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authorRoland McGrath <roland@redhat.com>2008-07-10 14:50:39 -0700
committerIngo Molnar <mingo@elte.hu>2008-07-12 07:11:10 +0200
commiteca91e7838ec92e8c12122849ec7539c4765b689 (patch)
treebfc7decbcb96189e40307ab80f96aac59526ecc7 /kernel/printk.c
parentda1f29f5dfcc8641f9ff6f3deaa9f03b57e63229 (diff)
downloadlinux-eca91e7838ec92e8c12122849ec7539c4765b689.tar.gz
x86_64: fix delayed signals
On three of the several paths in entry_64.S that call do_notify_resume() on the way back to user mode, we fail to properly check again for newly-arrived work that requires another call to do_notify_resume() before going to user mode. These paths set the mask to check only _TIF_NEED_RESCHED, but this is wrong. The other paths that lead to do_notify_resume() do this correctly already, and entry_32.S does it correctly in all cases. All paths back to user mode have to check all the _TIF_WORK_MASK flags at the last possible stage, with interrupts disabled. Otherwise, we miss any flags (TIF_SIGPENDING for example) that were set any time after we entered do_notify_resume(). More work flags can be set (or left set) synchronously inside do_notify_resume(), as TIF_SIGPENDING can be, or asynchronously by interrupts or other CPUs (which then send an asynchronous interrupt). There are many different scenarios that could hit this bug, most of them races. The simplest one to demonstrate does not require any race: when one signal has done handler setup at the check before returning from a syscall, and there is another signal pending that should be handled. The second signal's handler should interrupt the first signal handler before it actually starts (so the interrupted PC is still at the handler's entry point). Instead, it runs away until the next kernel entry (next syscall, tick, etc). This test behaves correctly on 32-bit kernels, and fails on 64-bit (either 32-bit or 64-bit test binary). With this fix, it works. #define _GNU_SOURCE #include <stdio.h> #include <signal.h> #include <string.h> #include <sys/ucontext.h> #ifndef REG_RIP #define REG_RIP REG_EIP #endif static sig_atomic_t hit1, hit2; static void handler (int sig, siginfo_t *info, void *ctx) { ucontext_t *uc = ctx; if ((void *) uc->uc_mcontext.gregs[REG_RIP] == &handler) { if (sig == SIGUSR1) hit1 = 1; else hit2 = 1; } printf ("%s at %#lx\n", strsignal (sig), uc->uc_mcontext.gregs[REG_RIP]); } int main (void) { struct sigaction sa; sigset_t set; sigemptyset (&sa.sa_mask); sa.sa_flags = SA_SIGINFO; sa.sa_sigaction = &handler; if (sigaction (SIGUSR1, &sa, NULL) || sigaction (SIGUSR2, &sa, NULL)) return 2; sigemptyset (&set); sigaddset (&set, SIGUSR1); sigaddset (&set, SIGUSR2); if (sigprocmask (SIG_BLOCK, &set, NULL)) return 3; printf ("main at %p, handler at %p\n", &main, &handler); raise (SIGUSR1); raise (SIGUSR2); if (sigprocmask (SIG_UNBLOCK, &set, NULL)) return 4; if (hit1 + hit2 == 1) { puts ("PASS"); return 0; } puts ("FAIL"); return 1; } Signed-off-by: Roland McGrath <roland@redhat.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'kernel/printk.c')
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