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Message-ID: <20230814152038.GA2367@redhat.com>
Date: Mon, 14 Aug 2023 17:20:38 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
brauner@...nel.org, ebiederm@...ssion.com, david@...hat.com,
akpm@...ux-foundation.org, linux-mm@...ck.org, koct9i@...il.com,
dave@...olabs.net
Subject: Re: [PATCH] kernel/fork: stop playing lockless games for exe_file
replacement
On 08/14, Oleg Nesterov wrote:
>
> On 08/13, Mateusz Guzik wrote:
> >
> > fe69d560b5bd ("kernel/fork: always deny write access to current MM
> > exe_file") added another lock trip to synchronize the state of exe_file
> > against fork, further defeating the point of xchg.
> >
> > As such I think the atomic here only adds complexity for no benefit.
> >
> > Just write-lock around the replacement.
>
> Well, I tend to agree but can't really comment because I forgot everything
> about these code paths.
>
> But I have to admit that I don't understand the code in replace_mm_exe_file()
> without this patch...
>
> old_exe_file = xchg(&mm->exe_file, new_exe_file);
> if (old_exe_file) {
> /*
> * Don't race with dup_mmap() getting the file and disallowing
> * write access while someone might open the file writable.
> */
> mmap_read_lock(mm);
> allow_write_access(old_exe_file);
> fput(old_exe_file);
> mmap_read_unlock(mm);
> }
>
> Can someone please explain me which exactly race this mmap_read_lock() tries
> to avoid and how ?
OK, I seem to understand... without mmap_read_lock() it is possible that
- dup_mm_exe_file() sees mm->exe_file = old_exe_file
- replace_mm_exe_file() does allow_write_access(old_exe_file)
- another process does get_write_access(old_exe_file)
- dup_mm_exe_file()->deny_write_access() fails
Right?
Or something else?
Well to me Mateusz's patch does make this logic more clear ;)
Oleg.
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