Re: [Virtio-fs] [PATCH] virtiofsd: Fix data corruption with O_APPEND wirte in writeback mode

[Vivek Goyal]

On Wed, Oct 23, 2019 at 09:25:23PM +0900, Misono Tomohiro wrote:
> When writeback mode is enabled (-o writeback), O_APPEND handling is
> done in kernel. Therefore virtiofsd clears O_APPEND flag when open.
> Otherwise O_APPEND flag takes precedence over pwrite() and write
> data may corrupt.
> 
> Currently clearing O_APPEND flag is done in lo_open(), but we also
> need the same operation in lo_create().

> So, factor out the flag
> update operation in lo_open() to update_open_flags() and call it
> in both lo_open() and lo_create().
> 
> This fixes the failure of xfstest generic/069 in writeback mode
> (which tests O_APPEND write data integrity).
> 

Hi,

Consolidating updation of flags both for lo_create() and lo_open() makes
sense to me. I will test it tomorrow.

Thanks
Vivek

> Signed-off-by: Misono Tomohiro <address@hidden>
> ---
>  contrib/virtiofsd/passthrough_ll.c | 56 +++++++++++++++---------------
>  1 file changed, 28 insertions(+), 28 deletions(-)
> 
> diff --git a/contrib/virtiofsd/passthrough_ll.c 
> b/contrib/virtiofsd/passthrough_ll.c
> index e8892c3c32..79fb78ecce 100644
> --- a/contrib/virtiofsd/passthrough_ll.c
> +++ b/contrib/virtiofsd/passthrough_ll.c
> @@ -1733,6 +1733,32 @@ static void lo_releasedir(fuse_req_t req, fuse_ino_t 
> ino, struct fuse_file_info
>       fuse_reply_err(req, 0);
>  }
>  
> +static void update_open_flags(int writeback, struct fuse_file_info *fi)
> +{
> +     /* With writeback cache, kernel may send read requests even
> +        when userspace opened write-only */
> +     if (writeback && (fi->flags & O_ACCMODE) == O_WRONLY) {
> +             fi->flags &= ~O_ACCMODE;
> +             fi->flags |= O_RDWR;
> +     }
> +
> +     /* With writeback cache, O_APPEND is handled by the kernel.
> +        This breaks atomicity (since the file may change in the
> +        underlying filesystem, so that the kernel's idea of the
> +        end of the file isn't accurate anymore). In this example,
> +        we just accept that. A more rigorous filesystem may want
> +        to return an error here */
> +     if (writeback && (fi->flags & O_APPEND))
> +             fi->flags &= ~O_APPEND;
> +
> +     /*
> +      * O_DIRECT in guest should not necessarily mean bypassing page
> +      * cache on host as well. If somebody needs that behavior, it
> +      * probably should be a configuration knob in daemon.
> +      */
> +     fi->flags &= ~O_DIRECT;
> +}
> +
>  static void lo_create(fuse_req_t req, fuse_ino_t parent, const char *name,
>                     mode_t mode, struct fuse_file_info *fi)
>  {
> @@ -1760,12 +1786,7 @@ static void lo_create(fuse_req_t req, fuse_ino_t 
> parent, const char *name,
>       if (err)
>               goto out;
>  
> -     /*
> -      * O_DIRECT in guest should not necessarily mean bypassing page
> -      * cache on host as well. If somebody needs that behavior, it
> -      * probably should be a configuration knob in daemon.
> -      */
> -     fi->flags &= ~O_DIRECT;
> +     update_open_flags(lo->writeback, fi);
>  
>       fd = openat(parent_inode->fd, name,
>                   (fi->flags | O_CREAT) & ~O_NOFOLLOW, mode);
> @@ -1966,28 +1987,7 @@ static void lo_open(fuse_req_t req, fuse_ino_t ino, 
> struct fuse_file_info *fi)
>  
>       fuse_log(FUSE_LOG_DEBUG, "lo_open(ino=%" PRIu64 ", flags=%d)\n", ino, 
> fi->flags);
>  
> -     /* With writeback cache, kernel may send read requests even
> -        when userspace opened write-only */
> -     if (lo->writeback && (fi->flags & O_ACCMODE) == O_WRONLY) {
> -             fi->flags &= ~O_ACCMODE;
> -             fi->flags |= O_RDWR;
> -     }
> -
> -     /* With writeback cache, O_APPEND is handled by the kernel.
> -        This breaks atomicity (since the file may change in the
> -        underlying filesystem, so that the kernel's idea of the
> -        end of the file isn't accurate anymore). In this example,
> -        we just accept that. A more rigorous filesystem may want
> -        to return an error here */
> -     if (lo->writeback && (fi->flags & O_APPEND))
> -             fi->flags &= ~O_APPEND;
> -
> -     /*
> -      * O_DIRECT in guest should not necessarily mean bypassing page
> -      * cache on host as well. If somebody needs that behavior, it
> -      * probably should be a configuration knob in daemon.
> -      */
> -     fi->flags &= ~O_DIRECT;
> +     update_open_flags(lo->writeback, fi);
>  
>       sprintf(buf, "%i", lo_fd(req, ino));
>       fd = openat(lo->proc_self_fd, buf, fi->flags & ~O_NOFOLLOW);
> -- 
> 2.21.0
> 
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