Re: [PATCH 07/14] block/blklogwrites: drop error propagation

[Markus Armbruster]

Markus Armbruster <armbru@redhat.com> writes:

> Greg Kurz <groug@kaod.org> writes:
>
>> On Wed,  9 Sep 2020 21:59:23 +0300
>> Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> wrote:
>>
>>> It's simple to avoid error propagation in blk_log_writes_open(), we
>>> just need to refactor blk_log_writes_find_cur_log_sector() a bit.
>>> 
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>> ---
>>>  block/blklogwrites.c | 23 +++++++++++------------
>>>  1 file changed, 11 insertions(+), 12 deletions(-)
>>> 
>>> diff --git a/block/blklogwrites.c b/block/blklogwrites.c
>>> index 7ef046cee9..c7da507b2d 100644
>>> --- a/block/blklogwrites.c
>>> +++ b/block/blklogwrites.c
>>> @@ -96,10 +96,10 @@ static inline bool 
>>> blk_log_writes_sector_size_valid(uint32_t sector_size)
>>>          sector_size < (1ull << 24);
>>>  }
>>>  
>>> -static uint64_t blk_log_writes_find_cur_log_sector(BdrvChild *log,
>>> -                                                   uint32_t sector_size,
>>> -                                                   uint64_t nr_entries,
>>> -                                                   Error **errp)
>>> +static int64_t blk_log_writes_find_cur_log_sector(BdrvChild *log,
>>> +                                                  uint32_t sector_size,
>>> +                                                  uint64_t nr_entries,
>>> +                                                  Error **errp)
>>>  {
>>>      uint64_t cur_sector = 1;
>>>      uint64_t cur_idx = 0;
>>> @@ -112,13 +112,13 @@ static uint64_t 
>>> blk_log_writes_find_cur_log_sector(BdrvChild *log,
>>>          if (read_ret < 0) {
>>>              error_setg_errno(errp, -read_ret,
>>>                               "Failed to read log entry %"PRIu64, cur_idx);
>>> -            return (uint64_t)-1ull;
>>> +            return read_ret;
>>
>> This changes the error status of blk_log_writes_open() from -EINVAL to
>> whatever is returned by bdrv_pread(). I guess this is an improvement
>> worth to be mentioned in the changelog.
>>
>>>          }
>>>  
>>>          if (cur_entry.flags & ~cpu_to_le64(LOG_FLAG_MASK)) {
>>>              error_setg(errp, "Invalid flags 0x%"PRIx64" in log entry 
>>> %"PRIu64,
>>>                         le64_to_cpu(cur_entry.flags), cur_idx);
>>> -            return (uint64_t)-1ull;
>>> +            return -EINVAL;
>>>          }
>>>  
>>>          /* Account for the sector of the entry itself */
>>> @@ -143,7 +143,6 @@ static int blk_log_writes_open(BlockDriverState *bs, 
>>> QDict *options, int flags,
>>>  {
>>>      BDRVBlkLogWritesState *s = bs->opaque;
>>>      QemuOpts *opts;
>>> -    Error *local_err = NULL;
>>>      int ret;
>>>      uint64_t log_sector_size;
>>>      bool log_append;
>>> @@ -215,15 +214,15 @@ static int blk_log_writes_open(BlockDriverState *bs, 
>>> QDict *options, int flags,
>>>          s->nr_entries = 0;
>>>  
>>>          if (blk_log_writes_sector_size_valid(log_sector_size)) {
>>> -            s->cur_log_sector =
>>> +            int64_t cur_log_sector =
>>>                  blk_log_writes_find_cur_log_sector(s->log_file, 
>>> log_sector_size,
>>> -                                    le64_to_cpu(log_sb.nr_entries), 
>>> &local_err);
>>> -            if (local_err) {
>>> -                ret = -EINVAL;
>>> -                error_propagate(errp, local_err);
>>> +                                    le64_to_cpu(log_sb.nr_entries), errp);
>>> +            if (cur_log_sector < 0) {
>>> +                ret = cur_log_sector;
>>
>> This is converting int64_t to int, which is usually not recommended.
>> In practice this is probably okay because cur_log_sector is supposed
>> to be a negative errno (ie. an int) in this case.
>
> It isn't: blk_log_writes_find_cur_log_sector() returns (uint64_t)-1ull
> on error.
>
> Aside: returning uint64_t is awkward.  We commonly use a signed integer
> for non-negative offset or negative error.
>
>> Maybe make this clear with a an assert(cur_log_sector >= INT_MIN) ?
>
> Unless we change blk_log_writes_find_cur_log_sector() to return a
> negative errno code,

Which the patch does.  I shouldn't review patches before breakfast.

> negative errno code, we need to ret = -EINVAL here.  Let's keep this
> series simple.

No, the patch is okay as is.

Dumbing it down to keep it simple would also be okay.

Regarding the proposed assertion: do we protect similar conversions from
over-wide negative errno int elsewhere?

>>>                  goto fail_log;
>>>              }
>>>  
>>> +            s->cur_log_sector = cur_log_sector;
>>>              s->nr_entries = le64_to_cpu(log_sb.nr_entries);
>>>          }
>>>      } else {