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

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Message-ID: <aR1awLOhdOXNMl9c@infradead.org>
Date: Tue, 18 Nov 2025 21:50:56 -0800
From: Christoph Hellwig <hch@...radead.org>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Matthew Wilcox <willy@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.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 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.,

> 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.