Re: [PATCH] fork: report pid exhaustion correctly

[Andrew Morton]

On Mon,  3 Sep 2018 04:10:16 -0700 ktsanaktsidis@xxxxxxxxxxx wrote:

> 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.

Thanks.  First ever kernel patch?  It was a damn good one!

> Fixes: 95846ecf9dac ("pid: replace pid bitmap implementation with IDR AIP")
> Signed-off-by: KJ Tsanaktsidis <ktsanaktsidis@xxxxxxxxxxx>

I'll add cc:stable here so the fix gets backported into earlier kernels