from http://www.x86-64.org/pipermail/discuss/attachments/20040816/914f9b7c/attachment.txt
EH_FRAME sections
=================
The call frame information needed for unwinding the stack is output in
an ELF section(s) of type SHT_AMD64_UNWIND. In the simplest case there
will be one such section per object file and it will be named
".eh_frame". An .eh_frame section consists of one or more
subsections. Each subsection contains a CIE (Common Information Entry)
followed by varying number of FDEs (Frame Descriptor Entry). A FDE
corresponds to an explicit or compiler generated function in a
compilation unit, all FDEs can access the CIE that begins their
subsection for data.
<<If the code for a function is not one contiguous block, there will
be a separate FDE for each contiguous sub-piece>>
If an object file contains C++ template instantiations there shall be
a separate CIE immediately preceding each FDE corresponding to an
instantiation.
<<Proposed standards and existing practice differs considerably in the
area of linker processing to eliminate duplicate instantiations. -
TBD>>
Using the preferred encoding specified below, the .eh_frame section can
be entirely resolved at link time and thus can become part of the
text segment.
.eh_frame Section Layout
------------------------
EH_PE encoding below refers to the pointer encoding as specified in the
enhanced LSB Chapter 7 for Eh_Frame_Hdr.
Common Information Entry (CIE)
------------------------------
CIE has the following format:
Length
in
Field Byte Description
----- ------ -----------
1. Length 4 Length of CIE (not including
this 4-byte field).
2. CIE id 4 Value Zero (0) for .eh_frame
(used to distinguish CIEs and
FDEs when scanning the section)
3. Version 1 Value One (1)
4. CIE Augmentation string Null-terminated string with legal
values being "" or 'z' optionally
followed by single occurrances of
'P', 'L', or 'R' in any order.
String The presence of character(s) in the
string dictates the content of
field 8, the Augmentation Section.
Each character has one or two
associated operands in the AS.
Operand order depends on
position in the string ('z' must
be first).
5. Code Align Factor uleb128 To be multiplied with the
"Advance Location" instructions in
the Call Frame Instructions
6. Data Align Factor sleb128 To be multiplied with all offset
in the Call Frame Instructions
7. Ret Address Reg 1 A "virtual' register representation
of the return address. In Dwarf V2,
this is a byte, otherwise it is
uleb128. It is a byte in gcc 3.3.x
8. Optional CIE varying Present if Augmentation String in
Augmentation Section field 4 is not 0.
z:
size uleb128 Length of the remainder of the
Augmentation Section
P:
personality_enc 1 Encoding specifier - preferred
value is a pc-relative, signed
4-byte
personality routine (encoded) Encoded pointer to personality
routine (actually to the PLT
entry for the personality
routine)
R:
code_enc 1 Non-default encoding for the
code-pointers (FDE members
"initial_location" and "address_range"
and the operand for DW_CFA_set_loc)
- preferred value is pc-relative,
signed 4-byte.
L:
lsda_enc 1 FDE augmentation bodies may contain
LSDA pointers. If so they are
encoded as specified here -
preferred value is pc-relative,
signed 4-byte possibly indirect
thru a GOT entry.
9. Optional Call Frame varying
Instructions
The size of the optional call frame instruction area must be computed
based on the overall size and the offset reached while scanning the
preceding fields of the CIE.
Frame Descriptor Entry (FDE)
----------------------------
FDE has the following format:
Length
in
Field Byte Description
----- ------ -----------
1. Length 4 Length of remainder of this FDE
2. CIE Pointer 4 Distance from this field to the
nearest preceding CIE
(the value is subtracted from the
current address). This value
can never be zero and thus can
be used to distinguish CIE's and
FDE's when scanning the
.eh_frame section
3. Initial Location varying Reference to the function code
corresponding to this FDE.
If 'R' is missing from the CIE
Augmentation String, the field is an
8-byte absolute pointer. Otherwise,
the corresponding EH_PE encoding in the
CIE Augmentation Section is used to
interpret the reference.
4. Address Range varying Size of the function code corresponding
to this FDE.
If 'R' is missing from the CIE
Augmentation String, the field is an
8-byte unsigned number. Otherwise,
the size is determined by the
corresponding EH_PE encoding in the
CIE Augmentation Section (the
value is always absolute).
5. Optional FDE varying present if CIE augmentation
Augmentation Section string is non-empty.
'z':
length uleb128 length of the remainder of the
FDE augmentation section
'L' (and length > 0):
LSDA varying LSDA pointer, encoded in the
format specified by the
corresponding operand in the CIE's
augmentation body.
6. Optional Call varying
Frame Instructions
The size of the optional call frame instruction area must be computed
based on the overall size and the offset reached while scanning the
preceding fields of the FDE.
The overall size of a .eh_frame section is given in the ELF section
header. The only way to determine the number of entries is to scan
the section till the end and count.
