Re: [PATCH] vl: Fix an assert failure in error path

[Markus Armbruster <armbru@redhat.com>]

Peng Liang <liangpeng10@huawei.com> writes:

> On 6/9/2021 8:15 PM, Daniel P. Berrangé wrote:
>> On Wed, Jun 09, 2021 at 02:09:47PM +0200, Markus Armbruster wrote:
>>> Paolo Bonzini <pbonzini@redhat.com> writes:
>>>
>>>> On 10/06/21 10:47, Zhenzhong Duan wrote:
>>>>> Based on the description of error_setg(), the local variable err in
>>>>> qemu_maybe_daemonize() should be initialized to NULL.
>>>>> Without fix, the uninitialized *errp triggers assert failure which
>>>>> doesn't show much valuable information.
>>>>> Before the fix:
>>>>> qemu-system-x86_64: ../util/error.c:59: error_setv: Assertion `*errp == NULL' failed.
>>>>> After fix:
>>>>> qemu-system-x86_64: cannot create PID file: Cannot open pid file: Permission denied
>>>>> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
>>>>> ---
>>>>>   softmmu/vl.c | 2 +-
>>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>>> diff --git a/softmmu/vl.c b/softmmu/vl.c
>>>>> index 326c1e9080..feb4d201f3 100644
>>>>> --- a/softmmu/vl.c
>>>>> +++ b/softmmu/vl.c
>>>>> @@ -2522,7 +2522,7 @@ static void qemu_process_help_options(void)
>>>>>     static void qemu_maybe_daemonize(const char *pid_file)
>>>>>   {
>>>>> -    Error *err;
>>>>> +    Error *err = NULL;
>>>
>>> Common mistake, I'm afraid.
>> 
>> Initializing isn't likely to be a performance impact, so I'd think
>> we should make 'checkpatch.pl' complain about any 'Error *' variable
>> that is not initialized to NULL, as a safety net, even if not technically
>> required in some cases.
>> 
>> Regards,
>> Daniel
>> 
>
> Hi,
> Could we add a coccinelle script to check (and fix) these problems?  e.g.:

Coccinelle is good for finding and fixing instances of bad patterns that
have crept in.  checkpatch is good for keeping them out.  Both has its
uses.

> @ r @
> identifier id;
> @@
>   Error *id
> + = NULL
>   ;
>
> Using this script, I found that local variable err in
> qemu_init_subsystems is not initialized to NULL too.

Crash bug, broken in commit efd7ab22fb "vl: extract
qemu_init_subsystems", v6.0.0.  Care to submit a fix?

>                                                       The script is not
> prefect though, it will initialize all global/static 'Error *' variables
> and all local 'Error *' variables in util/error.c to NULL, which is
> unnecessary.

Excluding util/error.c is easy once you know how to:

    @ depends on !(file in "util/error.c")@
    identifier id;
    @@
    ...

Excluding variable definitions with static storage duraction shouldn't
be too hard, either.  If Coccinelle is sufficiently clever, adding
keyword auto suffices.  Else, try matching keyword static separately,
like so:

    (
        static Error *id;
    |
    -   Error *id;
    +   Error *id = NULL;
    )

Completely untested.