https://sourceware.org/bugzilla/show_bug.cgi?id=21285
Bug ID: 21285
Summary: R_386_GOT32 for baseless case looks implemented
incorrectly.
Product: binutils
Version: 2.29 (HEAD)
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: gold
Assignee: ccoutant at gmail dot com
Reporter: georgerim at gmail dot com
CC: ian at airs dot com
Target Milestone: ---
Created attachment 9926
--> https://sourceware.org/bugzilla/attachment.cgi?id=9926&action=edit
got32-i386.s.tmp.o
According to ABI, R_386_GOT32
(https://github.com/hjl-tools/x86-psABI/wiki/intel386-psABI-draft.pdf) can be
calculated as G + A without base register when position-independent code is
disabled.
Imagine next asm code:
.text
.globl bar
.type bar, @function
bar:
.globl foo
.type foo, @function
foo:
_start:
movl bar@GOT, %eax
movl foo@GOT, %ebx
movl bar@GOT(%eax), %eax
movl foo@GOT(%ebx), %eax
I used llvm-mc -relax-relocations=false to compile it into got32-i386.s.tmp.o
(attached), its disassembly is:
Disassembly of section .text:
00000000 <bar>:
0: a1 00 00 00 00 mov 0x0,%eax
5: 8b 1d 00 00 00 00 mov 0x0,%ebx
b: 8b 80 00 00 00 00 mov 0x0(%eax),%eax
11: 8b 83 00 00 00 00 mov 0x0(%ebx),%eax
Relocation section '.rel.text' at offset 0x8c contains 4 entries:
Offset Info Type Sym.Value Sym. Name
00000001 00000203 R_386_GOT32 00000000 bar
00000007 00000303 R_386_GOT32 00000000 foo
0000000d 00000203 R_386_GOT32 00000000 bar
00000013 00000303 R_386_GOT32 00000000 foo
Notice that "mov 0x0,%eax" case is encoded using a1 and not 8b xx. Gold
implementation determines baseless case using following check:
baseless = (view[-1] & 0xc7) == 0x5;
(https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=blob;f=gold/i386.cc;h=0b447ef0d68f3ea8965091502d5a1302d69d1b32;hb=HEAD#l2895)
That works incorrectly with "a1 00 00 00 00", gold uses 0xa1 opcode in this
check, assuming it is ModRM byte, when it is not. And result calculation seems
to be broken:
08048094 <bar>:
8048094: a1 fc ff ff ff mov 0xfffffffc,%eax
8048099: 8d 1d 94 80 04 08 lea 0x8048094,%ebx
804809f: 8d 80 a0 e0 ff ff lea -0x1f60(%eax),%eax
80480a5: 8d 83 a0 e0 ff ff lea -0x1f60(%ebx),%eax
0xfffffffc is offset in GOT, calculated using "G + A - GOT", when it should be
value calculated using "G + A" I think.
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