Date: Mon, 7 Jan 2002 17:16:42 +1100 (EST) From: Bruce Evans <bde@zeta.org.au> 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 Message-ID: <20020107164848.J468-100000@gamplex.bde.org> In-Reply-To: <20020106232937.9F87D38CC@overcee.netplex.com.au>
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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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