linux-kernel - Re: [PATCH 3/6] exec: Stop open coding mutex_lock_killable of cred_guard

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Date:   Sat, 09 May 2020 14:57:33 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Rob Landley <rob@...dley.net>,
        Bernd Edlinger <bernd.edlinger@...mail.de>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/6] exec: Stop open coding mutex_lock_killable of cred_guard_mutex

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Fri, May 8, 2020 at 11:48 AM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>>
>>
>> Oleg modified the code that did
>> "mutex_lock_interruptible(&current->cred_guard_mutex)" to return
>> -ERESTARTNOINTR instead of -EINTR, so that userspace will never see a
>> failure to grab the mutex.
>>
>> Slightly earlier Liam R. Howlett defined mutex_lock_killable for
>> exactly the same situation but it does it a little more cleanly.
>
> What what what?
>
> None of this makes sense. Your commit message is completely wrong, and
> the patch is utter shite.
>
> mutex_lock_interruptible() and mutex_lock_killable() are completely
> different operations, and the difference has absolutely nothing to do
> with  -ERESTARTNOINTR or -EINTR.
>
> mutex_lock_interruptible() is interrupted by any signal.
>
> mutex_lock_killable() is - surprise surprise - only interrupted by
> SIGKILL (in theory any fatal signal, but we never actually implemented
> that logic, so it's only interruptible by the known-to-always-be-fatal
> SIGKILL).
>
>> Switch the code to mutex_lock_killable so that it is clearer what the
>> code is doing.
>
> This nonsensical patch makes me worry about all your other patches.
> The explanation is wrong, the patch is wrong, and it changes things to
> be fundamentally broken.
>
> Before this, ^C would break out of a blocked execve()/ptrace()
> situation. After this patch, you need special tools to do so.
>
> This patch is completely wrong.

Sigh.  Brain fart on my part. You are correct.

I saw the restart, and totally forgot that it allows the handling of a
signal before restarting the system call.

Except for the handling of the signal in userspace it is the same as
mutex_lock_killable but that is a big big big if.

My apologies.  I will drop this patch.

Eric