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The SFrame format
*****************

This manual describes version 1 of the SFrame file format.  SFrame stands for Simple Frame format.  SFrame format keeps track of the minimal necessary information needed for stack unwinding:

  - Canonical Frame Address (CFA).

  - Frame Pointer (FP).

  - Return Address (RA).

The reason for existence of the SFrame format is to support fast, online backtracing using a simple unwinder.

Overview
********

The SFrame unwind 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.

The contents of the SFrame section are stored in the target endianness, i.e., in the endianness of the system on which the section is targetted to be used. An SFrame section reader may use the magic number in the SFrame header to identify the endianness of the SFrame section.

Addresses in this specification are expressed in bytes.

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 descrepancies between the two, if any.

SFrame section
**************

The SFrame section consists of an SFrame header, starting with a preamble, and two other sub-sections, namely the SFrame Function Descriptor Entry (SFrame FDE) sub-section, and the SFrame Frame Row Entry (SFrame FRE) sub-section.

SFrame Preamble
===============

The preamble is a 32-bit packed structure; the only part of the SFrame 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;

All values are stored in the endianness of the target system for which the SFrame section is intended.  Further details:

Offset   Name   Description
0x00
uint16_t sfp_magic
The magic number for SFrame section: 0xdee2.  Defined as a macro SFRAME_MAGIC.

0x02
uint8_t sfp_version
The version number of this SFrame section.  See SFrame version, for the
set of valid values.  Current version is
SFRAME_VERSION_1.

0x03
uint8_t sfp_flags
Flags (section-wide) for this SFrame section.  See SFrame flags, for the
set of valid values.

SFrame endianness
-----------------

SFrame sections are stored in the target endianness of the system that consumes them.  The SFrame library (libsframe) can, however, detect whether to endian-flip an SFrame section at decode time, by inspecting the sfp_magic field in the SFrame header (If it appears as 0xe2de, endian-flipping is needed).

SFrame version
--------------

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

Version   Number   Description
SFRAME_VERSION_1
1   First version, under development.

This section documents SFRAME_VERSION_1.

SFrame flags
------------

The preamble contains bitflags in its sfp_flags field that describe various section-wide properties.

The following flags are currently defined.

Flag   Versions   Value   Meaning
SFRAME_F_FDE_SORTED   All   0x1   Function Descriptor
Entries are sorted on PC.
SFRAME_F_FRAME_POINTER   All   0x2
Functions preserve frame-pointer.

Further flags may be added in future.

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 SFrame Preamble) 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 the rest 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, the SFrame 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;

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.

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 this offset is in close vicinity with the CFA in most ABIs, sfh_cfa_fixed_fp_offset and sfh_cfa_fixed_ra_offset are limited to signed 8-bit integers.

SFrame format has provisioned for future ABIs/architectures that it may support.  The sframe_header structure provides an unsigned 8-bit integral field to denote the size of an auxilliary SFrame header.  The auxilliary SFrame header follows right after the sframe_header structure.  As for the offset calculations, the end of SFrame header must be the end of the auxilliary SFrame header, if the latter is present.

Tieing it all together:

Offset   Name   Description
0x00
sframe_preamble sfh_preamble
The SFrame preamble. See SFrame Preamble.

0x04
uint8_t sfh_abi_arch
The ABI/arch identifier.  See SFrame ABI/arch identifier.

0x05
int8_t sfh_cfa_fixed_fp_offset
The CFA fixed FP offset, if any.

0x06
int8_t sfh_cfa_fixed_ra_offset
The CFA fixed RA offset, if any.

0x07
uint8_t sfh_auxhdr_len
Size in bytes of the auxilliary header that follows the
sframe_header structure.

0x08
uint32_t sfh_num_fdes
The number of SFrame FDEs in the section.

0xc
uint32_t sfh_num_fres
The number of SFrame FREs in the section.

0x10
uint32_t sfh_fre_len
The length in bytes of the SFrame FRE sub-section.

0x14
uint32_t sfh_fdeoff
The offset in bytes of the SFrame FDE sub-section.  This sub-section
contains sfh_num_fdes number of fixed-length array elements.  The array
element is of type SFrame function desciptor entry, each providing a
high-level function description for backtracing.
See SFrame Function Descriptor Entries.

0x18
uint32_t sfh_freoff
The offset in bytes of the SFrame FRE sub-section, the core of the SFrame
section, which describes the unwind information using variable-length array
elements. See SFrame Frame Row Entries.

SFrame ABI/arch identifier
--------------------------

SFrame header identifies the ABI/arch of the target system for which the executable and it's unwind information 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
2   AARCH64 little-endian

SFRAME_ABI_AMD64_ENDIAN_LITTLE
3   AMD64 little-endian

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

SFrame FDE
==========

The SFrame Function Descriptor Entry sub-section is a sorted array of fixed-length SFrame function descriptor entries (SFrame FDEs).  Each SFrame FDE is a packed structure which contains information to describe a function's unwind information at a high-level.

     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;
     } ATTRIBUTE_PACKED sframe_func_desc_entry;

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 offsets in the SFrame header, which are relative to the end of the SFrame header).

sfde_func_info is the "info word", containing information on the FRE type and the FDE type for the function see The SFrame FDE info word.

Following table describes each component of the SFrame FDE structure:

Offset   Name   Description
0x00
int32_t sfde_func_start_address
Signed 32-bit integral field denoting the virtual memory address of the
described function.

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_t sfde_func_info
The SFrame FDE info word. See The SFrame FDE info word.

The SFrame FDE info word
------------------------

The info word is a bitfield split into three parts.  From MSB to LSB:

Bit offset   Name   Description
7--5
unused
Unused bits.

4
fdetype
SFRAME_FDE_TYPE_PCMASK (1) or SFRAME_FDE_TYPE_PCINC (0). See The SFrame FDE types.

0--3
fretype
Choice of three SFrame FRE types. See The SFrame FRE types.

The SFrame FDE types
--------------------

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 the FREs should be treated as increments in bytes.  This is used fo the the bulk of the executable code 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 the PCs in the FREs should be treated as masks.  This type is useful for the cases where a small 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   Unwinders perform a (PC >= FRE_START_ADDR) to look up a matching FRE.

SFRAME_FDE_TYPE_PCMASK
1    Unwinders perform a (PC & FRE_START_ADDR_AS_MASK >= FRE_START_ADDR_AS_MASK)
to look up a matching FRE.

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 represent the unwind information for such 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:

Name   Value   Description

SFRAME_FRE_TYPE_ADDR1
0
The start address offset (in bytes) of the SFrame FRE is an unsigned
8-bit value.

SFRAME_FRE_TYPE_ADDR2
1
The start address offset (in bytes) of the SFrame FRE is an unsigned
16-bit value.

SFRAME_FRE_TYPE_ADDR4
2
The start address offset (in bytes) of the SFrame FRE is an unsigned
32-bit value.

A single function must use the same type of FRE throughout.  The choice of which SFrame FRE is used to encode the unwind information of a function, is stored in the see The SFrame FDE info word.

SFrame FRE
==========

The SFrame Frame Row Entry sub-section contains the core of the unwind information.

An SFrame Frame Row Entry is a self-sufficient record containing SFrame unwind info for a range of contiguous addresses, starting at the specified offset from the start of the function.  Each SFrame Frame Row Entry is followed by S*N bytes, where:

  - S is the size of the stack frame offset for the FRE, and

  - N is the number of stack frame offsets in the FRE

The stack offsets, following the FRE, are interpreted in order as follows:

  - The first offset is always used to locate the CFA, by interpreting it as: CFA = BASE_REG + offset1.

  - If RA is being tracked, the second offset is always used to locate the RA, by interpreting it as: RA = CFA + offset2.  If RA is not being tracked and FP is being tracked, the second offset will be used to locate the FP, by interpreting it as: FP = CFA + offset2.

  - If both RA and FP are being tracked, the third offset will be used to locate the FP, by interpreting it as FP = CFA + offset3.

The entities S, N and BASE_REG are identified using the SFrame FRE info word, a.k.a. the sframe_fre_info see The SFrame FRE info word.

Following are the definitions of the allowed SFrame FRE:

     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
     {
       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;

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 the SFrame FRE, from the start address of the function.

Further FRE types may be added in future.

The SFrame FRE info word
------------------------

The SFrame FRE info word is a bitfield split into four parts.  From MSB to LSB:

Bit offset   Name   Description
7
unused
Unused bit.

5-6
fre_offset_size
Size of stack offsets in bytes.  Valid values are SFRAME_FRE_OFFSET_1B,
SFRAME_FRE_OFFSET_2B, and SFRAME_FRE_OFFSET_4B.

1-4
fre_offset_count
A value of upto 3 is allowed 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.

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.