linux-kernel - Re: regression: /proc/$pid/cmdline lacks trailing '\0' in 4.18-rc1

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Date:   Tue, 19 Jun 2018 11:22:55 +0200
From:   Michal Kubecek <mkubecek@...e.cz>
To:     linux-kernel@...r.kernel.org
Cc:     Alexey Dobriyan <adobriyan@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: regression: /proc/$pid/cmdline lacks trailing '\0' in 4.18-rc1

On Tue, Jun 19, 2018 at 08:07:55AM +0200, Michal Kubecek wrote:
> In v4.18-rc1, /proc/$pid/cmdline is missing final null byte which used
> to be there in v4.17 and older kernels:
> 
> 4.17.1:
> tweed:~ # cat /proc/self/cmdline | od -t c
> 0000000   c   a   t  \0   /   p   r   o   c   /   s   e   l   f   /   c
> 0000020   m   d   l   i   n   e  \0
> 0000027
> 
> 4.18-rc1:
> lion:~ # cat /proc/self/cmdline | od -t c
> 0000000   c   a   t  \0   /   p   r   o   c   /   s   e   l   f   /   c
> 0000020   m   d   l   i   n   e
> 0000026
> 
> The code has been rewritten quite a lot in 4.18-rc1 so I didn't find yet
> where exactly does the change come from. Still looking.

The issue was introduced by commit 5ab827189965 ("fs/proc: simplify and
clarify get_mm_cmdline() function"). The problem is that when looking
for the null character at or after args_end, strnlen() is used and it
returns the length _without_ the null character (if there is one) so
that we don't copy it.

I'll submit a patch once I test it.

Michal Kubecek