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From: Bryan Donlan <bdonlan@gmail.com>
Date: Sun, 4 Oct 2009 21:44:13 -0400
Message-ID: <3e8340490910041844w63720cfq7981c8f5364229de@mail.gmail.com>
Subject: Re: [PATCH] Added PR_SET_PROCTITLE_AREA option for prctl()
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>, Timo Sirainen <tss@iki.fi>,
       linux-kernel@vger.kernel.org, Ulrich Drepper <drepper@redhat.com>
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On Sun, Oct 4, 2009 at 9:38 PM, KOSAKI Motohiro
<kosaki.motohiro@jp.fujitsu.com> wrote:
>> The improvement idea is here.
>>
>> Changelog
>> ? - Added task_lock() to prctl(PR_SET_PROCTITLE_AREA)
>> ?- ?Added small input sanity check to prctl(PR_SET_PROCTITLE_AREA)
>
> Doh, task_lock() is obviously wrong. please forget this.

As another note, in general I think we'd need to hold a lock over the
entire operation. After all, if userspace changes its PROCTITLE_AREA,
and then reuses the memory for something else, we have an information
leak.

Perhaps a simpler approach would simply be to add a generation
counter. Read it once at the start, barrier, then grab the title. Then
at the end, read the generation counter again. If the value changed,
we need to start over. Also, in this case, an error when reading the
target process' memory should be ignored and retried, as we may have
hit a race in which the target process unmapped the proctitle area
after changing it.
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