[Ocfs2-devel] [PATCH] ocfs2: don't evaluate buffer head to NULL managed by caller

[Larry Chen]

Hi Changwei,

I found that your patch call put_bh function only if new_bh==1,
Will it cause buffer_head use count inconsistent??

Thanks
Larry


On 03/29/2018 10:06 AM, Changwei Ge wrote:
> ocfs2_read_blocks() is used to read several blocks from disk.
> Currently, the input argument *bhs* can be NULL or NOT. It depends on
> the caller's behavior. If the function fails in reading blocks from
> disk, the corresponding bh will be assigned to NULL and put.
>
> Obviously, above process for non-NULL input bh is not appropriate.
> Because the caller doesn't even know its bhs are put and re-assigned.
>
> If buffer head is managed by caller, ocfs2_read_blocks should not
> evaluate it to NULL. It will cause caller accessing illegal memory,
> thus crash.
>
> Signed-off-by: Changwei Ge <ge.changwei at h3c.com>
> ---
>   fs/ocfs2/buffer_head_io.c | 31 +++++++++++++++++++++++++------
>   1 file changed, 25 insertions(+), 6 deletions(-)
>
> diff --git a/fs/ocfs2/buffer_head_io.c b/fs/ocfs2/buffer_head_io.c
> index d9ebe11..17329b6 100644
> --- a/fs/ocfs2/buffer_head_io.c
> +++ b/fs/ocfs2/buffer_head_io.c
> @@ -188,6 +188,7 @@ int ocfs2_read_blocks(struct ocfs2_caching_info *ci, u64 block, int nr,
>   	int i, ignore_cache = 0;
>   	struct buffer_head *bh;
>   	struct super_block *sb = ocfs2_metadata_cache_get_super(ci);
> +	int new_bh = 0;
>   
>   	trace_ocfs2_read_blocks_begin(ci, (unsigned long long)block, nr, flags);
>   
> @@ -213,6 +214,18 @@ int ocfs2_read_blocks(struct ocfs2_caching_info *ci, u64 block, int nr,
>   		goto bail;
>   	}
>   
> +	/* Use below trick to check if all bhs are NULL or assigned.
> +	 * Basically, we hope all bhs are consistent so that we can
> +	 * handle exception easily.
> +	 */
> +	new_bh = (bhs[0] == NULL);
> +	for (i = 1 ; i < nr ; i++) {
> +		if ((new_bh && bhs[i]) || (!new_bh && !bhs[i])) {
> +			WARN(1, "Not all bhs are consistent\n");
> +			break;
> +		}
> +	}
> +
>   	ocfs2_metadata_cache_io_lock(ci);
>   	for (i = 0 ; i < nr ; i++) {
>   		if (bhs[i] == NULL) {
> @@ -324,8 +337,10 @@ int ocfs2_read_blocks(struct ocfs2_caching_info *ci, u64 block, int nr,
>   		if (!(flags & OCFS2_BH_READAHEAD)) {
>   			if (status) {
>   				/* Clear the rest of the buffers on error */
> -				put_bh(bh);
> -				bhs[i] = NULL;
> +				if (new_bh) {
> +					put_bh(bh);
> +					bhs[i] = NULL;
> +				}
>   				continue;
>   			}
>   			/* We know this can't have changed as we hold the
> @@ -342,8 +357,10 @@ int ocfs2_read_blocks(struct ocfs2_caching_info *ci, u64 block, int nr,
>   				 * for this bh as it's not marked locally
>   				 * uptodate. */
>   				status = -EIO;
> -				put_bh(bh);
> -				bhs[i] = NULL;
> +				if (new_bh) {
> +					put_bh(bh);
> +					bhs[i] = NULL;
> +				}
>   				continue;
>   			}
>   
> @@ -355,8 +372,10 @@ int ocfs2_read_blocks(struct ocfs2_caching_info *ci, u64 block, int nr,
>   				clear_buffer_needs_validate(bh);
>   				status = validate(sb, bh);
>   				if (status) {
> -					put_bh(bh);
> -					bhs[i] = NULL;
> +					if (new_bh) {
> +						put_bh(bh);
> +						bhs[i] = NULL;
> +					}
>   					continue;
>   				}
>   			}