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Re: [PATCH] block/vvfat: Fix ro shared folder
From: |
Philippe Mathieu-Daudé |
Subject: |
Re: [PATCH] block/vvfat: Fix ro shared folder |
Date: |
Tue, 31 Aug 2021 18:06:02 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 |
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;
> }
>