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

[Changwei Ge]

Hi Larry,

On 2018/3/29 18:33, Larry Chen wrote:
> Hi Changwei,
> 
> On 03/29/2018 05:50 PM, piaojun wrote:
>> Hi Changwei,
>>
>> On 2018/3/29 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;
>>> +				}
>>>    				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;
>>> +				}
>> How to make suer 'bhs[i]' is not allocated by user according to 'new_bh'?
>> 'new_bh' equis 1 only means 'bhs[0]' is allocated by ocfs2_read_blocks()
>> and we should put it here, right?
> Does your patch assumes that bhs refers to either an all-NULL-elements
> array or
> an all-preallocated-elements array?

True, I mean that. :)

Thanks,
Changwei

> 
> Thanks
> Larry
>> thanks,
>> Jun
>>>    				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;
>>>    				}
>>>    			}
>>>
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>>
>>
> 
> 
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