Commit 731ece41 authored by Olof Johansson's avatar Olof Johansson Committed by Michal Marek

modpost: Fix address calculation in reloc_location()

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 <>
Signed-off-by: default avatarOlof Johansson <>
Signed-off-by: default avatarMichal Marek <>
parent a3ba8113
......@@ -1442,7 +1442,7 @@ static unsigned int *reloc_location(struct elf_info *elf,
int section = shndx2secindex(sechdr->sh_info);
return (void *)elf->hdr + sechdrs[section].sh_offset +
r->r_offset - sechdrs[section].sh_addr;
static int addend_386_rel(struct elf_info *elf, Elf_Shdr *sechdr, Elf_Rela *r)
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