The userland_sysctl() function retries sysctl_root() until returned

error is not EAGAIN. Several sysctls that inspect another process use
p_candebug() for checking access right for the curproc. p_candebug()
returns EAGAIN for some reasons, in particular, for the process doing
exec() now. If execing process tries to lock Giant, we get a livelock,
because sysctl handlers are covered by Giant, and often do not sleep.

Break the livelock by dropping Giant and allowing other threads to
execute in the EAGAIN loop.

Also, do not return EAGAIN from p_candebug() when process is executing,
use more appropriate EBUSY error [1].

Reported and tested by:	pho
Suggested by:	rwatson [1]
Reviewed by:	rwatson, des
MFC after:	1 week

1 files changed, 8 insertions, 2 deletions

diff --git a/sys/kern/kern_sysctl.c b/sys/kern/kern_sysctl.c
index 5d79ca2..3e9878e 100644
--- a/sys/kern/kern_sysctl.c
+++ b/sys/kern/kern_sysctl.c
@@ -52,6 +52,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/mutex.h>
 #include <sys/sx.h>
 #include <sys/sysproto.h>
+#include <sys/uio.h>
 #include <sys/vimage.h>
 
 #include <security/mac/mac_framework.h>
@@ -1416,11 +1417,16 @@ userland_sysctl(struct thread *td, int *name, u_int namelen, void *old,
 	SYSCTL_LOCK();
 	CURVNET_SET(TD_TO_VNET(curthread));
 
-	do {
+	for (;;) {
 		req.oldidx = 0;
 		req.newidx = 0;
 		error = sysctl_root(0, name, namelen, &req);
-	} while (error == EAGAIN);
+		if (error != EAGAIN)
+			break;
+		DROP_GIANT();
+		uio_yield();
+		PICKUP_GIANT();
+	}
 
 	if (req.lock == REQ_WIRED && req.validlen > 0)
 		vsunlock(req.oldptr, req.validlen);