[PATCH 4.18 50/88] fork: report pid exhaustion correctly

[Greg Kroah-Hartman]

4.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: KJ Tsanaktsidis <ktsanaktsidis@xxxxxxxxxxx>

commit f83606f5eb007adc33bc8541ede00590f477bdeb upstream.

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 alloc_pid() uses the idr api to find
a new pid; when there are none available, idr_alloc_cyclic() returns
-ENOSPC, and this is being propagated back to 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.

Link: http://lkml.kernel.org/r/20180903111016.46461-1-ktsanaktsidis@xxxxxxxxxxx
Fixes: 95846ecf9dac ("pid: replace pid bitmap implementation with IDR AIP")
Signed-off-by: KJ Tsanaktsidis <ktsanaktsidis@xxxxxxxxxxx>
Reviewed-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Gargi Sharma <gs051095@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/pid.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -195,7 +195,7 @@ struct pid *alloc_pid(struct pid_namespa
 		idr_preload_end();
 
 		if (nr < 0) {
-			retval = nr;
+			retval = (nr == -ENOSPC) ? -EAGAIN : nr;
 			goto out_free;
 		}