Re: [Qemu-devel] [PATCH] qemu-img: initialize MapEntry object

[John Snow <jsnow@redhat.com>]

On 02/04/2016 07:43 AM, Kevin Wolf wrote:
> Am 04.02.2016 um 00:38 hat John Snow geschrieben:
>> Commit 16b0d555 introduced an issue where we are not initializing
>> has_filename for the 'next' MapEntry object, which leads to interesting
>> errors in Valgrind and Clang -fsanitize=undefined both.
>>
>> Zero the stack object at allocation AND make sure the utility to
>> populate the fields properly marks has_filename as false if applicable.
>>
>> Signed-off-by: John Snow <jsnow@redhat.com>
>> ---
>>  qemu-img.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/qemu-img.c b/qemu-img.c
>> index f121980..5a85178 100644
>> --- a/qemu-img.c
>> +++ b/qemu-img.c
>> @@ -2231,6 +2231,9 @@ static int get_block_status(BlockDriverState *bs, int64_t sector_num,
>>      if (file && e->has_offset) {
>>          e->has_filename = true;
>>          e->filename = file->filename;
>> +    } else {
>> +        e->has_filename = false;
>> +        e->filename = NULL;
>>      }
>>      return 0;
>>  }
> 
> I guess this fixes the bug, but wouldn't it actually be nicer to just
> reinitialise the whole object? As everyone knows, I love compound
> literals, so I'd make it one big assignment that zeroes everything that
> isn't specified:
> 
> *e = (MapEntry) {
>     ...
> };
> 

Yeah, that's more future proof isn't it.

>> @@ -2264,7 +2267,7 @@ static int img_map(int argc, char **argv)
>>      BlockDriverState *bs;
>>      const char *filename, *fmt, *output;
>>      int64_t length;
>> -    MapEntry curr = { .length = 0 }, next;
>> +    MapEntry curr = { .length = 0 }, next = { .length = 0 };
>>      int ret = 0;
> 
> At first I didn't quite understand what this was for, but I think you
> tried to cover newly added fields. If you overwrite the whole struct
> above, you wouldn't need to initialise it here any more.
> 
> Kevin
> 

Yeah, the intent was:

(1) Always make sure it starts out zeroed.
(2) In case someone tries to call get_block_status with an object that
isn't zeroed in the future, make sure has_filename is toggled back to false.



I'll send you a new one.