple, fastand low-overhead mechanism to generate stack traces. Needless to say that for generating
accurate and useful stack traces, several other aspects will need attention: finding anddecoding bits of SFrame section(s) in the program binary, symbolization of addresses, toname a few.
In the current context, a frame is the abstract construct that encapsulates the following
information:− program counter (PC),− stack pointer (SP), and− frame pointer (FP)With that said, establishing the first frame should be trivial:// frame 0
frame->pc = current_IP;
frame->sp = get_reg_value (REG_SP);frame->fp = get_reg_value (REG_FP);
where REG_SP and REG_FP are are ABI-designated stack pointer and frame pointerregisters respectively.
Next, given frame N, generating stack trace needs us to get frame N+1. This can be done
as follows:// Get the PC, SP, and FP for frame N.
pc = frame->pc;
sp = frame->sp;
fp = frame->fp;// Populate frame N+1.
int err = get_next_frame (&next_frame, pc, sp, fp);
where given the values of the program counter, stack pointer and frame pointer fromframe N, get_next_frame populates the provided next_frame object and returns the error
code, if any. In the following pseudocode for get_next_frame, the sframe_* functions fetch
information from the SFrame section.
fre = sframe_find_fre (pc);
if (fre)// Whether the base register for CFA tracking is REG_FP.
base_reg_val = sframe_fre_base_reg_fp_p (fre) ? fp : sp;// Get the CFA stack offset from the FRE.
cfa_offset = sframe_fre_get_cfa_offset (fre);// Get the fixed RA offset or FRE stack offset as applicable.
ra_offset = sframe_fre_get_ra_offset (fre);// Get the fixed FP offset or FRE stack offset as applicable.
fp_offset = sframe_fre_get_fp_offset (fre);cfa = base_reg_val + cfa_offset;
next_frame->sp = cfa;

Appendix A: Generating Stack Traces using SFrame

ra_stack_loc = cfa + ra_offset;// Get the address stored in the stack location.
next_frame->pc = read_value (ra_stack_loc);
if (fp_offset is VALID)
fp_stack_loc = cfa + fp_offset;// Get the value stored in the stack location.
next_frame->fp = read_value (fp_stack_loc);else// Continue to use the value of fp as it has not// been clobbered by the current frame yet.
next_frame->fp = fp;
else
ret = ERR_NO_SFRAME_FRE;

14

15

Index
A

P

ABI/arch-specific Definition . . . . . . . . . . . . . . . . . . . . 11

Provisions for future ABIs . . . . . . . . . . . . . . . . . . 4, 6, 8CS

Changes from Version 1 to Version 2 . . . . . . . . . . . . 1

SFrame ABI/arch Identifier . . . . . . . . . . . . . . . . . . . . . 5
SFrame FDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
SFrame Flags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
SFrame FRE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
SFrame header . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
SFrame magic number . . . . . . . . . . . . . . . . . . . . . . . . . . 2
SFrame preamble . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
SFrame Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
SFrame versions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Eendianness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

I
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

O
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

T
The SFrame FDE Info Word . . . . . . . . . . . . . . . . . . . . 6
The SFrame FRE Info Word . . . . . . . . . . . . . . . . . . . . 9