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

[Changwei Ge]

Hi Joseph,

On 2018/3/30 10:04, Joseph Qi wrote:
> 
> 
> On 18/3/30 09:31, Changwei Ge wrote:
>> 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.
>>
> 
> So this branch won't have risk, right?

Sorry... I'm not sure if I understand you correctly.
This branch will be walked through when previous part of bhs[] faces a read 
failure in order to put bh allocated in ocfs2_read_blocks().
And we assume all bh should be NULL or non-NULL, if new_bh is set, the back part 
should also be put to release those buffer heads.

If I made a mistake or misunderstand you, please let me know.

Thanks,
Changwei


> 
> Thanks,
> Joseph
>