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

[Greg Kurz]

On Fri, 11 Sep 2020 07:30:37 +0200
Markus Armbruster <armbru@redhat.com> wrote:

> 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.
> 

What about Ari's proposal to add ERRP_GUARD() and check errors
with "if (*errp)" like we do for void functions ?

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

We do protect int64_t->int conversions in a few places, eg.
qcow2_write_snapshots() or qemu_spice_create_host_primary(),
but I couldn't find one about a negarive errno.

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