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

[Gang He]

Hi Changwei,


>>> 
> Hi Gang,
> 
> On 2018/3/29 10:36, Gang He wrote:
>> Hello Changwei,
>> 
>> 
>> Do you have the related crash backtrace?
> This patch has been pending in my tree for quite a long time and sadly I 
> can't 
> find the back trace right now. But we can still find the risk by reviewing 
> related code. :)
> 
>> Maybe I feel that new adding check is not necessary.
> 
> Very true, but the check I add is for debug purpose.
> We can see that there are many places calling ocfs2_read_blocks(), some of 
> them 
> are passing only one bh while others are not.
> In order to handle potential exception easily, it's better for callers to 
> pass 
> bhs which are all null or assigned. So I add that trick to tell if some 
> callers 
> are doing stupid things.
> 
> Thanks,
> Changwei
> 
>> since the below code has make sure all buffer head is NOT NULL before 
> reading block.
>> 216         ocfs2_metadata_cache_io_lock(ci);
>> 217         for (i = 0 ; i < nr ; i++) {
>> 218                 if (bhs[i] == NULL) {
>> 219                         bhs[i] = sb_getblk(sb, block++);   <<= here
>> 220                         if (bhs[i] == NULL) {
>> 221                                 ocfs2_metadata_cache_io_unlock(ci);
>> 222                                 status = -ENOMEM;
>> 223                                 mlog_errno(status);
>> 224                                 goto bail;
>> 225                         }
>> 226                 }
>> 227                 bh = bhs[i];
>> 
>> 
>> Thanks
>> Gang
>> 
>> 
>>>>>
>>> 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;
>>> +		}
>>> +	}
Maybe just adding a buffer head array check is OK?
If not consistent, give a warning.
why do we need the below code change?
since all head buffers are always NOT NULL.

Thanks
Gang

>>> +
>>>   	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;
>>>   				}
>>>   			}
>>> -- 
>>> 2.7.4
>>>
>>>
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>> 
>>