Re: gcc behavior of init priority of .ctors and .dtors section

On Thu, May 16, 2024 at 08:06:46PM +0800, Zhenlei Huang wrote:
> 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.
Why do you add that switch?

> 
> 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.
Note that init array vs. init function behavior is encoded by a note added
by crt1.o.  I suspect that the problem is that gcc port is built without
--enable-initfini-array configure option.