• Olof Johansson's avatar
    modpost: Fix address calculation in reloc_location() · 731ece41
    Olof Johansson authored
    
    
    This patch fixes a segfault in modpost that is observed when the gold
    linker is used to link the input objects.
    
    The problem is that reloc_location (modpost.c) is computing the
    address of the relocation target incorrectly. Here, elf->hdr points
    to the beginning of the ELF file in memory, sechdr points to the
    relocation section header, section is the index of the section
    being relocated, and sechdrs[section].sh_offset would be the offset
    of that section, relative to the beginning of the ELF file. Adding
    elf->hdr + sechdrs[section].sh_offset gives you the address of the
    beginning of the section, and adding r->r_offset to that gives you the
    address of the location to be relocated. You do not need to subtract
    sechdrs[section].sh_addr from that -- the result of this is an address
    outside the file, and causes the segfault when addend_386_rel tries to
    dereference it.
    
    This bug is not observed when GNU ld is used to link the inputs. The
    object file ubuntu/omnibook/omnibook.o is the result of an ld -r of
    several other files.  When GNU ld does an ld -r, it sets the vaddr
    field for each section to 0, but gold lays out the section addresses
    sequentially instead:
    
    Section Headers:
     [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
     [ 0]                   NULL            00000000 000000 000000 00      0   0  0
     [ 1] .text             PROGBITS        00000000 000034 004794 00  AX  0   0  4
     [ 2] .data             PROGBITS        0000b9d0 0047c8 0009c0 00  WA  0   0  4
     [ 3] .bss              NOBITS          000162f8 005188 00013c 00  WA  0   0  4
     [ 4] .rodata.str1.1    PROGBITS        00004f2d 0052c4 001b1a 01 AMS  0   0  1
     [ 5] .init.text        PROGBITS        00004794 006dde 0005fa 00  AX  0   0  1
     [ 6] .exit.text        PROGBITS        00004d8e 0073d8 00018a 00  AX  0   0  1
      ...
    
    So the bug in the tool remained undiscovered because the section's vaddr
    always happened to be 0.
    Signed-off-by: default avatarRaymes Khoury <raymes@google.com>
    Signed-off-by: default avatarOlof Johansson <olof@lixom.net>
    Signed-off-by: default avatarMichal Marek <mmarek@suse.cz>
    731ece41
modpost.c 55.2 KB