QEMU exits in error when passing a vfat shared folder in read-only mode.

To fix this issue, this patch removes any potential write permission
from cumulative_perms, when a read-only block device is in use.

Buglink: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=918950

Signed-off-by: Guillaume Roche <groche@genymobile.com>
---
This is an attempt to fix this behavior, but it feels a bit hacky to me
since this patch checks for the vvfat format in a generic function.

However, I'd be glad to have some advice to make it better. Anyway, I
ran the block tests to ensure this does not introduce any regression.

To add some context: I know that this can be worked around by setting 
the shared folder in rw mode. But our use-case requires using both
shared and VM snapshots, and QEMU prevents using snapshot with a rw 
shared folder.

 block.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/block.c b/block.c
index e97ce0b1c8..3f59e3843f 100644
--- a/block.c
+++ b/block.c
@@ -2383,6 +2383,12 @@ void bdrv_get_cumulative_perm(BlockDriverState *bs, uint64_t *perm,
         cumulative_shared_perms &= c->shared_perm;
     }
 
+    /* Discard write permission if vvfat block device is read-only */
+    const char *format = bdrv_get_format_name(bs);
+    if (format != NULL && strncmp(format, "vvfat", 5) == 0 && bdrv_is_read_only(bs)) {
+        cumulative_perms &= ~BLK_PERM_WRITE;
+    }
+
     *perm = cumulative_perms;
     *shared_perm = cumulative_shared_perms;
 }
-- 
2.31.1

Hi Guillaume,

On 8/31/21 4:17 PM, Guillaume Roche wrote:
> QEMU exits in error when passing a vfat shared folder in read-only mode.
> 
> To fix this issue, this patch removes any potential write permission
> from cumulative_perms, when a read-only block device is in use.
> 
> Buglink: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=918950
> 
> Signed-off-by: Guillaume Roche <groche@genymobile.com>
> ---
> This is an attempt to fix this behavior, but it feels a bit hacky to me
> since this patch checks for the vvfat format in a generic function.

What about implementing bdrv_vvfat::bdrv_check_perm()?

> However, I'd be glad to have some advice to make it better. Anyway, I
> ran the block tests to ensure this does not introduce any regression.
> 
> To add some context: I know that this can be worked around by setting 
> the shared folder in rw mode. But our use-case requires using both
> shared and VM snapshots, and QEMU prevents using snapshot with a rw 
> shared folder.
> 
>  block.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/block.c b/block.c
> index e97ce0b1c8..3f59e3843f 100644
> --- a/block.c
> +++ b/block.c
> @@ -2383,6 +2383,12 @@ void bdrv_get_cumulative_perm(BlockDriverState *bs, uint64_t *perm,
>          cumulative_shared_perms &= c->shared_perm;
>      }
>  
> +    /* Discard write permission if vvfat block device is read-only */
> +    const char *format = bdrv_get_format_name(bs);
> +    if (format != NULL && strncmp(format, "vvfat", 5) == 0 && bdrv_is_read_only(bs)) {
> +        cumulative_perms &= ~BLK_PERM_WRITE;
> +    }
> +
>      *perm = cumulative_perms;
>      *shared_perm = cumulative_shared_perms;
>  }
> 

Hi Philippe,

Le mar. 31 août 2021 à 18:06, Philippe Mathieu-Daudé
<philmd@redhat.com> a écrit :
>
> Hi Guillaume,
>
> On 8/31/21 4:17 PM, Guillaume Roche wrote:
> > QEMU exits in error when passing a vfat shared folder in read-only mode.
> >
> > To fix this issue, this patch removes any potential write permission
> > from cumulative_perms, when a read-only block device is in use.
> >
> > Buglink: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=918950
> >
> > Signed-off-by: Guillaume Roche <groche@genymobile.com>
> > ---
> > This is an attempt to fix this behavior, but it feels a bit hacky to me
> > since this patch checks for the vvfat format in a generic function.
>
> What about implementing bdrv_vvfat::bdrv_check_perm()?

Thanks for this feedback. I had a look at your suggestion, but I'm a
bit confused.

As I understand it, bdrv_node_refresh_perm() calls
bdrv_get_cumulative_perm() (which I patched to remove the write
permission) then it checks the permissions.
Afterwards, it calls bdrv_drv_set_perm() that in turn calls
bs->drv->bdrv_check_perm(). So even if I implement
bdrv_vvfat::bdrv_check_perm(), I will get the permission error before
it is called.

Could you elaborate a bit on what you have in mind please?

Regards,
Guillaume

>
> > However, I'd be glad to have some advice to make it better. Anyway, I
> > ran the block tests to ensure this does not introduce any regression.
> >
> > To add some context: I know that this can be worked around by setting
> > the shared folder in rw mode. But our use-case requires using both
> > shared and VM snapshots, and QEMU prevents using snapshot with a rw
> > shared folder.
> >
> >  block.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/block.c b/block.c
> > index e97ce0b1c8..3f59e3843f 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -2383,6 +2383,12 @@ void bdrv_get_cumulative_perm(BlockDriverState *bs, uint64_t *perm,
> >          cumulative_shared_perms &= c->shared_perm;
> >      }
> >
> > +    /* Discard write permission if vvfat block device is read-only */
> > +    const char *format = bdrv_get_format_name(bs);
> > +    if (format != NULL && strncmp(format, "vvfat", 5) == 0 && bdrv_is_read_only(bs)) {
> > +        cumulative_perms &= ~BLK_PERM_WRITE;
> > +    }
> > +
> >      *perm = cumulative_perms;
> >      *shared_perm = cumulative_shared_perms;
> >  }
> >
>

Am 31.08.2021 um 16:17 hat Guillaume Roche geschrieben:
> QEMU exits in error when passing a vfat shared folder in read-only mode.
> 
> To fix this issue, this patch removes any potential write permission
> from cumulative_perms, when a read-only block device is in use.
> 
> Buglink: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=918950
> 
> Signed-off-by: Guillaume Roche <groche@genymobile.com>
> ---
> This is an attempt to fix this behavior, but it feels a bit hacky to me
> since this patch checks for the vvfat format in a generic function.
> 
> However, I'd be glad to have some advice to make it better. Anyway, I
> ran the block tests to ensure this does not introduce any regression.
> 
> To add some context: I know that this can be worked around by setting 
> the shared folder in rw mode. But our use-case requires using both
> shared and VM snapshots, and QEMU prevents using snapshot with a rw 
> shared folder.

I don't think the behaviour is actually a bug: ide-hd requires a
writable backend, so attaching a read-only vvfat node is a real error.

You can either specify -drive read-only=on and use a device that can
accept read-only backends (such as virtio-blk or ide-cd), or add a
temporary writable qcow2 overlay with -snapshot or -drive snapshot=on so
that the ide-hd device actually does get the writable backend it needs.

Kevin