From owner-freebsd-arch  Sun Jan  6 22:16:37 2002
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Date: Mon, 7 Jan 2002 17:16:42 +1100 (EST)
From: Bruce Evans <bde@zeta.org.au>
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To: Peter Wemm <peter@wemm.org>
Cc: Archie Cobbs <archie@dellroad.org>,
	Alfred Perlstein <bright@mu.org>, Dan Eischen <eischen@vigrid.com>,
	<arch@FreeBSD.ORG>
Subject: Re: Request for review: getcontext, setcontext, etc 
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On Sun, 6 Jan 2002, Peter Wemm wrote:

> The context argument passed in to signal handlers should be able to be used
> with setcontext() to return from the signal handler as an alternative to
> sigreturn().  Doing it in libc wont work there because you race with
> restoring the signal mask before you've finished setting the application
> state.

The i386 siglongjmp() gets this wrong despite using a syscall (sigprocmask):

%%%
#define	_POSIX_SOURCE	1

#include <setjmp.h>
#include <signal.h>
#include <string.h>
#include <unistd.h>

sigjmp_buf jb;

static void
catch(int sig)
{
	char buf[4 * 1024 * 1024];	/* Large to use up stack fast. */
	sigset_t omask;

	sigprocmask(SIG_BLOCK, NULL, &omask);
	sprintf(buf, "caught signal %d @ %p; signal mask %#x\n",
	    sig, (void *)&buf[0], *(unsigned *)&omask); /* XXX */
	write(1, buf, strlen(buf));
	kill(getpid(), sig);		/* Simulate a race. */
	sprintf(buf, "signals apparently masked correctly before siglongjmp\n");
	write(1, buf, strlen(buf));
	siglongjmp(jb, 1);
}

int
main(void)
{
	struct sigaction sa;

	sa.sa_flags = 0;
	sigemptyset(&sa.sa_mask);
	sa.sa_handler = catch;
	sigaction(SIGUSR1, &sa, NULL);
	if (sigsetjmp(jb, 1) == 0)
		kill(getpid(), SIGUSR1);
	return (0);
}
%%%

Myabe this can be fixed by restoring %esp before calling sigprocmask;
otherwise siglongjmp needs to use sigreturn (or the corresponding syscall
that restores the whole ucontext).

> Also, I noticed that the i386 patch doesn't save FP state (!) which is
> one of the primary reasons for get/setcontext().  I'm not sure if this
> can be efficiently done since this user-level function will not know if
> the current context has touched the FPU yet..

I think doing it in userland basically forces you to do physical FPU
accesses using fnsave/frstor (and something different for the SSE case?).
The case where a syscall would be most efficient (when the FPU is not used)
will be especially inefficient since it will have to trap to the kernel
anyway to access the state.

Bruce


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