Skip site navigation (1)Skip section navigation (2) 
Date:      Thu, 16 May 2024 20:06:46 +0800
From:      Zhenlei Huang <zlei@FreeBSD.org>
To:        FreeBSD Current <current@freebsd.org>
Subject:   gcc behavior of init priority of .ctors and .dtors section
Message-ID:  <3ECF8C28-D2D9-4212-B025-3EC64E46BADC@FreeBSD.org>
next in thread | raw e-mail | index | archive | help 
Hi,

I'm recently working on https://reviews.freebsd.org/D45194 and got =
noticed
that gcc behaves weirdly.

A simple source file to demonstrate that.

```
# cat ctors.c

#include <stdio.h>

__attribute__((constructor(101))) void init_101() { puts("init 1"); }
__attribute__((constructor(65535))) void init_65535() { puts("init 3"); =
}
__attribute__((constructor)) void init() { puts("init 4"); }
__attribute__((constructor(65535))) void init_65535_2() { puts("init =
5"); }
__attribute__((constructor(65534))) void init_65534() { puts("init 2"); =
}

int main() { puts("main"); }

__attribute__((destructor(65534))) void fini_65534() { puts("fini 2"); }
__attribute__((destructor(65535))) void fini_65535() { puts("fini 3"); }
__attribute__((destructor)) void fini() { puts("fini 4"); }
__attribute__((destructor(65535))) void fini_65535_2() { puts("fini 5"); =
}
__attribute__((destructor(101))) void fini_101() { puts("fini 1"); }

# clang ctors.c && ./a.out
init 1
init 2
init 3
init 4
init 5
main
fini 5
fini 4
fini 3
fini 2
fini 1
```

clang with the option -fno-use-init-array and run will produce the same =
result, which
is what I expected.

gcc13 from ports
```
# gcc ctors.c && ./a.out
init 1
init 2
init 5
init 4
init 3
main
fini 3
fini 4
fini 5
fini 2
fini 1
```

The above order is not expected. I think clang's one is correct.

Further hacking with readelf shows that clang produces the right order =
of
section .rela.ctors but gcc does not.

```
# clang -fno-use-init-array -c ctors.c && readelf -r ctors.o | grep =
'Relocation section with addend (.rela.ctors)' -A5 > clang.txt
# gcc -c ctors.c && readelf -r ctors.o | grep 'Relocation section with =
addend (.rela.ctors)' -A5 > gcc.txt
# diff clang.txt gcc.txt
3,5c3,5
< 000000000000 000800000001 R_X86_64_64         0000000000000060 =
init_65535_2 + 0
< 000000000008 000700000001 R_X86_64_64         0000000000000040 init + =
0
< 000000000010 000600000001 R_X86_64_64         0000000000000020 =
init_65535 + 0
---
> 000000000000 000600000001 R_X86_64_64         0000000000000011 =
init_65535 + 0
> 000000000008 000700000001 R_X86_64_64         0000000000000022 init + =
0
> 000000000010 000800000001 R_X86_64_64         0000000000000033 =
init_65535_2 + 0
```

The above show clearly gcc produces the wrong order of section =
`.rela.ctors`.

Is that expected behavior ?

I have not tried Linux version of gcc.


Best regards,
Zhenlei

Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?3ECF8C28-D2D9-4212-B025-3EC64E46BADC>