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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Vivek Goyal (vgoyal@redhat.com) wrote:
> 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).
> > 
> > Signed-off-by: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
> 
> Reviewed-by: Vivek Goyal <vgoyal@redhat.com>

Thanks, merged.

Dave

> Thanks
> Vivek
> 
> > ---
> >  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
> > 
> > _______________________________________________
> > Virtio-fs mailing list
> > Virtio-fs@redhat.com
> > https://www.redhat.com/mailman/listinfo/virtio-fs
> 
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--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK