> real code / testcase Should be easy.... yup. With an unpatched uclibc this code hangs in pselect() because the pending signal fires during the window between the sigprocmkas() and the select(). A proper pselect() returns with errno EINTR.
--------------------------------------------------------------------------------------------- #define _GNU_SOURCE // gimme the good stuff #include <stdio.h> #include <string.h> #include <unistd.h> #include <errno.h> #include <signal.h> #include <sys/types.h> #include <sys/select.h> // our SIGALRM handler void handler(int signum) { (void)signum; write(1, "got signal\n", 11); } int main(int argc, char** argv) { (void)argc; (void)argv; // block SIGALRM. We want to handle it only when we're ready sigset_t wait_mask, mask_sigchld; sigemptyset(&mask_sigchld); sigaddset(&mask_sigchld, SIGALRM); sigprocmask(SIG_BLOCK, &mask_sigchld, &wait_mask); sigdelset(&wait_mask, SIGALRM); // register a signal handler so we can see when the signal arrives struct sigaction act; memset(&act, 0, sizeof(act)); sigemptyset(&act.sa_mask); // just in case an empty set isn't all 0's (total paranoia) act.sa_handler = handler; sigaction(SIGALRM, &act, NULL); // send ourselves a SIGARLM. It will pend until we unblock that signal in pselect() printf("sending ourselves a signal\n"); kill(getpid(), SIGALRM); printf("signal is pending; calling pselect()\n"); int rc = pselect(0, NULL, NULL, NULL, NULL, &wait_mask); int e = errno; printf("pselect() returned %d, errno %d (%s)\n", rc, e, strerror(e)); return 0; } On Fri, Dec 18, 2015 at 10:37 AM, Rich Felker <dal...@libc.org> wrote: > On Thu, Dec 17, 2015 at 09:05:48PM +0100, Waldemar Brodkorb wrote: > > Hi Nicolas, > > Nicolas S. Dade wrote, > > > > > Linux has a pselect syscall since 2.6.something. Using it > > > rather than emulating it with sigprocmask+select+sigprocmask > > > is smaller code, and works properly. (The emulation has > > > race conditions when unblocked signals arrive before or > > > after the select) > > > > Did you see the race condition in real code or do you have > > a testcase for it? > > The race conditions should be easy to reproduce since you have a > relatively huge syscall-latency-length (several us) window to race > with. Fake pselect implementations are historically known to be > harmful; the whole reason pselect was invented was to fix these races > in old code that used sigprocmask followed by select. > > Rich > _______________________________________________ uClibc mailing list uClibc@uclibc.org http://lists.busybox.net/mailman/listinfo/uclibc