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

[Changwei Ge]

Hi Joseph,

On 2018/3/30 9:27, Joseph Qi wrote:
> 
> 
> On 18/3/29 10:06, 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;
>> +				}
> 
> Since we assume caller has to pass either all NULL or all non-NULL,
> here we will only put bh internal allocated. Am I missing something?

Thanks for your review.
Yes, we will only put bh internally allocated.
If bh is reserved in advance, we will not put it and re-assign it to NULL.

Thanks,
Changwei

> 
> Thanks,
> Joseph
> 
>>   				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;
>>   				}
>>   			}
>>
>