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

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Message-ID: <aRxr5l-usmPvenbM@casper.infradead.org>
Date: Tue, 18 Nov 2025 12:51:50 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Christoph Hellwig <hch@...radead.org>
Cc: 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
Subject: Re: [PATCH] mm/filemap: fix NULL pointer dereference in
 do_read_cache_folio()

On Mon, Nov 17, 2025 at 09:05:17PM -0800, Christoph Hellwig wrote:
> On Sun, Nov 16, 2025 at 10:32:12PM +0000, Matthew Wilcox wrote:
> > I don't think it's necessarily all that hard to make buildid work
> > for DAX.  It's probably something like:
> > 
> > 	if (IS_DAX(file_inode(file)))
> > 		kernel_read(file, buf, count, &pos);
> > 
> > but that's just off the top of my head.
> 
> The code should just unconditionally use kernel_read().  Relying
> on ->read_folio to just work is only something file system code and
> library code called by the file systems can assume.
> 
> Something reading ELF headers has no bunsiness poking into this layer.

Please read the rest of the thread; this code can be called in contexts
that can't block.  That was why I proposed the kiocb_read() refactoring
that I would expect you to have an opinion on.