The following program segfaults when you send SIGUSR1 to it.

I'm able to reproduce this on an AMD64 FC4 system with
kernel-smp-2.6.13-1.1526_FC4, and glibc-2.3.5-10.3, but only when compiled
without -m32 (the 32-bit compiled version works fine).

AFAICT, the call to setcontext jumps to the PLT, enters into ld.so for symbol
resolution, returns, and then jumps to nowhere.

Somebody else I talked to reported a segfault when signalling the program on an
i686 Debian system with kernel 2.6.12-1-686 and libc 2.3.5-6.

setcontext called with ucontext_t initialized by getcontext works fine, so I'm
not sure if this is actually a glibc problem or if the kernel-created ucontext
is bad. The fact that (I think) I'm seeing the program crash before it even
enters setcontext makes me suspect glibc.

---

#define _GNU_SOURCE
#include <signal.h>
#include <ucontext.h>
#include <stddef.h>

void handle_sig(int signo, siginfo_t *siginfo, void * context)
{
     ucontext_t *c = context;
     setcontext(c);
}


int main(int argc, char* argv[])
{
     sigset_t ss;
     sigemptyset(&ss);

     struct sigaction sa = {
          .sa_sigaction = handle_sig,
          .sa_mask = ss,
          .sa_flags = SA_SIGINFO
     };

     sigaction(SIGUSR1, &sa, NULL);

     while (1);

     return 0;
}