linux-kernel - Re: [PATCH] block: flip iter directions in blk_rq_integrity_map

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Message-ID: <aD-J9mzq_bJe26rD@kbusch-mbp>
Date: Tue, 3 Jun 2025 17:49:10 -0600
From: Keith Busch <kbusch@...nel.org>
To: Jens Axboe <axboe@...nel.dk>
Cc: Caleb Sander Mateos <csander@...estorage.com>,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: flip iter directions in
 blk_rq_integrity_map_user()

On Tue, Jun 03, 2025 at 12:54:05PM -0600, Jens Axboe wrote:
> On 6/3/25 12:47 PM, Caleb Sander Mateos wrote:
> > blk_rq_integrity_map_user() creates the ubuf iter with ITER_DEST for
> > write-direction operations and ITER_SOURCE for read-direction ones.
> > This is backwards; writes use the user buffer as a source for metadata
> > and reads use it as a destination. Switch to the rq_data_dir() helper,
> > which maps writes to ITER_SOURCE (WRITE) and reads to ITER_DEST(READ).
> 
> Was going to ask "how did this ever work without splats", but looks like
> a fairly recent change AND it's for integrity which isn't widely used.
> But it does show a gap in testing for sure.

The change is good and correct, but it doesn't look like normal tests
would find a problem here. The iter direction in this path only adds the
FOLL_WRITE flag, which appears to just check for writable access. Unless
you're specifically testing something using read-only PTE's, a test
wouldn't have triggered an early error. ?