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

[Linus Torvalds <torvalds@osdl.org>]

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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