https://sourceware.org/bugzilla/show_bug.cgi?id=28699
Bug ID: 28699
Summary: Bad interaction between .loc, .byte causes incorrect
DWARF line table, causes corruption under gdb
Product: binutils
Version: 2.38 (HEAD)
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: gas
Assignee: unassigned at sourceware dot org
Reporter: keno at juliacomputing dot com
Target Milestone: ---
Consider the following code which is reduced from wine:
minimal.c
```
void *wine_ntdll_get_thread_data() {
void *teb;
__asm__(".byte 0x65\n\tmovq (0x30),%0" : "=r" (teb));
return teb;
}
```
gcc -g3 -S -o minimal.S minimal.c
```
[snip]
.loc 1 3 5
#APP
# 3 "minimal.c" 1
.byte 0x65
movq (0x30),%rax
# 0 "" 2
#NO_APP
[snip]
```
$ as minimal.S -o minimal.o
objdump -d minimal.o
```
Disassembly of section .text:
0000000000000000 <wine_ntdll_get_thread_data>:
0: f3 0f 1e fa endbr64
4: 55 push %rbp
5: 48 89 e5 mov %rsp,%rbp
8: 65 48 8b 04 25 30 00 mov %gs:0x30,%rax
f: 00 00
11: 48 89 45 f8 mov %rax,-0x8(%rbp)
15: 48 8b 45 f8 mov -0x8(%rbp),%rax
19: 5d pop %rbp
1a: c3 retq
```
objdump --dwarf=line minimal.o
```
Line Number Statements:
[0x00000048] Set column to 36
[0x0000004a] Extended opcode 2: set Address to 0x0
[0x00000055] Copy
[0x00000056] Set column to 5
[0x00000058] Special opcode 133: advance Address by 9 to 0x9 and Line by 2
to 3
[0x00000059] Set column to 12
[0x0000005b] Special opcode 174: advance Address by 12 to 0x15 and Line by 1
to 4
[0x0000005c] Set column to 1
[0x0000005e] Special opcode 62: advance Address by 4 to 0x19 and Line by 1
to 5
[0x0000005f] Advance PC by 2 to 0x1b
[0x00000061] Extended opcode 1: End of Sequence
```
Note that the line table has an entry at address `0x9` which is
in the middle of the instruction at `0x8`. Because of this,
GDB will set a breakpoint there and subsequently execute
the instruction without the gs prefix causing crashes.
I believe GAS is wrong here to have put 0x9 as the line
table entry and instead should have put it before the `.byte`
at 0x8.
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