Re: [PATCH 09/13] parse-options API: don't restrict OPT_SUBCOMMAND() to one *_fn type

[Phillip Wood <phillip.wood123@gmail.com>]

On 05/11/2022 13:52, Ævar Arnfjörð Bjarmason wrote:
> 
> On Sat, Nov 05 2022, René Scharfe wrote:
> 
>> Am 04.11.22 um 14:22 schrieb Ævar Arnfjörð Bjarmason:
>>> diff --git a/parse-options.h b/parse-options.h
>>> index b6ef86e0d15..61e3016c3fc 100644
>>> --- a/parse-options.h
>>> +++ b/parse-options.h
>>> @@ -128,19 +128,24 @@ typedef int parse_opt_subcommand_fn(int argc, const char **argv,
>>>    *			 the option takes optional argument.
>>>    *
>>>    * `callback`::
>>> - *   pointer to the callback to use for OPTION_CALLBACK
>>> + *   pointer to the callback to use for OPTION_CALLBACK and OPTION_SUBCOMMAND.
>>>    *
>>>    * `defval`::
>>>    *   default value to fill (*->value) with for PARSE_OPT_OPTARG.
>>>    *   OPTION_{BIT,SET_INT} store the {mask,integer} to put in the value when met.
>>> + *   OPTION_SUBCOMMAND stores the pointer the function selected for
>>> + *   the subcommand.
>>> + *
>>>    *   CALLBACKS can use it like they want.
>>>    *
>>>    * `ll_callback`::
>>>    *   pointer to the callback to use for OPTION_LOWLEVEL_CALLBACK
>>>    *
>>>    * `subcommand_fn`::
>>> - *   pointer to a function to use for OPTION_SUBCOMMAND.
>>> - *   It will be put in value when the subcommand is given on the command line.
>>> + *   pointer to the callback used with OPT_SUBCOMMAND() and
>>> + *   OPT_SUBCOMMAND_F(). Internally we store the same value in
>>> + *   `defval`. This is only here to give the OPT_SUBCOMMAND{,_F}()
>>> + *   common case type safety.
>>>    */
>>>   struct option {
>>>   	enum parse_opt_type type;
>>> @@ -217,12 +222,24 @@ struct option {
>>>   #define OPT_ALIAS(s, l, source_long_name) \
>>>   	{ OPTION_ALIAS, (s), (l), (source_long_name) }
>>>
>>> +static inline int parse_options_pick_subcommand_cb(const struct option *option,
>>> +						   const char *arg UNUSED,
>>> +						   int unset UNUSED)
>>> +{
>>> +	parse_opt_subcommand_fn *fn = (parse_opt_subcommand_fn *)option->defval;
>>> +	*(parse_opt_subcommand_fn **)option->value = fn;
>>
>> ->defval is of type intptr_t and ->value is a void pointer.  The result
>> of converting a void pointer value to an intptr_t and back is a void
>> pointer equal to the original pointer if I read 6.3.2.3 (Pointers,
>> paragraphs 5 and 6) and 7.18.1.4 (Integer types capable of holding
>> object pointers) in C99 correctly.
>>
>> 6.3.2.3 paragraph 8 says that casting between function pointers of
>> different type is OK and you can get your original function pointer
>> back and use it in a call if you convert it back to the right type.
>>
>> Casting between a function pointer and an object pointer is undefined,
>> though.  They don't have to be of the same size, so a function pointer
>> doesn't have to fit into an intptr_t.  I wouldn't be surprised if CHERI
>> (https://www.cl.cam.ac.uk/research/security/ctsrd/cheri/) was an actual
>> example of that.
> 
> I should have called this out explicitly. I think you're right as far as
> what you're summarizing goes.
> 
> To elaborate on it, paragraph 8 of 6.3.2.3 says:
> 
> 	A pointer to a function of one type may be converted to a
> 	pointer to a function of another type and back again; the result
> 	shall compare equal to the original pointer. If a converted
> 	pointer is used to call a function whose type is not compatible
> 	with the pointed-to type, the behavior is undefined.
> 
> And 7.18.1.4 says, when discussing (among other things) "intptr_t"
> ("[such" added for clarity:
> 
> 	[...]any valid [such] pointer to void can be converted to this
> 	type, then converted back to pointer to void, and the result
> 	will compare equal to the original pointer:
> 
> But as you point out that doesn't say anything about whether a pointer
> to a function is a "valid .. pointer to void".
> 
> I think that's an "unportable" extension covered in "J.5 Common
> extensions", specifically "J.5.7 Function pointer casts":
> 
> 	A pointer to an object or to void may be cast to a pointer to a
> 	function, allowing data to be invoked as a function

This is a common extension, it is _not_ guaranteed by the standard and 
so still undefined behavior unless your compiler happens to have 
implemented that extension.

> Thus, since the standard already establishes that valid "void *" and
> "intptr_t" pointers can be cast'd back & forth, the J.5.7 bridges the
> gap between the two saying a function pointer can be converted to
> either.

How does J.5.7 bridge the gap when compilers are not required to 
implement it?

> Now, I may be missing something here, but I was under the impression
> that "intptr_t" wasn't special in any way here, and that any casting of
> a function pointer to either it or a "void *" was what was made portable
> by "J.5.7"

How is it made portable by an "unportable" extension?

> So I think aside from other concerns this should be safe to use, as
> real-world data backing that up we've had a intptr_t converted to a
> function pointer since v2.35.0: 5cb28270a1f (pack-objects: lazily set up
> "struct rev_info", don't leak, 2022-03-28).

Saying "it works so it is fine" is not a convincing argument that it is 
compliant with the standard. If it is undefined then it may work now but 
not in a future compiler.

>> Why is this trickery needed?  Above you write that callbacks in
>> builtin/bisect--helper.c can't use subcommand_fn because they need
>> their own argument.  Can we extend subcommand_fn or use a global
>> variable to pass that extra thing instead?  The latter may be ugly, but
>> at least it's valid C..

Unfortunately the current code relies on being able to cast function 
pointers to void* so it is not just this patch that is relying on 
undefined behavior.

Best Wishes

Phillip