[PATCH v3] direct-io: Fix unsigned comparison overflow

[PATCH v3] direct-io: Fix unsigned comparison overflow

[Harish Kasiviswanathan]

The first write after file create fails to take the direct IO
(Peer-to-Peer) path and falls back to slower software copy. The function
get_more_block() sets 'create' to 0 after comparing 'unsigned long
fs_startblk = 0' with 'long long (i_size_read(dio->inode) - 1) >>
i_blkbits = 0xfffffffffffff'.

v2: Instead of casting to loff_t check explicitly if i_size > 0
v3: Use cached dio->i_size instead of i_size_read() which isn't cheap on
    32-bit SMP

Signed-off-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
---
 fs/direct-io.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/direct-io.c b/fs/direct-io.c
index 3aafb33..1c33830 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -686,9 +686,8 @@ static int get_more_blocks(struct dio *dio, struct dio_submit *sdio,
 		 * buffer head.
 		 */
 		create = dio->op == REQ_OP_WRITE;
-		if (dio->flags & DIO_SKIP_HOLES) {
-			if (fs_startblk <= ((i_size_read(dio->inode) - 1) >>
-							i_blkbits))
+		if (dio->flags & DIO_SKIP_HOLES && dio->i_size) {
+			if (fs_startblk <= ((dio->i_size - 1) >> i_blkbits))
 				create = 0;
 		}
 
-- 
2.7.4

Re: [PATCH v3] direct-io: Fix unsigned comparison overflow

[Matthew Wilcox]

On Wed, Dec 06, 2017 at 11:57:18AM -0500, Harish Kasiviswanathan wrote:
> The first write after file create fails to take the direct IO
> (Peer-to-Peer) path and falls back to slower software copy. The function
> get_more_block() sets 'create' to 0 after comparing 'unsigned long
> fs_startblk = 0' with 'long long (i_size_read(dio->inode) - 1) >>
> i_blkbits = 0xfffffffffffff'.
> 
> v2: Instead of casting to loff_t check explicitly if i_size > 0
> v3: Use cached dio->i_size instead of i_size_read() which isn't cheap on
>     32-bit SMP
> 
> Signed-off-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>

Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>