linux-kernel - Re: [PATCH] mm/filemap: fix NULL pointer dereference in do_read_cache

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Message-ID: <CAEf4BzYWBUE0VseYgoa4r+UndxiOdCWazZ5FgZxi=dfhXaM16g@mail.gmail.com>
Date: Wed, 19 Nov 2025 09:12:13 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: Matthew Wilcox <willy@...radead.org>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	"Darrick J. Wong" <djwong@...nel.org>, SHAURYA RANE <ssrane_b23@...vjti.ac.in>, akpm@...ux-foundation.org, 
	shakeel.butt@...ux.dev, eddyz87@...il.com, andrii@...nel.org, ast@...nel.org, 
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, linux-kernel-mentees@...ts.linux.dev, 
	skhan@...uxfoundation.org, david.hunter.linux@...il.com, khalid@...nel.org, 
	syzbot+09b7d050e4806540153d@...kaller.appspotmail.com, 
	bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH] mm/filemap: fix NULL pointer dereference in do_read_cache_folio()

On Tue, Nov 18, 2025 at 9:50 PM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Tue, Nov 18, 2025 at 11:27:47AM -0800, Andrii Nakryiko wrote:
> > Then please help make it better, give us interfaces you think are
> > appropriate. People do use this functionality in production, it's
> > important and we are not going to drop it. In non-sleepable mode it's
> > best-effort, if the requested part of the file is paged in, we'll
> > successfully read data (such as ELF's build ID), and if not, we'll
> > report that to the BPF program as -EFAULT. In sleepable mode, we'll
> > wait for that part of the file to be paged in before proceeding.
> > PROCMAP_QUERY ioctl() is always in sleepable mode, so it will wait for
> > file data to be read.
>
> That's pretty demanding:  "If you don't give me the interface that I want
> I'll just poke into internals and do broken shit" isn't really the
> best way to make friends and win influence.,

Did you read the second part of my reply? The functionality in
question ([0]) was developed in the open, over multiple revisions,
with both mm and fsdevel mailing list CC'ed. Matthew Wilcox did look
at this, provided feedback and suggestion to use filemap_get_folio() +
read_cache_folio(), which I did incorporate.

  [0] https://lore.kernel.org/bpf/20240829174232.3133883-1-andrii@kernel.org/

>
> > If you don't like the implementation, please help improve it, don't
> > just request dropping it "because BPF folks" or anything like that.
>
> Again, you're trying to put a lot of work you should have done on
> others.  Everyone here is pretty helpful guiding when asking for help,
> but being asked at gunpoint to cleanup the mess your created is not
> going to get everyone drop their work and jump onto your project.

Gunpoint, really?.. Am I not asking for help to improve the code? This
functionality is being used, and we can't "just rip it out" as you
propose. Let's fix it instead.