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Message-ID: <CAGudoHFsAU_BDCOuz8UgDBLGEM8xg=aUGjaVoqkM_Zvxo2Re_g@mail.gmail.com>
Date: Sat, 5 Aug 2023 01:25:48 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
regressions@...mhuis.info, bagasdotme@...il.com,
jacobly.alt@...il.com, willy@...radead.org,
liam.howlett@...cle.com, david@...hat.com, peterx@...hat.com,
ldufour@...ux.ibm.com, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
linux-arm-kernel@...ts.infradead.org, gregkh@...uxfoundation.org,
regressions@...ts.linux.dev, Jiri Slaby <jirislaby@...nel.org>,
Holger Hoffstätte <holger@...lied-asynchrony.com>,
stable@...r.kernel.org
Subject: Re: [PATCH v2 3/3] fork: lock VMAs of the parent process when forking
On 8/5/23, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> On Fri, 4 Aug 2023 at 14:46, Mateusz Guzik <mjguzik@...il.com> wrote:
>>
>> I don't see it mentioned in the discussion, so at a risk of ruffling
>> feathers or looking really bad I'm going to ask: is the locking of any
>> use if the forking process is single-threaded? T
>
> Sadly, we've always been able to access the mm from other processes,
> so the locking is - I think - unavoidable.
>
> And some of those "access from other processes" aren't even uncommon
> or special. It's things like "ps" etc, that do it just to see the
> process name and arguments.
>
I know of these guys, I think they are excluded as is -- they go
through access_remote_vm, starting with:
if (mmap_read_lock_killable(mm))
return 0;
while dup_mmap already write locks the parent's mm.
I don't see any surprise relocks of the semaphore.
Granted, should someone *bypass* this mechanism the above would be moot.
--
Mateusz Guzik <mjguzik gmail.com>
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