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SubjectRe: [PATCH] i386: fix stack alignment for signal handlers


On Tue, 13 Sep 2005, Markus F.X.J. Oberhumer wrote:
>
> It seems that the current signal code always sets up a stack frame so that
> signal handlers are run with a somewhat mis-aligned stack, i.e. (esp % 8 == 4).

Actually, not really.

They get entered with the stack pointer aligned, at least in my tests.

#include <stdio.h>
#include <signal.h>
#include <unistd.h>

extern void handler(int);
void *saved_esp;

asm("handler: movl %esp,saved_esp; ret");

int main(int argc, char **argv)
{
signal(SIGALRM, handler);
alarm(1);
pause();
printf("%p\n", saved_esp);
return 0;
}

always prints out an aligned address for me.

You seem to be expecting that the address be aligned "before the return
address push", which is a totally different thing. Quite frankly, I don't
know which one gcc prefers or whether there's an ABI specifying any
preferences.

Linus
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