Re: [PATCH] virtiofsd: Fix side-effect in assert()

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Hi Stefan,

On 4/9/21 5:11 PM, Stefan Weil wrote:
> Am 09.04.21 um 12:06 schrieb Greg Kurz:
> 
>> It is bad practice to put an expression with a side-effect in
>> assert() because the side-effect won't happen if the code is
>> compiled with -DNDEBUG.
>>
>> Use an intermediate variable. Consolidate this in an macro to
>> have proper line numbers when the assertion is hit.
>>
>> virtiofsd: ../../tools/virtiofsd/passthrough_ll.c:2797: lo_getxattr:
>>   Assertion `fchdir_res == 0' failed.
>> Aborted
>>
>>    2796          /* fchdir should not fail here */
>> =>2797          FCHDIR_NOFAIL(lo->proc_self_fd);
>>    2798          ret = getxattr(procname, name, value, size);
>>    2799          FCHDIR_NOFAIL(lo->root.fd);
>>
>> Fixes: bdfd66788349 ("virtiofsd: Fix xattr operations")
>> Cc: misono.tomohiro@jp.fujitsu.com
>> Signed-off-by: Greg Kurz <groug@kaod.org>
>> ---
>>   tools/virtiofsd/passthrough_ll.c | 21 +++++++++++++--------
>>   1 file changed, 13 insertions(+), 8 deletions(-)
>>
>> diff --git a/tools/virtiofsd/passthrough_ll.c
>> b/tools/virtiofsd/passthrough_ll.c
>> index 1553d2ef454f..6592f96f685e 100644
>> --- a/tools/virtiofsd/passthrough_ll.c
>> +++ b/tools/virtiofsd/passthrough_ll.c
>> @@ -2723,6 +2723,11 @@ static int xattr_map_server(const struct lo_data 
> *lo, const char *server_name,
>>       return -ENODATA;
>>   }
>>   +#define FCHDIR_NOFAIL(fd) do {                         \
>> +        int fchdir_res = fchdir(fd);                   \
>> +        assert(fchdir_res == 0);                       \
>> +    } while (0)
>> +
>>   static void lo_getxattr(fuse_req_t req, fuse_ino_t ino, const char
>> *in_name,
>>   
> 
> 
> I am afraid that this will raise a compiler warning (or error with
> -Werror) when NDEBUG is defined because the variable is unused in that
> case ([-Wunused-variable]).
> 
> I suggest to use different implementations of the macro depending on
> NDEBUG.

QEMU doesn't build with NDEBUG, see commit 262a69f4282
("osdep.h: Prohibit disabling assert() in supported builds").

Regards,

Phil.