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

[Changwei Ge]

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;
>> +		}
>> +	}
>> +
>>   	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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> 
>