overhaul pthread cancellation

this patch improves the correctness, simplicity, and size of
cancellation-related code. modulo any small errors, it should now be
completely conformant, safe, and resource-leak free.

the notion of entering and exiting cancellation-point context has been
completely eliminated and replaced with alternative syscall assembly
code for cancellable syscalls. the assembly is responsible for setting
up execution context information (stack pointer and address of the
syscall instruction) which the cancellation signal handler can use to
determine whether the interrupted code was in a cancellable state.

these changes eliminate race conditions in the previous generation of
cancellation handling code (whereby a cancellation request received
just prior to the syscall would not be processed, leaving the syscall
to block, potentially indefinitely), and remedy an issue where
non-cancellable syscalls made from signal handlers became cancellable
if the signal handler interrupted a cancellation point.

x86_64 asm is untested and may need a second try to get it right.

1 files changed, 70 insertions, 0 deletions

diff --git a/src/thread/cancel_impl.c b/src/thread/cancel_impl.c
new file mode 100644
index 00000000..5ce545d7
--- /dev/null
+++ b/src/thread/cancel_impl.c
@@ -0,0 +1,70 @@
+#include "pthread_impl.h"
+
+long __syscall_cp_asm(volatile void *, long, long, long, long, long, long, long);
+
+long (__syscall_cp)(long nr, long u, long v, long w, long x, long y, long z)
+{
+	pthread_t self;
+	uintptr_t old_sp, old_ip;
+	long r;
+
+	if (!libc.lock || (self = __pthread_self())->canceldisable)
+		return __syscall(nr, u, v, w, x, y, z);
+
+	old_sp = self->cp_sp;
+	old_ip = self->cp_ip;
+	self->cp_sp = 0;
+	self->cp_ip = 0;
+	r = __syscall_cp_asm(&self->cp_sp, nr, u, v, w, x, y, z);
+	self->cp_sp = old_sp;
+	self->cp_ip = old_ip;
+	if (r == -EINTR && self->cancel) pthread_exit(PTHREAD_CANCELED);
+	return r;
+}
+
+static void cancel_handler(int sig, siginfo_t *si, void *ctx)
+{
+	pthread_t self = __pthread_self();
+	ucontext_t *uc = ctx;
+	uintptr_t sp = ((uintptr_t *)&uc->uc_mcontext)[CANCEL_REG_SP];
+	uintptr_t ip = ((uintptr_t *)&uc->uc_mcontext)[CANCEL_REG_IP];
+
+	if (!self->cancel || self->canceldisable) return;
+
+	if (self->cancelasync) pthread_exit(PTHREAD_CANCELED);
+
+	if (sp != self->cp_sp) {
+		if (!sp) return;
+		sigaddset(&uc->uc_sigmask, SIGCANCEL);
+		__syscall(SYS_tgkill, self->pid, self->tid, SIGCANCEL);
+		return;
+	}
+
+	if (ip <= self->cp_ip) pthread_exit(PTHREAD_CANCELED);
+}
+
+static void testcancel()
+{
+	pthread_t self = __pthread_self();
+	if (self->cancel && !self->canceldisable)
+		pthread_exit(PTHREAD_CANCELED);
+}
+
+static void init_cancellation()
+{
+	struct sigaction sa = {
+		.sa_flags = SA_SIGINFO | SA_RESTART,
+		.sa_sigaction = cancel_handler
+	};
+	sigfillset(&sa.sa_mask);
+	__libc_sigaction(SIGCANCEL, &sa, 0);
+	libc.testcancel = testcancel;
+}
+
+int pthread_cancel(pthread_t t)
+{
+	static pthread_once_t once;
+	pthread_once(&once, init_cancellation);
+	a_store(&t->cancel, 1);
+	return pthread_kill(t, SIGCANCEL);
+}