Re: [PATCH] block: note about cloned bios and bio_for_each_segment_all

[Jens Axboe <axboe@kernel.dk>]

On 07/14/2017 07:47 AM, Ming Lei wrote:
>> @@ -156,6 +156,9 @@ static inline void *bio_data(struct bio *bio)
>>  /*
>>   * drivers should _never_ use the all version - the bio may have been split
>>   * before it got to the driver and the driver won't own all of it
>> + *
>> + * Note that cloned bios must not use this as their bi_vcnt may be invalid and
>> + * this could lead to silent corruptions.
>>   */
>>  #define bio_for_each_segment_all(bvl, bio, i)                          \
>>         for (i = 0, bvl = (bio)->bi_io_vec; i < (bio)->bi_vcnt; i++, bvl++)
>> --
>> 2.13.0
>>
> 
> Maybe we can add a warning here if it is a cloned bio.

I think that's a good idea, it's easy for people to get this wrong, and
the consequences can be dire. How about something like this?

diff --git a/include/linux/bio.h b/include/linux/bio.h
index 7b1cf4ba0902..13b6ac6eae29 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -155,9 +155,12 @@ static inline void *bio_data(struct bio *bio)
 
 /*
  * drivers should _never_ use the all version - the bio may have been split
- * before it got to the driver and the driver won't own all of it
+ * before it got to the driver and the driver won't own all of it.
+ *
+ * Don't use this on cloned bio's.
  */
 #define bio_for_each_segment_all(bvl, bio, i)				\
+	WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED));			\
 	for (i = 0, bvl = (bio)->bi_io_vec; i < (bio)->bi_vcnt; i++, bvl++)
 
 static inline void bio_advance_iter(struct bio *bio, struct bvec_iter *iter,

-- 
Jens Axboe