Hello,
I'm not totally sure that GLibc's setcontext is safe to use in a
signal handler. So, I decided I was going to play things safe and let
rt_sigreturn switch stacks for me instead. However, rt_sigreturn seems
to reject my substitute stack frame as invalid and I'm not sure why.
Thank you,
Steven Stewart-Gallus
The code:
#include <stdio.h>
#include <signal.h>
#include <ucontext.h>
#include <unistd.h>
static ucontext_t alternate_context;
static char alternate_context_stack[SIGSTKSZ];
static char signal_stack[SIGSTKSZ];
static void alternate_context_func(void)
{
puts("alternate context!");
}
static void switch_stack(int signo, siginfo_t *infop, void *untyped_ucontextp)
{
ucontext_t * ucontextp = untyped_ucontextp;
/* I'm not sure if setcontext is async-signal-safe so set the
* context using the return from the signal handler.
*/
*ucontextp = alternate_context;
#ifdef __linux__
ucontextp->uc_mcontext.fpregs = &ucontextp->__fpregs_mem;
#endif
}
int main(void)
{
{
stack_t stack = { 0 };
stack.ss_sp = signal_stack;
stack.ss_size = sizeof signal_stack;
sigaltstack(&stack, NULL);
}
getcontext(&alternate_context);
alternate_context.uc_stack.ss_sp = alternate_context_stack;
alternate_context.uc_stack.ss_size = sizeof alternate_context_stack;
makecontext(&alternate_context, (void (*)(void))alternate_context_func, 0U);
{
struct sigaction action = { 0 };
action.sa_sigaction = switch_stack;
action.sa_flags = SA_SIGINFO;
sigfillset(&action.sa_mask);
sigaction(SIGRTMIN, &action, NULL);
}
raise(SIGRTMIN);
}
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