Ocfs2 devel - Jun 2013 - [PATCH] ocfs2: llseek requires to ocfs2 inode lock for the file in SEEK_END

llseek requires ocfs2 inode lock for updating the file size in SEEK_END.
because the file size maybe update on another node.
if it not . after call llseek in SEEK_END. the position is old.

this bug can be reproduce the following scenario:
at first ,we dd a test fileA,the file size is 10k.
on NodeA:
---------
1) open the test fileA, lseek the end of file. and print the position.
2) close the test fileA

on NodeB:
1) open the test fileA, append the 5k data to test FileA.
2) lseek the end of file. and print the position.
3) close file.

at first we run the test program1 on NodeA , the result is 10k.
and then run the test program2 on NodeB,  the result is 15k.
at last, we run the test program1 on NodeA again, the result is 10k.

after apply this patch.  the three step result is 15k.

Signed-off-by: jensen <shencanquan at huawei.com>
---
 fs/ocfs2/file.c |    9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/ocfs2/file.c b/fs/ocfs2/file.c
index ff54014..3afd24c 100644
--- a/fs/ocfs2/file.c
+++ b/fs/ocfs2/file.c
@@ -2626,7 +2626,16 @@ static loff_t ocfs2_file_llseek(struct file *file, loff_t
offset, int whence)
 	case SEEK_SET:
 		break;
 	case SEEK_END:
+		/* SEEK_END requires the OCFS2 inode lock for the file
+		 * because it references the file's size.
+		 */
+		ret = ocfs2_inode_lock(inode, NULL, 0);
+		if (ret < 0) {
+			mlog_errno(ret);
+			goto out;
+		}
 		offset += inode->i_size;
+		ocfs2_inode_unlock(inode, 0);
 		break;
 	case SEEK_CUR:
 		if (offset == 0) {
-- 
1.7.9.7

On Thu, 20 Jun 2013 16:23:59 +0800 shencanquan <shencanquan at huawei.com>
wrote:
> llseek requires ocfs2 inode lock for updating the file size in SEEK_END.
> because the file size maybe update on another node.
> if it not . after call llseek in SEEK_END. the position is old.
> 
> this bug can be reproduce the following scenario:
> at first ,we dd a test fileA,the file size is 10k.
> on NodeA:
> ---------
> 1) open the test fileA, lseek the end of file. and print the position.
> 2) close the test fileA
> 
> on NodeB:
> 1) open the test fileA, append the 5k data to test FileA.
> 2) lseek the end of file. and print the position.
> 3) close file.
> 
> at first we run the test program1 on NodeA , the result is 10k.
> and then run the test program2 on NodeB,  the result is 15k.
> at last, we run the test program1 on NodeA again, the result is 10k.
> 
> after apply this patch.  the three step result is 15k.
> 
> ...
>
> --- a/fs/ocfs2/file.c
> +++ b/fs/ocfs2/file.c
> @@ -2626,7 +2626,16 @@ static loff_t ocfs2_file_llseek(struct file *file,
loff_t offset, int whence)
>  	case SEEK_SET:
>  		break;
>  	case SEEK_END:
> +		/* SEEK_END requires the OCFS2 inode lock for the file
> +		 * because it references the file's size.
> +		 */
> +		ret = ocfs2_inode_lock(inode, NULL, 0);
> +		if (ret < 0) {
> +			mlog_errno(ret);
> +			goto out;
> +		}
>  		offset += inode->i_size;
> +		ocfs2_inode_unlock(inode, 0);
>  		break;
I don't understand this.  The lock for inode->i_size is
inode->i_mutex,
and we're already holding i_mutex here.  The current mainline code
looks correct.

My guess is that there is some other code path which is modifying
inode->i_size without holding inode->i_mutex, and while holding
ocfs2_inode_lock().  If so, that code is surely wrong - it should hold
i_mutex while modifying i_size.

Also, safely reading i_size should be performed via i_size_read(), and
modifications to i_size should use i_size_write().

And all this is only really applicable to 32-bit CPUs, which you
probably aren't using.

So.... please let's take a second look at this.

On Thu, Jun 20, 2013 at 04:23:59PM +0800, shencanquan
wrote:
> llseek requires ocfs2 inode lock for updating the file size in SEEK_END.
> because the file size maybe update on another node.
> if it not . after call llseek in SEEK_END. the position is old.
> 
> this bug can be reproduce the following scenario:
> at first ,we dd a test fileA,the file size is 10k.
> on NodeA:
> ---------
> 1) open the test fileA, lseek the end of file. and print the position.
> 2) close the test fileA
> 
> on NodeB:
> 1) open the test fileA, append the 5k data to test FileA.
> 2) lseek the end of file. and print the position.
> 3) close file.
> 
> at first we run the test program1 on NodeA , the result is 10k.
> and then run the test program2 on NodeB,  the result is 15k.
> at last, we run the test program1 on NodeA again, the result is 10k.
> 
> after apply this patch.  the three step result is 15k.
> 
> Signed-off-by: jensen <shencanquan at huawei.com>
> ---
>  fs/ocfs2/file.c |    9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/fs/ocfs2/file.c b/fs/ocfs2/file.c
> index ff54014..3afd24c 100644
> --- a/fs/ocfs2/file.c
> +++ b/fs/ocfs2/file.c
> @@ -2626,7 +2626,16 @@ static loff_t ocfs2_file_llseek(struct file *file,
loff_t offset, int whence)
>  	case SEEK_SET:
>  		break;
>  	case SEEK_END:
> +		/* SEEK_END requires the OCFS2 inode lock for the file
> +		 * because it references the file's size.
> +		 */
> +		ret = ocfs2_inode_lock(inode, NULL, 0);
> +		if (ret < 0) {
> +			mlog_errno(ret);
> +			goto out;
> +		}
>  		offset += inode->i_size;
> +		ocfs2_inode_unlock(inode, 0);
Why wouldn't ocfs2_rw_lock() work?  Just because we dont get the LVB
from it?

Joel
>  		break;
>  	case SEEK_CUR:
>  		if (offset == 0) {
> -- 
> 1.7.9.7
> 
> 
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