Re: [PATCH] fsdax: Fix infinite loop in dax_iomap_rw()

[Darrick J. Wong]

On Mon, Jul 25, 2022 at 11:20:50AM +0800, Li Jinlin wrote:
> I got an infinite loop and a WARNING report when executing a tail command
> in virtiofs.
> 
>   WARNING: CPU: 10 PID: 964 at fs/iomap/iter.c:34 iomap_iter+0x3a2/0x3d0
>   Modules linked in:
>   CPU: 10 PID: 964 Comm: tail Not tainted 5.19.0-rc7
>   Call Trace:
>   <TASK>
>   dax_iomap_rw+0xea/0x620
>   ? __this_cpu_preempt_check+0x13/0x20
>   fuse_dax_read_iter+0x47/0x80
>   fuse_file_read_iter+0xae/0xd0
>   new_sync_read+0xfe/0x180
>   ? 0xffffffff81000000
>   vfs_read+0x14d/0x1a0
>   ksys_read+0x6d/0xf0
>   __x64_sys_read+0x1a/0x20
>   do_syscall_64+0x3b/0x90
>   entry_SYSCALL_64_after_hwframe+0x63/0xcd
> 
> The tail command will call read() with a count of 0. In this case,
> iomap_iter() will report this WARNING, and always return 1 which casuing
> the infinite loop in dax_iomap_rw().
> 
> Fixing by checking count whether is 0 in dax_iomap_rw().
> 
> Fixes: ca289e0b95af ("fsdax: switch dax_iomap_rw to use iomap_iter")
> Signed-off-by: Li Jinlin <lijinlin3@xxxxxxxxxx>

Huh, I didn't know FUSE supports DAX and iomap now...

> ---
>  fs/dax.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index 4155a6107fa1..7ab248ed21aa 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -1241,6 +1241,9 @@ dax_iomap_rw(struct kiocb *iocb, struct iov_iter *iter,
>  	loff_t done = 0;
>  	int ret;
>  
> +	if (!iomi.len)
> +		return 0;

Hmm, most of the callers of dax_iomap_rw skip the whole call if
iov_iter_count(to)==0, so I wonder if fuse_dax_read_iter should do the
same?

That said, iomap_dio_rw bails early if you pass it iomi.len, so I don't
have any real objections to this.

Reviewed-by: Darrick J. Wong <djwong@xxxxxxxxxx>

--D


> +
>  	if (iov_iter_rw(iter) == WRITE) {
>  		lockdep_assert_held_write(&iomi.inode->i_rwsem);
>  		iomi.flags |= IOMAP_WRITE;
> -- 
> 2.30.2
>