Re: [Qemu-devel] [PATCH v3 05/19] nbd/server: Favor [u]int64_t over off_t

[Eric Blake <eblake@redhat.com>]

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On 1/15/19 4:19 AM, Vladimir Sementsov-Ogievskiy wrote:
> 12.01.2019 20:57, Eric Blake wrote:
>> Although our compile-time environment is set up so that we always
>> support long files with 64-bit off_t, we have no guarantee whether
>> off_t is the same type as int64_t.  This requires casts when
>> printing values, and prevents us from directly using qemu_strtoi64().
>> Let's just flip to [u]int64_t (signed for length, because we have to
>> detect failure of blk_getlength()
> 
> we have not, after previous patch

nbd/server.c no longer has to check for blk_getlength() failures, but
blockdev-nbd.c and qemu-nbd.c still do.  Since the callers have to use
an int64_t type for the length as part of their error checking, it's
easier to accept an int64_t length to nbd_export_new(), even if
nbd_export_new() could also use an unsigned type.

> 
> and because off_t was signed;
>> unsigned for offset because it lets us simplify some math without
>> having to worry about signed overflow).
>>
>> Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>> Signed-off-by: Eric Blake <eblake@redhat.com>
>>
>> ---
>> v3: new patch
>> ---
>>   include/block/nbd.h |  4 ++--
>>   nbd/server.c        | 14 +++++++-------
>>   qemu-nbd.c          | 26 ++++++++++----------------
>>   3 files changed, 19 insertions(+), 25 deletions(-)
>>
>> diff --git a/include/block/nbd.h b/include/block/nbd.h
>> index 1971b557896..0f252829376 100644
>> --- a/include/block/nbd.h
>> +++ b/include/block/nbd.h
>> @@ -294,8 +294,8 @@ int nbd_errno_to_system_errno(int err);
>>   typedef struct NBDExport NBDExport;
>>   typedef struct NBDClient NBDClient;
>>
>> -NBDExport *nbd_export_new(BlockDriverState *bs, off_t dev_offset, off_t size,
>> -                          const char *name, const char *description,
>> +NBDExport *nbd_export_new(BlockDriverState *bs, uint64_t dev_offset,
>> +                          int64_t size, const char *name, const char *desc,
> 
> in previous patch you drop use of negative @size parameter, so it looks better
> to use unsigned for size like for offset

You can't have a size larger than 2^63; but an unsigned type holds
nearly 2^64.  I prefer a signed size, for the same reason that off_t is
signed, in that checking for a negative size is easier to rule out sizes
that are too large.


>>
>>   static int find_partition(BlockBackend *blk, int partition,
>> -                          off_t *offset, off_t *size)
>> +                          uint64_t *offset, int64_t *size)
> 
> function never return negative @size, so what is the reason to keep it signed?

Because the C compiler does NOT like:

int64_t len;
find_partition(..., &len);

with a uint64_t* parameter type - you HAVE to match the signed-ness of
your caller's parameter with your pointer type. Since the caller already
has to use a signed type (to check for blk_getlength() failure AND
because sizes really cannot exceed 2^63), it's easier to keep it signed
here.

> 
> Also, type conversion (uint64_t) should be dropped from the function code I think.

Are you talking about this part:

                if ((ext_partnum + j + 1) == partition) {
                    *offset = (uint64_t)ext[j].start_sector_abs << 9;
                    *size = (uint64_t)ext[j].nb_sectors_abs << 9;
                    return 0;
                }
            }
            ext_partnum += 4;
        } else if ((i + 1) == partition) {
            *offset = (uint64_t)mbr[i].start_sector_abs << 9;
            *size = (uint64_t)mbr[i].nb_sectors_abs << 9;
            return 0;

No - that has to keep the cast, because .start_sector_abs and
.nb_sectors_abs are uint32_t values, but we want to shift into 64-bit
results.  You need the cast to force the correct arithmetic rather than
truncating into a 32-bit value that then gets widened into 64-bit storage.

>> @@ -665,10 +665,6 @@ int main(int argc, char **argv)
>>                   error_report("Invalid offset `%s'", optarg);
>>                   exit(EXIT_FAILURE);
>>               }
>> -            if (dev_offset < 0) {
>> -                error_report("Offset must be positive `%s'", optarg);
>> -                exit(EXIT_FAILURE);
>> -            }
> 
> hm, then, may be, s/strtoll/strtoull before this?

I clean that up in patch 6/19.

> 
>>               break;
>>           case 'l':
>>               if (strstart(optarg, SNAPSHOT_OPT_BASE, NULL)) {
>> @@ -1005,15 +1001,14 @@ int main(int argc, char **argv)
>>       }
>>
>>       if (dev_offset >= fd_size) {
>> -        error_report("Offset (%lld) has to be smaller than the image size "
>> -                     "(%lld)",
>> -                     (long long int)dev_offset, (long long int)fd_size);
>> +        error_report("Offset (%" PRIu64 ") has to be smaller than the image "
>> +                     "size (%" PRIu64 ")", dev_offset, fd_size);
> 
> PRId64 for fd_size

Sure.


>> @@ -1026,12 +1021,11 @@ int main(int argc, char **argv)
>>               exit(EXIT_FAILURE);
>>           }
>>           /* partition limits are (32-bit << 9); can't overflow 64 bits */
>> -        assert(dev_offset >= 0 && dev_offset + limit >= dev_offset);
>> +        assert(dev_offset + limit >= dev_offset);
>>           if (dev_offset + limit > fd_size) {
>> -            error_report("Discovered partition %d at offset %lld size %lld, "
>> -                         "but size exceeds file length %lld", partition,
>> -                         (long long int) dev_offset, (long long int) limit,
>> -                         (long long int) fd_size);
>> +            error_report("Discovered partition %d at offset %" PRIu64
>> +                         " size %" PRId64 ", but size exceeds file length %"
>> +                         PRId64, partition, dev_offset, limit, fd_size);
>>               exit(EXIT_FAILURE);
> 
> hmm, it may be better to place this patch before [03], to squash this chunk into [03]

I didn't mind the churn; also, I prefer patch 3 first, because it's more
likely to get backported as a bug fix than the rest of the series (and
the earlier you stick backport candidates in a series, the easier it is
to backport).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org


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