| Issue |
135223
|
| Summary |
[LLD] relinking objects that contain COMMON symbols adds superfluous COMMON section.
|
| Labels |
lld:ELF
|
| Assignees |
|
| Reporter |
mandlebug
|
When performing incremental linking on object files that contain symbols with COMMON linkage, the output object will contain the original COMMON symbols but will add a COMMON section. This COMMON section ends up getting copied into the .bss section of the final binary.
Reproducer:
header.h
```
int a;
int b;
void foo(void);
void bar(int*);
```
main.c:
```
#include "header.h"
int main(void) {
a = 1;
foo();
return a;
}
```
foo.c:
```
#include "header.h"
void foo(void) {
bar(&a);
}
```
bar.c:
```
#include "header.h"
void bar(int *ip) {
*ip = 2;
}
```
build and link commands:
```
clang -c -fcommon main.c foo.c bar.c
clang main.o foo.o bar.o -o no_relinking.out
ld.lld --hash-style=gnu --eh-frame-hdr -m elf64lppc -o relink1.o main.o foo.o -r
ld.lld --hash-style=gnu --eh-frame-hdr -m elf64lppc -o relink2.o relink1.o bar.o -r
clang relink2.o -o with_relinking.out
```
examining relink1.o we have both 'a' and 'b' as COMMON symbols still defined, and a section named COMMON with the same length of sizeof(a) + sizeof(b).
`readelf --sections --wide --syms relink1.o | grep COM`
```
[ 1] COMMON NOBITS 0000000000000000 000040 000008 00 WA 0 0 4
9: 0000000000000004 4 OBJECT GLOBAL DEFAULT COM a
11: 0000000000000004 4 OBJECT GLOBAL DEFAULT COM b
```
Similarly for relink2.o we have 'a' and 'b' still defined as tentative definitions, and a 16 byte long COMMON section.
Comparing with_relinking.out and no_relinking.out bss sections we see the COMMON section has been retained in the final image.
no_relinking.out:
```
[26] .bss NOBITS 0000000000030da0 000da0 000009 00 WA 0 0 4
no_relinking.out: file format elf64-powerpcle
Disassembly of section .bss:
0000000000030da0 <a>:
30da0: 00 00 00 00 .long 0x0
0000000000030da4 <b>:
30da4: 00 00 00 00 .long 0x0
0000000000030da8 <completed.0>:
30da8: Address 0x0000000000030da8 is out of bounds.
```
with_relinking.out:
```
[27] .bss NOBITS 0000000000030da0 000da0 00001c 00 WA 0 0 4
Disassembly of section .bss:
0000000000030da0 <a>:
30da0: 00 00 00 00 .long 0x0
0000000000030da4 <b>:
30da4: 00 00 00 00 .long 0x0
0000000000030da8 <completed.0>:
30da8: 00 00 00 00 .long 0x0
30dac: 00 00 00 00 .long 0x0
30db0: 00 00 00 00 .long 0x0
30db4: 00 00 00 00 .long 0x0
30db8: 00 00 00 00 .long 0x0
```
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