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

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Message-ID: <CAEf4BzYkPxUcQK2VWEE+8N=U5CXjtUNs6GfbfW2+GoTDebk19A@mail.gmail.com>
Date: Tue, 18 Nov 2025 11:38:36 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: Matthew Wilcox <willy@...radead.org>, Christoph Hellwig <hch@...radead.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 8:12 AM Darrick J. Wong <djwong@...nel.org> wrote:
>
> On Tue, Nov 18, 2025 at 03:37:09PM +0000, Matthew Wilcox wrote:
> > On Tue, Nov 18, 2025 at 05:03:24AM -0800, Christoph Hellwig wrote:
> > > On Mon, Nov 17, 2025 at 10:45:31AM -0800, Andrii Nakryiko wrote:
> > > > As I replied on another email, ideally we'd have some low-level file
> > > > reading interface where we wouldn't have to know about secretmem, or
> > > > XFS+DAX, or whatever other unusual combination of conditions where
> > > > exposed internal APIs like filemap_get_folio() + read_cache_folio()
> > > > can crash.
> > >
> > > The problem is that you did something totally insane and it kinda works
> > > most of the time.
> >
> > ... on 64-bit systems.  The HIGHMEM handling is screwed up too.
> >
> > > But bpf or any other file system consumer has
> > > absolutely not business poking into the page cache to start with.
> >
> > Agreed.
> >
> > > And I'm really pissed off that you wrote and merged this code without
> > > ever bothering to talk to a FS or MM person who have immediately told
> > > you so.  Let's just rip out this buildid junk for now and restart
> > > because the problem isn't actually that easy.
> >
> > Oh, they did talk to fs & mm people originally and were told NO, so they
> > sneaked it in through the BPF tree.
> >
> > https://lore.kernel.org/all/20230316170149.4106586-1-jolsa@kernel.org/
> >
> > > > The only real limitation is that we'd like to be able to control
> > > > whether we are ok sleeping or not, as this code can be called from
> > > > pretty much anywhere BPF might run, which includes NMI context.
> > > >
> > > > Would this kiocb_read() approach work under those circumstances?
> > >
> > > No.  IOCB_NOWAIT is just a hint to avoid blocking function calls.
> > > It is not guarantee and a guarantee is basically impossible.
> >
> > I'm not sure I'd go that far -- I think we're pretty good about not
> > sleeping when IOCB_NOWAIT is specified and any remaining places can
> > be fixed up.
> >
> > But I am inclined to rip out the buildid code, just because the
> > authors have been so rude.
>
> Which fstest actually checks the functionality of the buildid code?
> I don't find any, which means none of the fs people have a good signal
> for breakage in this, um, novel file I/O path.

We have plenty of build ID tests in BPF selftest that validate this
functionality:

  - tools/testing/selftests/bpf/prog_tests/stacktrace_build_id.c
  - tools/testing/selftests/bpf/prog_tests/stacktrace_build_id_nmi.c
  - tools/testing/selftests/bpf/prog_tests/build_id.c

This functionality is exposed to BPF (and PROCMAP_QUERY, which has its
own mm selftests), so that's where we test this. So we'll know at the
very least when trees merge that something is broken.

>
> --D