https://sourceware.org/bugzilla/show_bug.cgi?id=20117
Bug ID: 20117
Summary: GOT relocation changes another instruction
Product: binutils
Version: 2.25
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: ld
Assignee: unassigned at sourceware dot org
Reporter: louis.granboulan.developer at gmail dot com
Target Milestone: ---
Created attachment 9278
--> https://sourceware.org/bugzilla/attachment.cgi?id=9278&action=edit
Almost minimal example that triggers the bug
Using the attached file, on a recent Debian (with gcc 4.9.2 as the default
compiler), we can reproduce the bug with the following steps.
echo 'long DEBUGLEVEL; int main(){return 66;}' > main.c
gcc -m32 -c -o main.o main.c
gcc -m32 -c -o bug-ld.o bug-ld.s
gcc -m32 -o main main.o bug-ld.o
The bug can be seen using objdump.
objdump -d bug-ld.o | grep jmp
returns
73: eb 8b jmp 0 <bug>
but
objdump -d main | grep jmp
80482a6: ff 25 bc 96 04 08 jmp *0x80496bc
80482b0: ff 25 c0 96 04 08 jmp *0x80496c0
80482bb: e9 e0 ff ff ff jmp 80482a0 <_init+0x2c>
80482c0: ff 25 c4 96 04 08 jmp *0x80496c4
80482cb: e9 d0 ff ff ff jmp 80482a0 <_init+0x2c>
80483ab: eb 93 jmp 8048340 <register_tm_clones>
80483c6: e9 75 ff ff ff jmp 8048340 <register_tm_clones>
8048448: eb 8d jmp 80483d7 <bug+0x2>
80484b1: eb 0d jmp 80484c0 <__libc_csu_fini>
We can see that "eb 8b" has been transformed to "eb 8d".
This appears to be a bug of ld. The last command can be replaced by
ld -m elf_i386 -o main main.o bug-ld.o
/usr/lib/gcc/x86_64-linux-gnu/4.9/32/crtend.o /usr/lib32/crtn.o
/usr/lib32/crt1.o /usr/lib32/crti.o
/usr/lib/gcc/x86_64-linux-gnu/4.9/32/crtbegin.o -lc
The bug is diffult to trigger: if you replace in bug-ld.s 0xeb, 0x8b by
something else, e.g. 0xeb, 0x8c, then the bug is not triggered.
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