[PATCH] dm ebs: drop dirty bufio state when discarding blocks

[PATCH] dm ebs: drop dirty bufio state when discarding blocks

[Samuel Moelius]

dm-ebs can discard a block while a dirty dm-bufio buffer for the
same block is still cached.  If that buffer is later written back,
stale data can be written over the discarded state.

That resurrects data that userspace explicitly discarded and breaks
the expected discard semantics of the target.

Invalidate or clean the matching bufio state when processing discards
so that old dirty buffers cannot be written back after the discard.

Assisted-by: Codex:gpt-5.5-cyber-preview
Signed-off-by: Samuel Moelius <sam.moelius@trailofbits.com>
---
 drivers/md/dm-ebs-target.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/md/dm-ebs-target.c b/drivers/md/dm-ebs-target.c
index 1e52bde48b91..9fece535a0b3 100644
--- a/drivers/md/dm-ebs-target.c
+++ b/drivers/md/dm-ebs-target.c
@@ -212,8 +212,12 @@ static void __ebs_process_bios(struct work_struct *ws)
 			write = true;
 			r = __ebs_rw_bio(ec, REQ_OP_WRITE, bio);
 		} else if (bio_op(bio) == REQ_OP_DISCARD) {
-			__ebs_forget_bio(ec, bio);
-			r = __ebs_discard_bio(ec, bio);
+			if (write)
+				r = dm_bufio_write_dirty_buffers(ec->bufio);
+			if (!r) {
+				__ebs_forget_bio(ec, bio);
+				r = __ebs_discard_bio(ec, bio);
+			}
 		}
 
 		if (r < 0)
-- 
2.43.0

Re: [PATCH] dm ebs: drop dirty bufio state when discarding blocks

[Mikulas Patocka]

On Wed, 3 Jun 2026, Samuel Moelius wrote:

> dm-ebs can discard a block while a dirty dm-bufio buffer for the
> same block is still cached.  If that buffer is later written back,
> stale data can be written over the discarded state.
> 
> That resurrects data that userspace explicitly discarded and breaks
> the expected discard semantics of the target.
> 
> Invalidate or clean the matching bufio state when processing discards
> so that old dirty buffers cannot be written back after the discard.
> 
> Assisted-by: Codex:gpt-5.5-cyber-preview
> Signed-off-by: Samuel Moelius <sam.moelius@trailofbits.com>
> ---
>  drivers/md/dm-ebs-target.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/md/dm-ebs-target.c b/drivers/md/dm-ebs-target.c
> index 1e52bde48b91..9fece535a0b3 100644
> --- a/drivers/md/dm-ebs-target.c
> +++ b/drivers/md/dm-ebs-target.c
> @@ -212,8 +212,12 @@ static void __ebs_process_bios(struct work_struct *ws)
>  			write = true;
>  			r = __ebs_rw_bio(ec, REQ_OP_WRITE, bio);
>  		} else if (bio_op(bio) == REQ_OP_DISCARD) {
> -			__ebs_forget_bio(ec, bio);
> -			r = __ebs_discard_bio(ec, bio);
> +			if (write)
> +				r = dm_bufio_write_dirty_buffers(ec->bufio);
> +			if (!r) {
> +				__ebs_forget_bio(ec, bio);
> +				r = __ebs_discard_bio(ec, bio);
> +			}
>  		}
>  
>  		if (r < 0)

Hi

There's a bug in the patch - when the "write" variable is false, "r" is 
equal to -EIO, thus __ebs_forget_bio and __ebs_discard_bio is skipped at 
all.

If dm_bufio_write_dirty_buffers returns non-zero, the error is erroneously 
reported on the discard bio (that is innocent) and not on the write bios 
that triggered the error.

Mikulas

Re: [PATCH] dm ebs: drop dirty bufio state when discarding blocks

[Samuel Moelius]

On Mon, Jun 8, 2026 at 10:16 AM Mikulas Patocka <mpatocka@redhat.com> wrote:
>
>
>
> On Wed, 3 Jun 2026, Samuel Moelius wrote:
>
> > dm-ebs can discard a block while a dirty dm-bufio buffer for the
> > same block is still cached.  If that buffer is later written back,
> > stale data can be written over the discarded state.
> >
> > That resurrects data that userspace explicitly discarded and breaks
> > the expected discard semantics of the target.
> >
> > Invalidate or clean the matching bufio state when processing discards
> > so that old dirty buffers cannot be written back after the discard.
> >
> > Assisted-by: Codex:gpt-5.5-cyber-preview
> > Signed-off-by: Samuel Moelius <sam.moelius@trailofbits.com>
> > ---
> >  drivers/md/dm-ebs-target.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/md/dm-ebs-target.c b/drivers/md/dm-ebs-target.c
> > index 1e52bde48b91..9fece535a0b3 100644
> > --- a/drivers/md/dm-ebs-target.c
> > +++ b/drivers/md/dm-ebs-target.c
> > @@ -212,8 +212,12 @@ static void __ebs_process_bios(struct work_struct *ws)
> >                       write = true;
> >                       r = __ebs_rw_bio(ec, REQ_OP_WRITE, bio);
> >               } else if (bio_op(bio) == REQ_OP_DISCARD) {
> > -                     __ebs_forget_bio(ec, bio);
> > -                     r = __ebs_discard_bio(ec, bio);
> > +                     if (write)
> > +                             r = dm_bufio_write_dirty_buffers(ec->bufio);
> > +                     if (!r) {
> > +                             __ebs_forget_bio(ec, bio);
> > +                             r = __ebs_discard_bio(ec, bio);
> > +                     }
> >               }
> >
> >               if (r < 0)
>
> Hi
>
> There's a bug in the patch - when the "write" variable is false, "r" is
> equal to -EIO, thus __ebs_forget_bio and __ebs_discard_bio is skipped at
> all.
>
> If dm_bufio_write_dirty_buffers returns non-zero, the error is erroneously
> reported on the discard bio (that is innocent) and not on the write bios
> that triggered the error.

v2 forthcoming.