[PATCH] fuse: do not write whole page while page straddles i_size

[PATCH] fuse: do not write whole page while page straddles i_size

[Liu Bo]

From: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>

If page straddles i_size and we write the whole page, the fuse
user-space filesystem may extend file size, it will confuse users.

Before this patch:
xfs_io -t -f \
        -c "truncate 5120" \
        -c "pwrite -S 0x58 0 5120"      \
        -c "mmap -rw 0 5120"            \
        -c "mwrite -S 0x59 2048 3072" \
        -c "close"      \
        testfile
testfile's length will be 8192 bytes, with this patch, testfile's
length will be 5120 bytes.

Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
---
 fs/fuse/file.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 06096b60f1df..44e76b00e985 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1863,6 +1863,8 @@ static int fuse_writepages_fill(struct page *page,
 	struct page *tmp_page;
 	bool is_writeback;
 	int err;
+	loff_t size;
+	unsigned int len;
 
 	if (!data->ff) {
 		err = -EIO;
@@ -1940,7 +1942,12 @@ static int fuse_writepages_fill(struct page *page,
 	copy_highpage(tmp_page, page);
 	req->pages[req->num_pages] = tmp_page;
 	req->page_descs[req->num_pages].offset = 0;
-	req->page_descs[req->num_pages].length = PAGE_SIZE;
+	size = i_size_read(inode);
+	if (page->index == size >> PAGE_SHIFT)
+		len = size & ~PAGE_MASK;
+	else
+		len = PAGE_SIZE;
+	req->page_descs[req->num_pages].length = len;
 
 	inc_wb_stat(&inode_to_bdi(inode)->wb, WB_WRITEBACK);
 	inc_node_page_state(tmp_page, NR_WRITEBACK_TEMP);
-- 
2.20.1.2.gb21ebb6

Re: [PATCH] fuse: do not write whole page while page straddles i_size

[Miklos Szeredi]

On Wed, Apr 17, 2019 at 10:04 PM Liu Bo <bo.liu@linux.alibaba.com> wrote:
>
> From: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
>
> If page straddles i_size and we write the whole page, the fuse
> user-space filesystem may extend file size, it will confuse users.
>
> Before this patch:
> xfs_io -t -f \
>         -c "truncate 5120" \
>         -c "pwrite -S 0x58 0 5120"      \
>         -c "mmap -rw 0 5120"            \
>         -c "mwrite -S 0x59 2048 3072" \
>         -c "close"      \
>         testfile
> testfile's length will be 8192 bytes, with this patch, testfile's
> length will be 5120 bytes.

Cannot reproduce this.  The file size should be cropped in
fuse_flush_writepages() and the above test case produces the expected
result, so I'm not sure how were you able to get 8192 byte file.

Thanks,
Miklos

Re: [PATCH] fuse: do not write whole page while page straddles i_size

[Liu Bo]

On Thu, Apr 18, 2019 at 01:19:10PM +0200, Miklos Szeredi wrote:
> On Wed, Apr 17, 2019 at 10:04 PM Liu Bo <bo.liu@linux.alibaba.com> wrote:
> >
> > From: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
> >
> > If page straddles i_size and we write the whole page, the fuse
> > user-space filesystem may extend file size, it will confuse users.
> >
> > Before this patch:
> > xfs_io -t -f \
> >         -c "truncate 5120" \
> >         -c "pwrite -S 0x58 0 5120"      \
> >         -c "mmap -rw 0 5120"            \
> >         -c "mwrite -S 0x59 2048 3072" \
> >         -c "close"      \
> >         testfile
> > testfile's length will be 8192 bytes, with this patch, testfile's
> > length will be 5120 bytes.
> 
> Cannot reproduce this.  The file size should be cropped in
> fuse_flush_writepages() and the above test case produces the expected
> result, so I'm not sure how were you able to get 8192 byte file.

Hmm, this was found by fstests while testing against virtiofs 0.1
daemon code, which doesn't handle file size from fuse_in_head.  but
now with 0.2, /me couldn't reproduce it, either.

Please ignore the patch as well.

thanks,
-liubo