Re: [PATCH v3] qga: fence guest-set-time if hwclock not available

[Laszlo Ersek]

On 12/06/19 08:15, Markus Armbruster wrote:
> Laszlo Ersek <address@hidden> writes:
> 
>> On 12/05/19 14:05, Philippe Mathieu-Daudé wrote:
>>> Hi Cornelia,
>>>
>>> On 12/5/19 12:53 PM, Cornelia Huck wrote:
>>>> The Posix implementation of guest-set-time invokes hwclock to
>>>> set/retrieve the time to/from the hardware clock. If hwclock
>>>> is not available, the user is currently informed that "hwclock
>>>> failed to set hardware clock to system time", which is quite
>>>> misleading. This may happen e.g. on s390x, which has a different
>>>> timekeeping concept anyway.
>>>>
>>>> Let's check for the availability of the hwclock command and
>>>> return QERR_UNSUPPORTED for guest-set-time if it is not available.
>>>>
>>>> Reviewed-by: Laszlo Ersek <address@hidden>
>>>> Reviewed-by: Daniel P. Berrangé <address@hidden>
>>>> Reviewed-by: Michael Roth <address@hidden>
>>>> Signed-off-by: Cornelia Huck <address@hidden>
>>>> ---
>>>>
>>>> v2->v3:
>>>>    - added 'static' keyword to hwclock_path
>>>>
>>>> Not sure what tree this is going through; if there's no better place,
>>>> I can also take this through the s390 tree.
>>>
>>> s390 or trivial trees seems appropriate.
>>>
>>>>
>>>> ---
>>>>   qga/commands-posix.c | 13 ++++++++++++-
>>>>   1 file changed, 12 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/qga/commands-posix.c b/qga/commands-posix.c
>>>> index 1c1a165daed8..0be301a4ea77 100644
>>>> --- a/qga/commands-posix.c
>>>> +++ b/qga/commands-posix.c
>>>> @@ -156,6 +156,17 @@ void qmp_guest_set_time(bool has_time, int64_t
>>>> time_ns, Error **errp)
>>>>       pid_t pid;
>>>>       Error *local_err = NULL;
>>>>       struct timeval tv;
>>>> +    static const char hwclock_path[] = "/sbin/hwclock";
>>>> +    static int hwclock_available = -1;
>>>> +
>>>> +    if (hwclock_available < 0) {
>>>> +        hwclock_available = (access(hwclock_path, X_OK) == 0);
>>>> +    }
>>>> +
>>>> +    if (!hwclock_available) {
>>>> +        error_setg(errp, QERR_UNSUPPORTED);
>>>
>>> In include/qapi/qmp/qerror.h we have:
>>>
>>> /*
>>>  * These macros will go away, please don't use in new code, and do not
>>>  * add new ones!
>>>  */
>>
>> Obviously, the last word on this belongs to Markus (CC'd) -- he added
>> that comment. I'd just like to point out *when* that comment was added:
>> approx. four and half years ago. (See commit 4629ed1e9896.)
> 
> Yes, with a big push to finally kill qerror_report().  I was too
> exhausted to finish the job and kill all the remaining QERR_, too.
> 
> I'm dead serious on the "do not add new ones" part.
> 
> On the "don't use in new code" part, I'm quite willing to tolerate
> reasonable exceptions, e.g. to maintain consistency with old code.
> 
> This one looks like a reasonable exception to me.
> 
>> I've always associated QERR_UNSUPPORTED with QMP interfaces rejecting
>> invocation due to lack of support. I don't think one more instance of
>> QERR_UNSUPPORTED will matter much, when we'll "finally" :) convert or
>> eliminate the other 35! (Yes, I've counted.)
>>
>> In case it's unacceptable to add one more QERR_UNSUPPORTED: what is the
>> official solution that replaces it?
>>
>> I assume it was explained in the series that included commit
>> 4629ed1e9896, but I can't easily tell. (And, there is no "QERR_" match
>> in docs/.)
> 
> See "exhausted" above.
> 
>> Hmmm, more history digging... In the 4629ed1e9896..v4.2.0-rc4 set of
>> commits, the following commits introduced new instances of
>> QERR_UNSUPPORTED:
>>
>> - e09484efbc9d ("qmp: add QMP interface "query-cpu-model-expansion"", 
>> 2016-09-06)
>> - 0031e0d68339 ("qmp: add QMP interface "query-cpu-model-comparison"", 
>> 2016-09-06)
>> - b18b6043341d ("qmp: add QMP interface "query-cpu-model-baseline"", 
>> 2016-09-06)
>> - 1007a37e2082 ("smbios: filter based on CONFIG_SMBIOS rather than TARGET", 
>> 2017-01-16)
>> - 9f57061c3555 ("acpi: filter based on CONFIG_ACPI_X86 rather than TARGET", 
>> 2017-01-16)
>> - 39164c136cba ("qmp/hmp: add query-vm-generation-id and 'info 
>> vm-generation-id' commands", 2017-03-02)
>> - 161a56a9065f ("qga: Add 'guest-get-users' command", 2017-04-26)
>> - 53c58e64d0a2 ("qga: Add `guest-get-timezone` command", 2017-04-27)
>> - e674605f9821 ("qemu-ga: check if utmpx.h is available on the system", 
>> 2017-07-17)
>>
>> I don't claim that all of those additions have stuck with us, to
>> v4.2.0-rc4. Yet, in general, practice doesn't seem to have followed the
>> intended deprecation.
>>
>>>
>>> Maybe we can replace it by "this feature is not supported on this
>>> architecture"? (or without 'on this architecture').
>>
>> I think if we replace QERR_UNSUPPORTED with anything, it should be
>> "similarly standardized". (Lack of support for a given QMP interface is
>> pretty common, I think.)
> 
> Here's my the general idea on getting rid of qerror.h.  The QERR_ macros
> effectively factor out common error message format strings.  DRY is a
> legitimate concern.  However, I dislike (1) passing anything but string
> literals as format to printf()-style function, and (2) tempting people
> to reuse existing error messages where a new error message would be more
> helpful.
> 
> Note that the error_setg() is *also* common.  So take DRY to the next
> level: factor out the common error_setg(errp, "literal error format
> string", ...) along with whatever error handling is also common in a
> helper function, and call that.

Good point, I didn't think of (e.g.) error_set_qmp_unsupported().

Thank you for the details!
Laszlo

> 
> However, do that only where the errors are truly common.  Where they're
> just similar, duplicate the error message, and maybe specialize it for
> specific error situations.
> 
>>>> +        return;
>>>> +    }
>>>>         /* If user has passed a time, validate and set it. */
>>>>       if (has_time) {
>>>> @@ -195,7 +206,7 @@ void qmp_guest_set_time(bool has_time, int64_t
>>>> time_ns, Error **errp)
>>>>             /* Use '/sbin/hwclock -w' to set RTC from the system time,
>>>>            * or '/sbin/hwclock -s' to set the system time from RTC. */
>>>> -        execle("/sbin/hwclock", "hwclock", has_time ? "-w" : "-s",
>>>> +        execle(hwclock_path, "hwclock", has_time ? "-w" : "-s",
>>>>                  NULL, environ);
>>>>           _exit(EXIT_FAILURE);
>>>>       } else if (pid < 0) {
>>>>
>>>