Re: [PATCH] Btrfs: always copy scrub arguments back to user space

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2020/1/16 下午7:29, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> If scrub returns an error we are not copying back the scrub arguments
> structure to user space. This prevents user space to know how much progress
> scrub has done if an error happened - this includes -ECANCELED which is
> returned when users ask for scrub to stop. A particular use case, which is
> used in btrfs-progs, is to resume scrub after it is canceled, in that case
> it relies on checking the progress from the scrub arguments structure and
> then use that progress in a call to resume scrub.
> 
> So fix this by always copying the scrub arguments structure to user space,
> overwriting the value returned to user space with -EFAULT only if copying
> the structure failed to let user space know that either that copying did
> not happen, and therefore the structure is stale, or it happened partially
> and the structure is probably not valid and corrupt due to the partial
> copy.
> 
> Reported-by: Graham Cobb <g.btrfs@cobb.uk.net>
> Link: https://lore.kernel.org/linux-btrfs/d0a97688-78be-08de-ca7d-bcb4c7fb397e@cobb.uk.net/
> Fixes: 06fe39ab15a6a4 ("Btrfs: do not overwrite scrub error with fault error in scrub ioctl")
> Signed-off-by: Filipe Manana <fdmanana@suse.com>

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu

> ---
>  fs/btrfs/ioctl.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index 3a4bd5cd67fa..173758d86feb 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -4253,7 +4253,19 @@ static long btrfs_ioctl_scrub(struct file *file, void __user *arg)
>  			      &sa->progress, sa->flags & BTRFS_SCRUB_READONLY,
>  			      0);
>  
> -	if (ret == 0 && copy_to_user(arg, sa, sizeof(*sa)))
> +	/*
> +	 * Copy scrub args to user space even if btrfs_scrub_dev() returned an
> +	 * error. This is important as it allows user space to know how much
> +	 * progress scrub has done. For example, if scrub is canceled we get
> +	 * -ECANCELED from btrfs_scrub_dev() and return that error back to user
> +	 * space. Later user space can inspect the progress from the structure
> +	 * btrfs_ioctl_scrub_args and resume scrub from where it left off
> +	 * previously (btrfs-progs does this).
> +	 * If we fail to copy the btrfs_ioctl_scrub_args structure to user space
> +	 * then return -EFAULT to signal the structure was not copied or it may
> +	 * be corrupt and unreliable due to a partial copy.
> +	 */
> +	if (copy_to_user(arg, sa, sizeof(*sa)))
>  		ret = -EFAULT;
>  
>  	if (!(sa->flags & BTRFS_SCRUB_READONLY))
> 


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