Re: [Qemu-devel] [PATCH v3 1/7] qapi: use qemu_strtoi64() in parse_str

[David Hildenbrand <david@redhat.com>]

On 07.11.18 16:29, Markus Armbruster wrote:
> David Hildenbrand <david@redhat.com> writes:
> 
>> On 05.11.18 21:43, Markus Armbruster wrote:
>>> David Hildenbrand <david@redhat.com> writes:
>>>
>>>> On 05.11.18 16:37, Markus Armbruster wrote:
>>>>> David Hildenbrand <david@redhat.com> writes:
>>>>>
>>>>>> On 31.10.18 18:55, Markus Armbruster wrote:
>>>>>>> David Hildenbrand <david@redhat.com> writes:
>>>>>>>
>>>>>>>> On 31.10.18 15:40, Markus Armbruster wrote:
>>>>>>>>> David Hildenbrand <david@redhat.com> writes:
>>>>>>>>>
>>>>>>>>>> The qemu api claims to be easier to use, and the resulting code seems to
>>>>>>>>>> agree.
>>>>>>> [...]
>>>>>>>>>> @@ -60,9 +61,7 @@ static int parse_str(StringInputVisitor *siv, const char *name, Error **errp)
>>>>>>>>>>      }
>>>>>>>>>>  
>>>>>>>>>>      do {
>>>>>>>>>> -        errno = 0;
>>>>>>>>>> -        start = strtoll(str, &endptr, 0);
>>>>>>>>>> -        if (errno == 0 && endptr > str) {
>>>>>>>>>> +        if (!qemu_strtoi64(str, &endptr, 0, &start)) {
>>>>>>>>>>              if (*endptr == '\0') {
>>>>>>>>>>                  cur = g_malloc0(sizeof(*cur));
>>>>>>>>>>                  range_set_bounds(cur, start, start);
>>>>>>>>>> @@ -71,11 +70,7 @@ static int parse_str(StringInputVisitor *siv, const char *name, Error **errp)
>>>>>>>>>>                  str = NULL;
>>>>>>>>>>              } else if (*endptr == '-') {
>>>>>>>>>>                  str = endptr + 1;
>>>>>>>>>> -                errno = 0;
>>>>>>>>>> -                end = strtoll(str, &endptr, 0);
>>>>>>>>>> -                if (errno == 0 && endptr > str && start <= end &&
>>>>>>>>>> -                    (start > INT64_MAX - 65536 ||
>>>>>>>>>> -                     end < start + 65536)) {
>>>>>>>>>> +                if (!qemu_strtoi64(str, &endptr, 0, &end) && start < end) {
>>>>>>>>>
>>>>>>>>> You deleted (start > INT64_MAX - 65536 || end < start + 65536).  Can you
>>>>>>>>> explain that to me?  I'm feeling particularly dense today...
>>>>>>>>
>>>>>>>> qemu_strtoi64 performs all different kinds of error handling completely
>>>>>>>> internally. This old code here was an attempt to filter out -EWHATEVER
>>>>>>>> from the response. No longer needed as errors and the actual value are
>>>>>>>> reported via different ways.
>>>>>>>
>>>>>>> I understand why errno == 0 && endptr > str go away.  They also do in
>>>>>>> the previous hunk.
>>>>>>>
>>>>>>> The deletion of (start > INT64_MAX - 65536 || end < start + 65536) is
>>>>>>> unobvious.  What does it do before the patch?
>>>>>>>
>>>>>>> The condition goes back to commit 659268ffbff, which predates my watch
>>>>>>> as maintainer.  Its commit message is of no particular help.  Its code
>>>>>>> is... allright, the less I say about that, the better.
>>>>>>>
>>>>>>> We're parsing a range here.  We already parsed its lower bound into
>>>>>>> @start (and guarded against errors), and its upper bound into @end (and
>>>>>>> guarded against errors).
>>>>>>>
>>>>>>> If the condition you delete is false, we goto error.  So the condition
>>>>>>> is about range validity.  I figure it's an attempt to require valid
>>>>>>> ranges to be no "wider" than 65535.  The second part end < start + 65536
>>>>>>> checks exactly that, except shit happens when start + 65536 overflows.
>>>>>>> The first part attempts to guard against that, but
>>>>>>>
>>>>>>> (1) INT64_MAX is *wrong*, because we compute in long long, and
>>>>>>>
>>>>>>> (2) it rejects even small ranges like INT64_MAX - 2 .. INT64_MAX - 1.
>>>>>>>
>>>>>>> WTF?!?
>>>>>>>
>>>>>>> Unless I'm mistaken, the condition is not about handling any of the
>>>>>>> errors that qemu_strtoi64() handles for us.
>>>>>>>
>>>>>>> The easiest way for you out of this morass is probably to keep the
>>>>>>> condition exactly as it was, then use the "my patch doesn't make things
>>>>>>> any worse" get-out-of-jail-free card.
>>>>>>>
>>>>>>
>>>>>> Looking at the code in qapi/string-output-visitor.c related to range and
>>>>>> list handling I feel like using the get-out-of-jail-free card to get out
>>>>>> of qapi code now :) Too much magic in that code and too little time for
>>>>>> me to understand it all.
>>>>>>
>>>>>> Thanks for your time and review anyway. My time is better invested in
>>>>>> other parts of QEMU. I will drop both patches from this series.
>>>>>
>>>>> Understand.
>>>>>
>>>>> When I first looked at the ranges stuff in the string input visitor, I
>>>>> felt the urge to clean it up, then sat on my hands until it passed.
>>>>>
>>>>> The rest is reasonable once you understand how it works.  The learning
>>>>> curve is less than pleasant, though.
>>>>>
>>>>
>>>> Maybe I'll pick this up again when I have more time to invest.
>>>>
>>>> The general concept
>>>>
>>>> 1. of having an input visitor that is able to parse different types
>>>> (expected by e.g. a property) sounds sane to me.
>>>>
>>>> 2. of having a list of *something*, assuming it is int64_t, and assuming
>>>> it is to be parsed into a list of ranges sounds completely broken to me.
>>>
>>> Starting point: the string visitors can only do scalars.  We have a need
>>> for lists of integers (see below).  The general solution would be
>>> generalizing these visitors to lists (and maybe objects while we're at
>>> it).  YAGNI.  So we put in a quick hack that can do just lists of
>>> integers.
>>>
>>> Except applying YAGNI to stable interfaces is *bonkers*.
>>>
>>>> I was not even able to find an example QEMU comand line for 2. Is this
>>>> maybe some very old code that nobody actually uses anymore? (who uses
>>>> list of ranges?)
>>>
>>> The one I remember offhand is -numa node,cpus=..., but that one's
>>> actually parsed with the options visitor.  Which is even hairier, but at
>>> least competently coded.
>>>
>>> To find uses, we need to follow the uses of the string visitors.
>>>
>>> Of the callers of string_input_visitor_new(),
>>> object_property_get_uint16List() is the only one that deals with lists.
>>> It's used by query_memdev() for property host-nodes.
>>>
>>> The callers of string_output_visitor_new() lead to MigrationInfo member
>>> postcopy-vcpu-blocktime, and Memdev member host-nodes again.
>>>
>>> Searching the QAPI schema for lists of integers coughs up a few more
>>> candidates: NumaNodeOptions member cpus (covered above), RxFilterInfo
>>> member vlan-table (unrelated, as far as I can tell), RockerOfDpaGroup
>>> (likewise), block latency histogram stuff (likewise).
>>>
>>
>> As Eric pointed out, tests/test-string-input-visitor.c actually tests
>> for range support in test_visitor_in_intList.
>>
>> I might be completely wrong, but actually the string input visitor
>> should not pre-parse stuff into a list of ranges, but instead parse on
>> request (parse_type_...) and advance in the logical list on "next_list".
>> And we should parse ranges *only* if we are expecting a list. Because a
>> range is simply a short variant of a list. A straight parse_type_uint64
>> should bail out if we haven't started a list.
> 
> Yes, parse_type_int64() & friends should simply parse the appropriate
> integer, *except* when we're working on a list.  Then they should return
> the next integer, which may or may not require parsing.
> 
> Say, input is "1-3,5", and the visitor is called like
> 
>     visit_start_list()
>     visit_next_list()   more input, returns "there's more"
>     visit_type_int()    parses "1-3,", buffers 2-3, returns 1
>     visit_next_list()   buffer not empty, returns "there's more"
>     visit_type_int()    unbuffers and returns 2
>     visit_next_list()   buffer not empty, returns "there's more"
>     visit_type_int()    unbuffers and returns 3 
>     visit_next_list()   more input, returns "there's more"
>     visit_type_int()    parses "5", returns 5
>     visit_next_list()   buffer empty and no more input, returns "done"
>     visit_end_list()   
> 

Would it be valid to do something like this (skipping elements without a
proper visit_type_int)

visit_start_list();
visit_next_list();  more input, returns "there's more"
visit_next_list();  parses "1-3,", buffers 2-3, skips over 1
visit_type_int();   returns 2
...

Or mixing types

visit_start_list();
visit_next_list();
visit_type_int64();
visit_next_list();
visit_type_uint64();

IOW, can I assume that after every visit_next_list(), visit_type_X is
called, and that X remains the same for one pass over the list?

Also, I assume it is supposed to work like this

visit_start_list();
visit_next_list();  more input, returns "there's more"
visit_type_int();   returns 1 (parses 1-3, buffers 1-3)
visit_type_int();   returns 1
visit_type_int();   returns 1
visit_next_list();  more input, unbuffers 1
visit_type_int();   returns 2

So unbuffering actually happens on visit_next_list()?

-- 

Thanks,

David / dhildenb