On 2015-12-01 at 6:59 'Davide Libenzi' via Akaros wrote:
> Since I am currently using dummies in other code, can you check if
> something like this pleases the linker, without screwing up with
> section size constraints (multiple of object size):
>
> #define INIT_SECTION(sect) char __attribute__((__section__( #sect )))
> __init_section_##sect[0]
My usual approach is to try something and see if the compiler/linker
yell at me, then do an objdump to see if it worked. =)
> A better alternative would be weaks for __start_/__stop_. This
> generates no linker errors, even though foo is never defined:
>
> extern int __attribute__((weak)) foo[];
I like this approach.
> int main(void)
> {
> if (foo)
> return 0;
>
> return 1;
> }
>
> Look at what GCC (and linker missing relocation) does with rax/eax ☺
>
>
> 00000000004004ed <main>:
> 4004ed: 55 push %rbp
> 4004ee: 48 89 e5 mov %rsp,%rbp
> 4004f1: b8 00 00 00 00 mov $0x0,%eax
> 4004f6: 48 85 c0 test %rax,%rax
> 4004f9: 74 07 je 400502 <main+0x15>
> 4004fb: b8 00 00 00 00 mov $0x0,%eax
> 400500: eb 05 jmp 400507 <main+0x1a>
> 400502: b8 01 00 00 00 mov $0x1,%eax
> 400507: 5d pop %rbp
> 400508: c3 retq
> 400509: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
That 0x0 gets overriden at link time, so it seems to work.
foo.c:
extern int __attribute__((weak)) foo[];
int main(void)
{
if (foo)
return 0;
return 1;
}
bar.c:
int foo[5];
$ gcc -c foo.c -o foo.o
$ objdump -d foo.o
foo.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <main>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: b8 00 00 00 00 mov $0x0,%eax
9: 48 85 c0 test %rax,%rax
c: 74 07 je 15 <main+0x15>
e: b8 00 00 00 00 mov $0x0,%eax
13: eb 05 jmp 1a <main+0x1a>
15: b8 01 00 00 00 mov $0x1,%eax
1a: 5d pop %rbp
1b: c3 retq
Still has $0x0 for the instruction at byte 0x4.
$ objdump -x foo.o
foo.o: file format elf64-x86-64
foo.o
architecture: i386:x86-64, flags 0x00000011:
HAS_RELOC, HAS_SYMS
start address 0x0000000000000000
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 0000001c 0000000000000000 0000000000000000 00000040 2**0
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
1 .data 00000000 0000000000000000 0000000000000000 0000005c 2**0
CONTENTS, ALLOC, LOAD, DATA
2 .bss 00000000 0000000000000000 0000000000000000 0000005c 2**0
ALLOC
3 .comment 0000002b 0000000000000000 0000000000000000 0000005c 2**0
CONTENTS, READONLY
4 .note.GNU-stack 00000000 0000000000000000 0000000000000000 00000087 2**0
CONTENTS, READONLY
5 .eh_frame 00000038 0000000000000000 0000000000000000 00000088 2**3
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
SYMBOL TABLE:
0000000000000000 l df *ABS* 0000000000000000 foo.c
0000000000000000 l d .text 0000000000000000 .text
0000000000000000 l d .data 0000000000000000 .data
0000000000000000 l d .bss 0000000000000000 .bss
0000000000000000 l d .note.GNU-stack 0000000000000000 .note.GNU-stack
0000000000000000 l d .eh_frame 0000000000000000 .eh_frame
0000000000000000 l d .comment 0000000000000000 .comment
0000000000000000 g F .text 000000000000001c main
0000000000000000 w *UND* 0000000000000000 foo
RELOCATION RECORDS FOR [.text]:
OFFSET TYPE VALUE
0000000000000005 R_X86_64_32 foo
RELOCATION RECORDS FOR [.eh_frame]:
OFFSET TYPE VALUE
0000000000000020 R_X86_64_PC32 .text
It has a relocation record at offset 0x5, which is where the $0x0 is in
the instruction mov $0x0, %eax.
$ gcc -c bar.c -o bar.o
$ gcc -o foobar foo.o bar.o
$ objdump -d foobar
(edited to just show main)
0000000000400526 <main>:
400526: 55 push %rbp
400527: 48 89 e5 mov %rsp,%rbp
40052a: b8 50 10 60 00 mov $0x601050,%eax
40052f: 48 85 c0 test %rax,%rax
400532: 74 07 je 40053b <main+0x15>
400534: b8 00 00 00 00 mov $0x0,%eax
400539: eb 05 jmp 400540 <main+0x1a>
40053b: b8 01 00 00 00 mov $0x1,%eax
400540: 5d pop %rbp
400541: c3 retq
Barret
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