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...

1 files changed, 7 insertions, 1 deletions

diff --git a/src/process/waitid.c b/src/process/waitid.c
index b1e5e9b1..4fa7c024 100644
--- a/src/process/waitid.c
+++ b/src/process/waitid.c
@@ -1,7 +1,13 @@
 #include <sys/wait.h>
 #include "syscall.h"
+#include "libc.h"
 
 int waitid(idtype_t type, id_t id, siginfo_t *info, int options)
 {
-	return syscall(SYS_waitid, type, id, info, options, 0);
+	int r;
+	CANCELPT_BEGIN;
+	r = syscall(SYS_waitid, type, id, info, options, 0);
+	if (r<0) CANCELPT_TRY;
+	CANCELPT_END;
+	return r;
 }