[PATCH 0/2] fuse: fix regression in fuse_get_user_pages()

Hi,

The patchset fixes a regression introduced by the following commits:

c7f3888ad7f0 ("switch iov_iter_get_pages() to passing maximal number of pages")
c9c37e2e6378 ("fuse: switch to iov_iter_get_pages()")

The regression manifests itslef like this (thanks to Werner Baumann for reporting):

davfs2 uses the fuse kernel module directly (not using the fuse
userspace library). A user of davfs2 reported this problem
(http://savannah.nongnu.org/support/?108640):

dd if=/dev/zero of=/mnt/owncloud/test.txt bs=20416 count=1
works fine, but
dd if=/dev/zero of=/mnt/owncloud/test.txt bs=20417 count=1
fails.

Thanks,
Maxim

---

Maxim Patlasov (2):
      vfs: switch iov_iter_get_pages() to passing maximal size
      fuse: fuse_get_user_pages(): do not pack more data than requested


 fs/direct-io.c      |    2 +-
 fs/fuse/file.c      |   13 +++++++++----
 include/linux/uio.h |    2 +-
 mm/iov_iter.c       |   17 +++++++++--------
 4 files changed, 20 insertions(+), 14 deletions(-)

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The patch fixes a bug introduced by commit c9c37e2e6378 ("fuse: switch to
iov_iter_get_pages()"):

The third argument of fuse_get_user_pages() "nbytesp" refers to the number of
bytes a caller asked to pack into fuse request. This value may be lesser
than capacity of fuse request or iov_iter. So fuse_get_user_pages() must
ensure that *nbytesp won't grow. Before that commit, it was ensured by:

	ret = get_user_pages_fast(user_addr, npages, !write,
				  &req->pages[req->num_pages]);
	...
	npages = ret;
	frag_size = min_t(size_t, frag_size,
			  (npages << PAGE_SHIFT) - offset);

Now, when helper iov_iter_get_pages() performs all hard work of extracting
pages from iov_iter, it can be done by passing properly calculated "maxsize"
to the helper.

Reported-by: Werner Baumann <redacted>
Signed-off-by: Maxim Patlasov <redacted>
---
 fs/fuse/file.c |   11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 40ac262..1d2bb70 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c

@@ -1303,10 +1303,15 @@ static int fuse_get_user_pages(struct fuse_req *req, struct iov_iter *ii,
 	while (nbytes < *nbytesp && req->num_pages < req->max_pages) {
 		unsigned npages;
 		size_t start;
+		ssize_t ret;
 		unsigned n = req->max_pages - req->num_pages;
-		ssize_t ret = iov_iter_get_pages(ii,
-					&req->pages[req->num_pages],
-					n * PAGE_SIZE, &start);
+		size_t frag_size = fuse_get_frag_size(ii, *nbytesp - nbytes);
+
+		frag_size = min_t(size_t, frag_size, n << PAGE_SHIFT);
+
+		ret = iov_iter_get_pages(ii,
+				&req->pages[req->num_pages],
+				frag_size, &start);
 		if (ret < 0)
 			return ret;
 

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On Wed, Sep 03, 2014 at 02:10:23PM +0400, Maxim Patlasov wrote:

Hi,

The patchset fixes a regression introduced by the following commits:

c7f3888ad7f0 ("switch iov_iter_get_pages() to passing maximal number of pages")
c9c37e2e6378 ("fuse: switch to iov_iter_get_pages()")

Hmm, instead of reverting to passing maxbytes *instead* of maxpages, I think the
right fix is to *add* the maxbytes argument.

Just maxbytes alone doesn't have enough information in it.  E.g. 4096 contiguous
bytes could occupy 1 or 2 pages, depending on the starting offset.

So how about the following (untested) patch?

Thanks,
Miklos

diff --git a/fs/direct-io.c b/fs/direct-io.c
index c3116404ab49..e181b6b2e297 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c

@@ -158,7 +158,7 @@ static inline int dio_refill_pages(struct dio *dio, struct dio_submit *sdio)
 {
 	ssize_t ret;
 
-	ret = iov_iter_get_pages(sdio->iter, dio->pages, DIO_PAGES,
+	ret = iov_iter_get_pages(sdio->iter, dio->pages, LONG_MAX, DIO_PAGES,
 				&sdio->from);
 
 	if (ret < 0 && sdio->blocks_available && (dio->rw & WRITE)) {

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 912061ac4baf..caa8d95b24e8 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c

@@ -1305,6 +1305,7 @@ static int fuse_get_user_pages(struct fuse_req *req, struct iov_iter *ii,
 		size_t start;
 		ssize_t ret = iov_iter_get_pages(ii,
 					&req->pages[req->num_pages],
+					*nbytesp - nbytes,
 					req->max_pages - req->num_pages,
 					&start);
 		if (ret < 0)

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 48d64e6ab292..290fbf0b6b8a 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h

@@ -84,7 +84,7 @@ unsigned long iov_iter_alignment(const struct iov_iter *i);
 void iov_iter_init(struct iov_iter *i, int direction, const struct iovec *iov,
 			unsigned long nr_segs, size_t count);
 ssize_t iov_iter_get_pages(struct iov_iter *i, struct page **pages,
-			unsigned maxpages, size_t *start);
+			size_t maxsize, unsigned maxpages, size_t *start);
 ssize_t iov_iter_get_pages_alloc(struct iov_iter *i, struct page ***pages,
 			size_t maxsize, size_t *start);
 int iov_iter_npages(const struct iov_iter *i, int maxpages);

diff --git a/mm/iov_iter.c b/mm/iov_iter.c
index ab88dc0ea1d3..9a09f2034fcc 100644
--- a/mm/iov_iter.c
+++ b/mm/iov_iter.c

@@ -310,7 +310,7 @@ void iov_iter_init(struct iov_iter *i, int direction,
 EXPORT_SYMBOL(iov_iter_init);
 
 static ssize_t get_pages_iovec(struct iov_iter *i,
-		   struct page **pages, unsigned maxpages,
+		   struct page **pages, size_t maxsize, unsigned maxpages,
 		   size_t *start)
 {
 	size_t offset = i->iov_offset;

@@ -323,6 +323,8 @@ static ssize_t get_pages_iovec(struct iov_iter *i,
 	len = iov->iov_len - offset;
 	if (len > i->count)
 		len = i->count;
+	if (len > maxsize)
+		len = maxsize;
 	addr = (unsigned long)iov->iov_base + offset;
 	len += *start = addr & (PAGE_SIZE - 1);
 	if (len > maxpages * PAGE_SIZE)

@@ -588,13 +590,15 @@ static unsigned long alignment_bvec(const struct iov_iter *i)
 }
 
 static ssize_t get_pages_bvec(struct iov_iter *i,
-		   struct page **pages, unsigned maxpages,
+		   struct page **pages, size_t maxsize, unsigned maxpages,
 		   size_t *start)
 {
 	const struct bio_vec *bvec = i->bvec;
 	size_t len = bvec->bv_len - i->iov_offset;
 	if (len > i->count)
 		len = i->count;
+	if (len > maxsize)
+		len = maxsize;
 	/* can't be more than PAGE_SIZE */
 	*start = bvec->bv_offset + i->iov_offset;
 

@@ -711,13 +715,13 @@ unsigned long iov_iter_alignment(const struct iov_iter *i)
 EXPORT_SYMBOL(iov_iter_alignment);
 
 ssize_t iov_iter_get_pages(struct iov_iter *i,
-		   struct page **pages, unsigned maxpages,
+		   struct page **pages, size_t maxsize, unsigned maxpages,
 		   size_t *start)
 {
 	if (i->type & ITER_BVEC)
-		return get_pages_bvec(i, pages, maxpages, start);
+		return get_pages_bvec(i, pages, maxsize, maxpages, start);
 	else
-		return get_pages_iovec(i, pages, maxpages, start);
+		return get_pages_iovec(i, pages, maxsize, maxpages, start);
 }
 EXPORT_SYMBOL(iov_iter_get_pages);
 

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On 09/10/2014 01:51 PM, Miklos Szeredi wrote:

On Wed, Sep 03, 2014 at 02:10:23PM +0400, Maxim Patlasov wrote:

quoted

Hi,

The patchset fixes a regression introduced by the following commits:

c7f3888ad7f0 ("switch iov_iter_get_pages() to passing maximal number of pages")
c9c37e2e6378 ("fuse: switch to iov_iter_get_pages()")

Hmm, instead of reverting to passing maxbytes *instead* of maxpages, I think the
right fix is to *add* the maxbytes argument.

Just maxbytes alone doesn't have enough information in it.  E.g. 4096 contiguous
bytes could occupy 1 or 2 pages, depending on the starting offset.

Yes, you are right. I missed that c7f3888ad7f0 fixed a subtle bug in 
get_pages_iovec().

So how about the following (untested) patch?

Your patch works fine in my tests.

Thanks,
Maxim

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