linux-kernel - Re: [PATCH v2 1/6] device-mapper: Check that target specs are sufficiently aligned

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Message-ID: <ZJSmYgvwoSY6Gb4f@itl-email>
Date:   Thu, 22 Jun 2023 15:51:59 -0400
From:   Demi Marie Obenour <demi@...isiblethingslab.com>
To:     Mike Snitzer <snitzer@...nel.org>, mpatocka@...hat.com
Cc:     Alasdair Kergon <agk@...hat.com>, dm-devel@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/6] device-mapper: Check that target specs are
 sufficiently aligned

On Thu, Jun 22, 2023 at 12:28:28PM -0400, Mike Snitzer wrote:
> On Sat, Jun 03 2023 at 10:52P -0400,
> Demi Marie Obenour <demi@...isiblethingslab.com> wrote:
> 
> > Otherwise subsequent code will dereference a misaligned
> > `struct dm_target_spec *`, which is undefined behavior.
> > 
> > Signed-off-by: Demi Marie Obenour <demi@...isiblethingslab.com>
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > Cc: stable@...r.kernel.org
> > ---
> >  drivers/md/dm-ioctl.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
> > index cc77cf3d410921432eb0c62cdede7d55b9aa674a..34fa74c6a70db8aa67aaba3f6a2fc4f38ef736bc 100644
> > --- a/drivers/md/dm-ioctl.c
> > +++ b/drivers/md/dm-ioctl.c
> > @@ -1394,6 +1394,13 @@ static inline fmode_t get_mode(struct dm_ioctl *param)
> >  static int next_target(struct dm_target_spec *last, uint32_t next, void *end,
> >  		       struct dm_target_spec **spec, char **target_params)
> >  {
> > +	static_assert(_Alignof(struct dm_target_spec) <= 8,
> > +		      "struct dm_target_spec has excessive alignment requirements");
> 
> Really not sure what you mean by "has excessive alignment requirements"...

This patch checks that struct dm_target_spec is 8-byte aligned.  That is
okay if its alignment is 8 or less, but not if is 16 or more, so I added
a static assert to check that struct dm_target_spec indeed requires at
most 8-byte alignment.  That said, “excessive alignment requirements” is
(as shown by you having to ask this question) a bad error message.
Would “must not require more than 8-byte alignment” be better?

> > +	if (next % 8) {
> > +		DMERR("Next target spec (offset %u) is not 8-byte aligned", next);
> > +		return -EINVAL;
> > +	}
> > +
> >  	*spec = (struct dm_target_spec *) ((unsigned char *) last + next);
> >  	*target_params = (char *) (*spec + 1);
> >  
> 
> But this patch and patches 2 and 3 need more review. I'd like Mikulas to review.
> 
> I did pick up patches 4-6 for the upcoming 6.5 merge window.

Thanks!

> Note, please prefix with "dm ioctl" instead of "device-mapper".

Good to know, thanks!  I have several additional patches written that
require patch 4.  Should I send patches 1 through 3 in the same series
as well?
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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