[PATCH] block/vvfat: Fix ro shared folder

[Guillaume Roche <groche@genymobile.com>]

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