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I would like to know how GCC implements exception handling for C++ programs. I couldn't find an easy-to-understand and self-explanatory article on the Web (although there are many such articles for Visual C++). All I know is that GCC's implementation is called DWARF exception handling.
I have written a small C++ program and translated it into assembly with the command:
g++ main.cpp -S -masm=intel -fno-dwarf2-cfi-asm
The main.cpp and main.s files are given here. Could anyone please explain the contents of the main.s file, especially the sections .gcc_except_table and .eh_frame line-by-line? (My OS is Ubuntu 13.04 32-bit.) Thanks!
main.cpp:
void f()
{
throw 1;
}
int main()
{
int j;
try {
f();
} catch (int i) {
j = i;
}
return 0;
}
main.s:
.file "main.cpp"
.intel_syntax noprefix
.text
.globl _Z1fv
.type _Z1fv, @function
_Z1fv:
.LFB0:
push ebp
.LCFI0:
mov ebp, esp
.LCFI1:
sub esp, 24
mov DWORD PTR [esp], 4
call __cxa_allocate_exception
mov DWORD PTR [eax], 1
mov DWORD PTR [esp+8], 0
mov DWORD PTR [esp+4], OFFSET FLAT:_ZTIi
mov DWORD PTR [esp], eax
call __cxa_throw
.LFE0:
.size _Z1fv, .-_Z1fv
.globl main
.type main, @function
main:
.LFB1:
push ebp
.LCFI2:
mov ebp, esp
.LCFI3:
and esp, -16
sub esp, 32
.LEHB0:
call _Z1fv
.LEHE0:
.L7:
mov eax, 0
jmp .L9
.L8:
cmp edx, 1
je .L6
mov DWORD PTR [esp], eax
.LEHB1:
call _Unwind_Resume
.LEHE1:
.L6:
mov DWORD PTR [esp], eax
call __cxa_begin_catch
mov eax, DWORD PTR [eax]
mov DWORD PTR [esp+24], eax
mov eax, DWORD PTR [esp+24]
mov DWORD PTR [esp+28], eax
call __cxa_end_catch
jmp .L7
.L9:
leave
.LCFI4:
ret
.LFE1:
.globl __gxx_personality_v0
.section .gcc_except_table,"a",@progbits
.align 4
.LLSDA1:
.byte 0xff
.byte 0
.uleb128 .LLSDATT1-.LLSDATTD1
.LLSDATTD1:
.byte 0x1
.uleb128 .LLSDACSE1-.LLSDACSB1
.LLSDACSB1:
.uleb128 .LEHB0-.LFB1
.uleb128 .LEHE0-.LEHB0
.uleb128 .L8-.LFB1
.uleb128 0x1
.uleb128 .LEHB1-.LFB1
.uleb128 .LEHE1-.LEHB1
.uleb128 0
.uleb128 0
.LLSDACSE1:
.byte 0x1
.byte 0
.align 4
.long _ZTIi
.LLSDATT1:
.text
.size main, .-main
.section .eh_frame,"a",@progbits
.Lframe1:
.long .LECIE1-.LSCIE1
.LSCIE1:
.long 0
.byte 0x1
.string "zPL"
.uleb128 0x1
.sleb128 -4
.byte 0x8
.uleb128 0x6
.byte 0
.long __gxx_personality_v0
.byte 0
.byte 0xc
.uleb128 0x4
.uleb128 0x4
.byte 0x88
.uleb128 0x1
.align 4
.LECIE1:
.LSFDE1:
.long .LEFDE1-.LASFDE1
.LASFDE1:
.long .LASFDE1-.Lframe1
.long .LFB0
.long .LFE0-.LFB0
.uleb128 0x4
.long 0
.byte 0x4
.long .LCFI0-.LFB0
.byte 0xe
.uleb128 0x8
.byte 0x85
.uleb128 0x2
.byte 0x4
.long .LCFI1-.LCFI0
.byte 0xd
.uleb128 0x5
.align 4
.LEFDE1:
.LSFDE3:
.long .LEFDE3-.LASFDE3
.LASFDE3:
.long .LASFDE3-.Lframe1
.long .LFB1
.long .LFE1-.LFB1
.uleb128 0x4
.long .LLSDA1
.byte 0x4
.long .LCFI2-.LFB1
.byte 0xe
.uleb128 0x8
.byte 0x85
.uleb128 0x2
.byte 0x4
.long .LCFI3-.LCFI2
.byte 0xd
.uleb128 0x5
.byte 0x4
.long .LCFI4-.LCFI3
.byte 0xc5
.byte 0xc
.uleb128 0x4
.uleb128 0x4
.align 4
.LEFDE3:
.ident "GCC: (Ubuntu/Linaro 4.7.3-1ubuntu1) 4.7.3"
.section .note.GNU-stack,"",@progbits
709
.eh_frame layout is described briefly in the LSB documentation. Ian Lance Taylor (author of the gold linker) also made some blog posts on .eh_frame and .gcc_except_table layout.
For a more reference-like description, check my Recon 2012 slides (start at 37 or so).
EDIT: here's the commented structures from your sample. First, the .eh_table (some parts omitted for clarity):
.Lframe1: # start of CFI 1
.long .LECIE1-.LSCIE1 # length of CIE 1 data
.LSCIE1: # start of CIE 1 data
.long 0 # CIE id
.byte 0x1 # Version
.string "zPL" # augmentation string:
# z: has augmentation data
# P: has personality routine pointer
# L: has LSDA pointer
.uleb128 0x1 # code alignment factor
.sleb128 -4 # data alignment factor
.byte 0x8 # return address register no.
.uleb128 0x6 # augmentation data length (z)
.byte 0 # personality routine pointer encoding (P): DW_EH_PE_ptr|DW_EH_PE_absptr
.long __gxx_personality_v0 # personality routine pointer (P)
.byte 0 # LSDA pointer encoding: DW_EH_PE_ptr|DW_EH_PE_absptr
.byte 0xc # Initial CFI Instructions
[...]
.align 4
.LECIE1: # end of CIE 1
[...]
.LSFDE3: # start of FDE 3
.long .LEFDE3-.LASFDE3 # length of FDE 3
.LASFDE3: # start of FDE 3 data
.long .LASFDE3-.Lframe1 # Distance to parent CIE from here
.long .LFB1 # initial location
.long .LFE1-.LFB1 # range length
.uleb128 0x4 # Augmentation data length (z)
.long .LLSDA1 # LSDA pointer (L)
.byte 0x4 # CFI instructions
.long .LCFI2-.LFB1
[...]
.align 4
.LEFDE3: # end of FDE 3
Next, the LSDA (language-specific data area) referenced by FDE 3:
.LLSDA1: # LSDA 1
.byte 0xff # LPStart encoding: DW_EH_PE_omit
.byte 0 # TType encoding: DW_EH_PE_ptr|DW_EH_PE_absptr
.uleb128 .LLSDATT1-.LLSDATTD1 # TType offset
.LLSDATTD1: # LSDA 1 action table
.byte 0x1 # call site encoding: DW_EH_PE_uleb128|DW_EH_PE_absptr
.uleb128 .LLSDACSE1-.LLSDACSB1 # call site table length
.LLSDACSB1: # LSDA 1 call site entries
.uleb128 .LEHB0-.LFB1 # call site 0 start
.uleb128 .LEHE0-.LEHB0 # call site 0 length
.uleb128 .L8-.LFB1 # call site 0 landing pad
.uleb128 0x1 # call site 0 action (1=action 1)
.uleb128 .LEHB1-.LFB1 # call site 1 start
.uleb128 .LEHE1-.LEHB1 # call site 1 length
.uleb128 0 # call site 1 landing pad
.uleb128 0 # call site 1 action (0=no action)
.LLSDACSE1: # LSDA 1 action table entries
.byte 0x1 # action 1 filter (1=T1 typeinfo)
.byte 0 # displacement to next action (0=end of chain)
.align 4
.long _ZTIi # T1 typeinfo ("typeinfo for int")
.LLSDATT1: # LSDA 1 TTBase
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