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Index > Compiler Internals > [bug] Unable to use negative 32-bit offsets with extrn

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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 20176
Location: In your JS exploiting you and your system
revolution 11 Jul 2023, 13:15
Code:
format elf64
extrn foo
dd foo+1  ;  this is fine
dd foo+0  ;  this is fine
dd foo-1  ;  <--- error: value out of range.    
This can be tracked to a change at version 1.69.41 (Mar 01, 2012).

In exprcalc.inc line 1330:
Code:
get_dword_value:
        mov     [value_size],4
        or      [operand_flags],1
        call    calculate_value
        cmp     al,4
        jne     check_dword_value
        mov     [value_type],2
        mov     eax,[edi]
        cdq
        cmp     edx,[edi+4]
        jne     range_exceeded
        mov     ecx,edx
        shr     ecx,31       ; <--- using sar fixes the bug
        cmp     cl,[value_sign]
        jne     range_exceeded
        ret    
The relocation table is correct for positive offsets.
Code:
Relocation section '.rela.flat' at offset 0x48 contains 1 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000000000  00020000000b R_X86_64_32S      0000000000000000 foo + 1    
But is impossible to generate for negative offsets.
Post 11 Jul 2023, 13:15
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 20176
Location: In your JS exploiting you and your system
revolution 12 Jul 2023, 16:17
Fixed in 1.73.31
Code:
Relocation section '.rela.flat' at offset 0x50 contains 3 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000000000  00020000000b R_X86_64_32S      0000000000000000 foo + 1
000000000004  00020000000b R_X86_64_32S      0000000000000000 foo + 0
000000000008  00020000000b R_X86_64_32S      0000000000000000 foo - 1    
Post 12 Jul 2023, 16:17
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