Re: [PATCH] fs: move the bdex_statx call to vfs_getattr_nosec

[Christian Brauner <brauner@kernel.org>]

On Tue, Apr 22, 2025 at 07:51:49AM +0200, hch wrote:
> On Tue, Apr 22, 2025 at 05:03:19AM +0000, Shinichiro Kawasaki wrote:
> > I ran blktests with the kernel v6.15-rc3, and found the test case md/001 hangs.
> > The hang is recreated in stable manner. I bisected and found this patch as the
> > commit 777d0961ff95 is the trigger. When I revert the commit from v6.15-rc3
> > kernel, the hang disappeared.
> > 
> > Actions for fix will be appreciated.
> > 
> > FYI, the kernel INFO messages recorded functions relevant to the trigger commit,
> > such as bdev_statx or vfs_getattr_nosec [1].
> 
> This should fix it:
> 
> diff --git a/block/bdev.c b/block/bdev.c
> index 6a34179192c9..97d4c0ab1670 100644
> --- a/block/bdev.c
> +++ b/block/bdev.c
> @@ -1274,18 +1274,23 @@ void sync_bdevs(bool wait)
>   */
>  void bdev_statx(const struct path *path, struct kstat *stat, u32 request_mask)
>  {
> -	struct inode *backing_inode;
>  	struct block_device *bdev;
>  
> -	backing_inode = d_backing_inode(path->dentry);
> -
>  	/*
> -	 * Note that backing_inode is the inode of a block device node file,
> -	 * not the block device's internal inode.  Therefore it is *not* valid
> -	 * to use I_BDEV() here; the block device has to be looked up by i_rdev
> -	 * instead.
> +	 * Note that d_backing_inode() returnsthe inode of a block device node
> +	 * file, not the block device's internal inode.
> +	 *
> +	 * Therefore it is *not* valid to use I_BDEV() here; the block device
> +	 * has to be looked up by i_rdev instead.
> +	 *
> +	 * Only do this lookup if actually needed to avoid the performance
> +	 * overhead of the lookup, and to avoid injecting bdev lifetime issues
> +	 * into devtmpfs.
>  	 */
> -	bdev = blkdev_get_no_open(backing_inode->i_rdev);
> +	if (!(request_mask & (STATX_DIOALIGN | STATX_WRITE_ATOMIC)))
> +		return;

This leaks the block device reference if blkdev_get_no_open() succeeds.

> +
> +	bdev = blkdev_get_no_open(d_backing_inode(path->dentry)->i_rdev);
>  	if (!bdev)
>  		return;
>