[PATCH] fork: report pid exhaustion correctly

[ktsanaktsidis]

From: KJ Tsanaktsidis <ktsanaktsidis@xxxxxxxxxxx>

Make the clone and fork syscalls return EAGAIN when the limit on the
number of pids /proc/sys/kernel/pid_max is exceeded.

Currently, when the pid_max limit is exceeded, the kernel will return
ENOSPC from the fork and clone syscalls. This is contrary to the
documented behaviour, which explicitly calls out the pid_max case as one
where EAGAIN should be returned. It also leads to really confusing error
messages in userspace programs which will complain about a lack of disk
space when they fail to create processes/threads for this reason.

This error is being returned because the alloc_pid function uses the idr
api to find a new pid; when there are none available, idr_alloc_cyclic
is returns -ENOSPC, and this is being propagated back into userspace.

This behaviour has been broken before, and was explicitly fixed in
commit 35f71bc0a09a ("fork: report pid reservation failure properly"),
so I think -EAGAIN is definitely the right thing to return in this case.
The current behaviour change dates from commit 95846ecf9dac ("pid:
replace pid bitmap implementation with IDR AIP") and was I believe
unintentional.

This patch has no impact on the case where allocating a pid fails
because the child reaper for the namespace is dead; that case will still
return -ENOMEM.

Fixes: 95846ecf9dac ("pid: replace pid bitmap implementation with IDR AIP")
Signed-off-by: KJ Tsanaktsidis <ktsanaktsidis@xxxxxxxxxxx>
---
 kernel/pid.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/pid.c b/kernel/pid.c
index de1cfc4f75a2..cdf63e53a014 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -195,7 +195,7 @@ struct pid *alloc_pid(struct pid_namespace *ns)
 		idr_preload_end();
 
 		if (nr < 0) {
-			retval = nr;
+			retval = (nr == -ENOSPC) ? -EAGAIN : nr;
 			goto out_free;
 		}
 
-- 
2.17.1