path: root/src/thread/pthread_create.c
diff options
authorRich Felker <>2011-03-24 14:18:00 -0400
committerRich Felker <>2011-03-24 14:18:00 -0400
commitb470030f839a375e5030ec9d44903ef7581c15a2 (patch)
tree462b1df89a3ea45bcf50b9d0a844472576ed6585 /src/thread/pthread_create.c
parent095820016689dfdc9141f477a86de22054c86078 (diff)
overhaul cancellation to fix resource leaks and dangerous behavior with signals
this commit addresses two issues: 1. a race condition, whereby a cancellation request occurring after a syscall returned from kernelspace but before the subsequent CANCELPT_END would cause cancellable resource-allocating syscalls (like open) to leak resources. 2. signal handlers invoked while the thread was blocked at a cancellation point behaved as if asynchronous cancellation mode wer in effect, resulting in potentially dangerous state corruption if a cancellation request occurs. the glibc/nptl implementation of threads shares both of these issues. with this commit, both are fixed. however, cancellation points encountered in a signal handler will not be acted upon if the signal was received while the thread was already at a cancellation point. they will of course be acted upon after the signal handler returns, so in real-world usage where signal handlers quickly return, it should not be a problem. it's possible to solve this problem too by having sigaction() wrap all signal handlers with a function that uses a pthread_cleanup handler to catch cancellation, patch up the saved context, and return into the cancellable function that will catch and act upon the cancellation. however that would be a lot of complexity for minimal if any benefit...
Diffstat (limited to 'src/thread/pthread_create.c')
1 files changed, 11 insertions, 5 deletions
diff --git a/src/thread/pthread_create.c b/src/thread/pthread_create.c
index 17a47f6a..785a82b8 100644
--- a/src/thread/pthread_create.c
+++ b/src/thread/pthread_create.c
@@ -42,6 +42,12 @@ void __pthread_unwind_next(struct __ptcb *cb)
static void docancel(struct pthread *self)
struct __ptcb cb = { .__next = self->cancelbuf };
+ sigset_t set;
+ self->canceldisable = 1;
+ self->cancelasync = 0;
+ sigemptyset(&set);
+ sigaddset(&set, SIGCANCEL);
+ __libc_sigprocmask(SIG_UNBLOCK, &set, 0);
@@ -50,17 +56,17 @@ static void cancel_handler(int sig, siginfo_t *si, void *ctx)
struct pthread *self = __pthread_self();
if (si->si_code > 0 || si->si_pid != self->pid) return;
self->cancel = 1;
- if (self->canceldisable || (!self->cancelasync && !self->cancelpoint))
- return;
- docancel(self);
+ if (self->canceldisable) return;
+ if (self->cancelasync || (self->cancelpoint==1 && PC_AT_SYS(ctx)))
+ docancel(self);
static void cancelpt(int x)
struct pthread *self = __pthread_self();
if (self->canceldisable) return;
- self->cancelpoint = x;
- if (self->cancel) docancel(self);
+ if ((self->cancelpoint+=x)==1 && x>=0 && self->cancel)
+ docancel(self);
/* "rsyscall" is a mechanism by which a thread can synchronously force all