libsframe/doc/sframe-spec.texi

\input texinfo       @c                    -*- Texinfo -*-
@setfilename sframe-spec.info
@settitle The SFrame Format

@copying
Copyright @copyright{} 2021-2022 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of 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''.

@end copying

@dircategory Software development
@direntry
* SFrame: (sframe-spec).         The Simple Frame format.
@end direntry

@titlepage
@title The SFrame Format
@subtitle Version 1
@author Indu Bhagat

@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage
@contents

@ifnottex
@node Top
@top 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:

@itemize @minus
@item
Canonical Frame Address (CFA).
@item
Frame Pointer (FP).
@item
Return Address (RA).
@end itemize

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

@menu
* Overview::
* SFrame section::
* Index::
@end menu

@end ifnottex

@node Overview
@unnumbered Overview
@cindex Overview
@tindex PT_GNU_SFRAME

The SFrame unwind information is provided in a loaded section, known as the
@code{.sframe} section.  When available, the @code{.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
@code{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 @file{sframe.h}.
Please report descrepancies between the two, if any.

@node SFrame section
@chapter SFrame section
@cindex 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.

@menu
* SFrame Preamble::
* SFrame Header::
* SFrame Function Descriptor Entries::
* SFrame Frame Row Entries::
@end menu

@node SFrame Preamble
@section SFrame Preamble
@cindex SFrame preamble

The preamble is a 32-bit packed structure; the only part of the SFrame whose
format cannot vary between versions.

@example
typedef struct sframe_preamble
@{
  uint16_t sfp_magic;
  uint8_t sfp_version;
  uint8_t sfp_flags;
@} ATTRIBUTE_PACKED sframe_preamble;
@end example

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

@multitable {Offset} {@code{uint8_t sfp_version}} {The magic number for SFrame section: 0xdee2.  Defined}
@headitem Offset @tab Name @tab Description
@item 0x00
@tab @code{uint16_t sfp_magic}
@tab The magic number for SFrame section: 0xdee2.  Defined as a macro @code{SFRAME_MAGIC}.
@tindex SFRAME_MAGIC

@item 0x02
@tab @code{uint8_t sfp_version}
@tab The version number of this SFrame section.  @xref{SFrame version}, for the
set of valid values.  Current version is
@code{SFRAME_VERSION_1}.

@item 0x03
@tab @code{uint8_t sfp_flags}
@tab Flags (section-wide) for this SFrame section.  @xref{SFrame flags}, for the
set of valid values.
@end multitable

@menu
* SFrame endianness::
* SFrame version::
* SFrame flags::
@end menu

@node SFrame endianness
@subsection SFrame endianness

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

@node SFrame version
@subsection SFrame version

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

@tindex SFRAME_VERSION_1
@cindex SFrame versions
@multitable {SFRAME_VERSION_1} {Number} {First version, under development.}
@headitem Version @tab Number @tab Description
@item @code{SFRAME_VERSION_1}
@tab 1 @tab First version, under development.
@end multitable

This section documents @code{SFRAME_VERSION_1}.

@node SFrame flags
@subsection SFrame flags
@cindex SFrame flags
@comment @vindex sfp_flags
@comment @vindex SFrame section-wide flags
@comment @subsection SFrame section-wide flags

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

The following flags are currently defined.

@multitable {@code{SFRAME_F_FRAME_POINTER}} {Versions} {Value} {Function Descriptor Entries}
@headitem Flag @tab Versions @tab Value @tab Meaning
@tindex SFRAME_F_FDE_SORTED
@item @code{SFRAME_F_FDE_SORTED} @tab All @tab 0x1 @tab Function Descriptor
Entries are sorted on PC.
@tindex SFRAME_F_FRAME_POINTER
@item @code{SFRAME_F_FRAME_POINTER} @tab All @tab 0x2
@tab Functions preserve frame-pointer.
@end multitable

Further flags may be added in future.

@node SFrame Header
@section SFrame Header
@cindex 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
(@pxref{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.

@example
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;
@end example

The sub-section offsets, namely @code{sfh_fdeoff} and @code{sfh_freoff}, in the
SFrame header are relative to the @emph{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 @code{CFA - 8}.  Since this offset is in close
vicinity with the CFA in most ABIs, @code{sfh_cfa_fixed_fp_offset} and
@code{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 @code{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 @code{sframe_header}
structure.  As for the offset calculations, the @emph{end} of SFrame header
must be the end of the auxilliary SFrame header, if the latter is present.

Tieing it all together:

@multitable {Offset} {@code{int8_t sfh_cfa_fixed_fp_offset}} {The ABI/arch identifier. See above}
@headitem Offset @tab Name @tab Description
@item 0x00
@tab @code{sframe_preamble sfh_preamble}
@tab The SFrame preamble. @xref{SFrame Preamble}.

@item 0x04
@tab @code{uint8_t sfh_abi_arch}
@tab The ABI/arch identifier.  @xref{SFrame ABI/arch identifier}.

@item 0x05
@tab @code{int8_t sfh_cfa_fixed_fp_offset}
@tab The CFA fixed FP offset, if any.

@item 0x06
@tab @code{int8_t sfh_cfa_fixed_ra_offset}
@tab The CFA fixed RA offset, if any.

@item 0x07
@tab @code{uint8_t sfh_auxhdr_len}
@tab Size in bytes of the auxilliary header that follows the
@code{sframe_header} structure.

@item 0x08
@tab @code{uint32_t sfh_num_fdes}
@tab The number of SFrame FDEs in the section.

@item 0xc
@tab @code{uint32_t sfh_num_fres}
@tab The number of SFrame FREs in the section.

@item 0x10
@tab @code{uint32_t sfh_fre_len}
@tab The length in bytes of the SFrame FRE sub-section.

@item 0x14
@tab @code{uint32_t sfh_fdeoff}
@tab The offset in bytes of the SFrame FDE sub-section.  This sub-section
contains @code{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.
@xref{SFrame Function Descriptor Entries}.

@item 0x18
@tab @code{uint32_t sfh_freoff}
@tab 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. @xref{SFrame Frame Row Entries}.

@end multitable

@menu
* SFrame ABI/arch identifier::
@end menu

@node SFrame ABI/arch identifier
@subsection SFrame ABI/arch identifier
@cindex 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.

@multitable {SFRAME_ABI_AARCH64_ENDIAN_LITTLE} {Value} {@code{AARCH64 little-endian}}
@headitem ABI/arch Identifier @tab Value @tab Description

@tindex SFRAME_ABI_AARCH64_ENDIAN_BIG
@item @code{SFRAME_ABI_AARCH64_ENDIAN_BIG}
@tab 1 @tab AARCH64 big-endian

@tindex SFRAME_ABI_AARCH64_ENDIAN_LITTLE
@item @code{SFRAME_ABI_AARCH64_ENDIAN_LITTLE}
@tab 2 @tab AARCH64 little-endian

@tindex SFRAME_ABI_AMD64_ENDIAN_LITTLE
@item @code{SFRAME_ABI_AMD64_ENDIAN_LITTLE}
@tab 3 @tab AMD64 little-endian

@end multitable

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

@node SFrame Function Descriptor Entries
@section SFrame FDE
@cindex 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.

@example
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;
@end example

@code{sfde_func_start_fre_off} is the offset to the first SFrame FRE for the
function.  This offset is relative to the @emph{end of the SFrame FDE}
sub-section (unlike the offsets in the SFrame header, which are relative to the
@emph{end} of the SFrame header).

@code{sfde_func_info} is the "info word", containing information on the FRE
type and the FDE type for the function @xref{The SFrame FDE info word}.

Following table describes each component of the SFrame FDE structure:

@multitable {Offset} {@code{uint32_t sfde_func_start_fre_off}} {The ABI/arch identifier. See above}
@headitem Offset @tab Name @tab Description
@item 0x00
@tab @code{int32_t sfde_func_start_address}
@tab Signed 32-bit integral field denoting the virtual memory address of the
described function.

@item 0x04
@tab @code{uint32_t sfde_func_size}
@tab Unsigned 32-bit integral field specifying the size of the function in
bytes.

@item 0x08
@tab @code{uint32_t sfde_func_start_fre_off}
@tab Unsigned 32-bit integral field specifying the offset in bytes of the
function's first SFrame FRE in the SFrame section.

@item 0x0c
@tab @code{uint32_t sfde_func_num_fres}
@tab Unsigned 32-bit integral field specifying the total number of SFrame FREs
used for the function.

@item 0x10
@tab @code{uint8_t sfde_func_info}
@tab The SFrame FDE info word. @xref{The SFrame FDE info word}.

@end multitable

@menu
* The SFrame FDE info word::
* The SFrame FDE types::
* The SFrame FRE types::
@end menu

@cindex The SFrame FDE info word
@node The SFrame FDE info word
@subsection The SFrame FDE info word

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

@multitable {Bit offset} {@code{isroot}} {Length of variable-length data for this type (some kinds only).}
@headitem Bit offset @tab Name @tab Description
@item 7--5
@tab @code{unused}
@tab Unused bits.

@item 4
@tab @code{fdetype}
@tab SFRAME_FDE_TYPE_PCMASK (1) or SFRAME_FDE_TYPE_PCINC (0). @xref{The SFrame FDE types}.

@item 0--3
@tab @code{fretype}
@tab Choice of three SFrame FRE types. @xref{The SFrame FRE types}.
@end multitable

@node The SFrame FDE types
@subsection The SFrame FDE types
@tindex SFRAME_FDE_TYPE_PCMASK
@tindex SFRAME_FDE_TYPE_PCINC

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 @code{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 @code{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.

@multitable {Name of SFrame FDE type} {Value} {Unwinders perform a (PC >= FRE_START_ADDR)}
@headitem Name of SFrame FDE type @tab Value @tab Description

@item SFRAME_FDE_TYPE_PCINC
@tab 0 @tab Unwinders perform a (PC >= FRE_START_ADDR) to look up a matching FRE.

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

@end multitable

@node The SFrame FRE types
@subsection 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:

@multitable {SFRAME_FRE_TYPE_ADDR1} {@code{Value}} {The start address offset of FRE is an}
@headitem Name @tab Value @tab Description

@tindex SFRAME_FRE_TYPE_ADDR1
@item @code{SFRAME_FRE_TYPE_ADDR1}
@tab 0
@tab The start address offset (in bytes) of the SFrame FRE is an unsigned
8-bit value.

@tindex SFRAME_FRE_TYPE_ADDR2
@item @code{SFRAME_FRE_TYPE_ADDR2}
@tab 1
@tab The start address offset (in bytes) of the SFrame FRE is an unsigned
16-bit value.

@tindex SFRAME_FRE_TYPE_ADDR4
@item @code{SFRAME_FRE_TYPE_ADDR4}
@tab 2
@tab The start address offset (in bytes) of the SFrame FRE is an unsigned
32-bit value.
@end multitable

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 @xref{The SFrame FDE info word}.

@node SFrame Frame Row Entries
@section SFrame FRE
@cindex 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:

@itemize @minus
@item
@code{S} is the size of the stack frame offset for the FRE, and
@item
@code{N} is the number of stack frame offsets in the FRE
@end itemize

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

@itemize @minus
@item
The first offset is always used to locate the CFA, by interpreting it as:
CFA = @code{BASE_REG} + offset1.
@item
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 @emph{not} being tracked
@emph{and} FP is being tracked, the second offset will be used to locate the
FP, by interpreting it as: FP = CFA + offset2.
@item
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.
@end itemize

The entities @code{S}, @code{N} and @code{BASE_REG} are identified using the
SFrame FRE info word, a.k.a. the @code{sframe_fre_info}
@xref{The SFrame FRE info word}.

Following are the definitions of the allowed SFrame FRE:

@example
typedef struct sframe_frame_row_entry_addr1
@{
  uint8_t sfre_start_address;
  sframe_fre_info sfre_info;
@} ATTRIBUTE_PACKED sframe_frame_row_entry_addr1;
@end example

@example
typedef struct sframe_frame_row_entry_addr2
@{
  uint16_t sfre_start_address;
  sframe_fre_info sfre_info;
@} ATTRIBUTE_PACKED sframe_frame_row_entry_addr2;
@end example

@example
typedef struct sframe_frame_row_entry_addr4
@{
  uint32_t sfre_start_address;
  sframe_fre_info sfre_info;
@} ATTRIBUTE_PACKED sframe_frame_row_entry_addr4;
@end example

@code{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 @code{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.

@menu
* The SFrame FRE info word::
@end menu

@cindex The SFrame FRE info word
@node The SFrame FRE info word
@subsection The SFrame FRE info word

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

@multitable {Bit offset} {@code{fre_cfa_base_reg_id}} {Size of stack offsets in bytes.  Valid values}
@headitem Bit offset @tab Name @tab Description
@item 7
@tab @code{fre_mangled_ra_p}
@tab Indicate whether the return address is mangled with any authorization bits (signed RA).

@item 5-6
@tab @code{fre_offset_size}
@tab Size of stack offsets in bytes.  Valid values are SFRAME_FRE_OFFSET_1B,
SFRAME_FRE_OFFSET_2B, and SFRAME_FRE_OFFSET_4B.

@item 1-4
@tab @code{fre_offset_count}
@tab A value of upto 3 is allowed to track all three of CFA, FP and RA.

@item 0
@tab @code{fre_cfa_base_reg_id}
@tab Distinguish between SP or FP based CFA recovery.

@end multitable

@multitable {SFRAME_FRE_OFFSET_4B} {@code{Value}} {All stack offsets following the fixed-length}
@headitem Name @tab Value @tab Description

@tindex SFRAME_FRE_OFFSET_1B
@item @code{SFRAME_FRE_OFFSET_1B}
@tab 0
@tab All stack offsets following the fixed-length FRE structure are 1 byte
long.

@tindex SFRAME_FRE_OFFSET_2B
@item @code{SFRAME_FRE_OFFSET_2B}
@tab 1
@tab All stack offsets following the fixed-length FRE structure are 2 bytes
long.

@tindex SFRAME_FRE_OFFSET_4B
@item @code{SFRAME_FRE_OFFSET_4B}
@tab 2
@tab All stack offsets following the fixed-length FRE structure are 4 bytes
long.

@end multitable

@node Index
@unnumbered Index

@syncodeindex tp cp
@printindex cp

@bye