Re: [PATCH] prctl: remove one-shot limitation for changing exe link

From: Oleg Nesterov <oleg@redhat.com>
To: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>
Cc: peterz@infradead.org, mingo@redhat.com, mhocko@suse.com,
	keescook@chromium.org, linux-kernel@vger.kernel.org,
	mguzik@redhat.com, bsegall@google.com, john.stultz@linaro.org,
	ebiederm@xmission.com, gorcunov@openvz.org, matthltc@us.ibm.com,
	akpm@linux-foundation.org, luto@amacapital.net, vbabka@suse.cz,
	xemul@virtuozzo.com
Subject: Re: [PATCH] prctl: remove one-shot limitation for changing exe link
Date: Tue, 12 Jul 2016 19:01:45 +0200	[thread overview]
Message-ID: <20160712170144.GC4557@redhat.com> (raw)
In-Reply-To: <69b3bfd5-02aa-51ad-12b4-dc8d330b6c67@virtuozzo.com>

On 07/12, Stanislav Kinsburskiy wrote:
>
> 12.07.2016 18:42, Oleg Nesterov пишет:
>> But I would like to ack it in any case. I never understood why do we
>> want/need this MMF_EXE_FILE_CHANGED check, I suggested to remove it
>> many times.
>>
>> And can't resist, please note the xchg() below. Currently (before this
>> patch) we do not need it. I was specially added to ensure that we can
>> just remove this test_and_set_bit(MMF_EXE_FILE_CHANGED) without adding
>> a race.
>
> Thanks, Oleg. I'll take a look.
> But should this be addressed in this patch? Especially if it's not
> needed even now (before this patch)?

Sorry for confusion...

Yes, it is not needed now (before this patch). Because only one caller
of prctl_set_mm_exe_file() can succced and update mm->exe_file. So we
could just do

	fput(mm->exe_file);
	mm->exe_file = get_file(exe.file);

But after this patch we do need this xchg(), otherwise 2 callers of
prctl_set_mm_exe_file() can race with each other. And this was the
actual reason for xchg: simplify the MMF_EXE_FILE_CHANGED removal in
future.

Oleg.