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From: Phillip Wood <phillip.wood123@gmail.com>
To: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"René Scharfe" <l.s.r@web.de>
Cc: git@vger.kernel.org, "Junio C Hamano" <gitster@pobox.com>,
	"Jeff King" <peff@peff.net>, "Taylor Blau" <me@ttaylorr.com>,
	"Đoàn Trần Công Danh" <congdanhqx@gmail.com>,
	"SZEDER Gábor" <szeder.dev@gmail.com>,
	"Lukáš Doktor" <ldoktor@redhat.com>,
	"Johannes Schindelin" <Johannes.Schindelin@gmx.de>
Subject: Re: [PATCH 09/13] parse-options API: don't restrict OPT_SUBCOMMAND() to one *_fn type
Date: Sat, 5 Nov 2022 16:36:19 +0000	[thread overview]
Message-ID: <1b158749-44b1-34e0-5b52-1d3bfad9bc9a@dunelm.org.uk> (raw)
In-Reply-To: <221105.861qqhxz0o.gmgdl@evledraar.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

  reply	other threads:[~2022-11-05 16:36 UTC|newest]

Thread overview: 106+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-11-04  6:31 "git bisect run" strips "--log" from the list of arguments Lukáš Doktor
2022-11-04  9:45 ` Jeff King
2022-11-04 11:10   ` Đoàn Trần Công Danh
2022-11-04 12:51     ` Jeff King
2022-11-04 11:36   ` Ævar Arnfjörð Bjarmason
2022-11-04 12:45     ` Jeff King
2022-11-04 13:07       ` Ævar Arnfjörð Bjarmason
2022-11-04 12:37   ` SZEDER Gábor
2022-11-04 12:44     ` Jeff King
2022-11-04 11:40 ` [PATCH 0/3] Convert git-bisect--helper to OPT_SUBCOMMAND Đoàn Trần Công Danh
2022-11-04 11:40   ` [PATCH 1/3] bisect--helper: remove unused options Đoàn Trần Công Danh
2022-11-04 12:53     ` Jeff King
2022-11-04 11:40   ` [PATCH 2/3] bisect--helper: move all subcommands into their own functions Đoàn Trần Công Danh
2022-11-04 12:55     ` Jeff King
2022-11-04 13:32     ` Ævar Arnfjörð Bjarmason
2022-11-04 14:03       ` Đoàn Trần Công Danh
2022-11-04 11:40   ` [PATCH 3/3] bisect--helper: parse subcommand with OPT_SUBCOMMAND Đoàn Trần Công Danh
2022-11-04 13:00     ` Jeff King
2022-11-04 13:46     ` Ævar Arnfjörð Bjarmason
2022-11-04 14:07       ` Đoàn Trần Công Danh
2022-11-04 13:55   ` [PATCH 0/3] Convert git-bisect--helper to OPT_SUBCOMMAND Ævar Arnfjörð Bjarmason
2022-11-05 17:03   ` [PATCH v2 " Đoàn Trần Công Danh
2022-11-05 17:03     ` [PATCH v2 1/3] bisect--helper: remove unused options Đoàn Trần Công Danh
2022-11-05 17:03     ` [PATCH v2 2/3] bisect--helper: move all subcommands into their own functions Đoàn Trần Công Danh
2022-11-05 17:13       ` Đoàn Trần Công Danh
2022-11-05 17:03     ` [PATCH v2 3/3] bisect--helper: parse subcommand with OPT_SUBCOMMAND Đoàn Trần Công Danh
2022-11-05 17:07     ` [PATCH 00/13] Turn git-bisect to be builtin Đoàn Trần Công Danh
2022-11-05 17:07       ` [PATCH 01/13] bisect tests: test for v2.30.0 "bisect run" regressions Đoàn Trần Công Danh
2022-11-07 21:31         ` Ævar Arnfjörð Bjarmason
2022-11-08  1:17           ` Đoàn Trần Công Danh
2022-11-05 17:07       ` [PATCH 02/13] bisect: refactor bisect_run() to match CodingGuidelines Đoàn Trần Công Danh
2022-11-05 17:07       ` [PATCH 03/13] bisect--helper: pass arg[cv] down to do_bisect_run Đoàn Trần Công Danh
2022-11-05 17:07       ` [PATCH 04/13] bisect: fix output regressions in v2.30.0 Đoàn Trần Công Danh
2022-11-05 17:07       ` [PATCH 05/13] bisect run: keep some of the post-v2.30.0 output Đoàn Trần Công Danh
2022-11-07 21:40         ` Ævar Arnfjörð Bjarmason
2022-11-08  1:26           ` Đoàn Trần Công Danh
2022-11-08  3:11             ` Ævar Arnfjörð Bjarmason
2022-11-05 17:07       ` [PATCH 06/13] bisect--helper: remove unused arguments from do_bisect_run Đoàn Trần Công Danh
2022-11-05 17:07       ` [PATCH 07/13] bisect--helper: pretend we're real bisect when report error Đoàn Trần Công Danh
2022-11-07 21:29         ` Ævar Arnfjörð Bjarmason
2022-11-05 17:07       ` [PATCH 08/13] bisect test: test exit codes on bad usage Đoàn Trần Công Danh
2022-11-05 17:07       ` [PATCH 09/13] bisect--helper: emit usage for "git bisect" Đoàn Trần Công Danh
2022-11-05 17:07       ` [PATCH 10/13] bisect--helper: make `state` optional Đoàn Trần Công Danh
2022-11-05 17:07       ` [PATCH 11/13] bisect--helper: remove subcommand state Đoàn Trần Công Danh
2022-11-07 21:45         ` Ævar Arnfjörð Bjarmason
2022-11-08  1:27           ` Đoàn Trần Công Danh
2022-11-05 17:07       ` [PATCH 12/13] bisect--helper: log: allow arbitrary number of arguments Đoàn Trần Công Danh
2022-11-05 17:07       ` [PATCH 13/13] Turn `git bisect` into a full built-in Đoàn Trần Công Danh
2022-11-05 23:18     ` [PATCH v2 0/3] Convert git-bisect--helper to OPT_SUBCOMMAND Taylor Blau
2022-11-10 16:36   ` [PATCH v3 " Đoàn Trần Công Danh
2022-11-10 16:36     ` [PATCH v3 1/3] bisect--helper: remove unused options Đoàn Trần Công Danh
2022-11-11 12:42       ` Ævar Arnfjörð Bjarmason
2022-11-10 16:36     ` [PATCH v3 2/3] bisect--helper: move all subcommands into their own functions Đoàn Trần Công Danh
2022-11-11 13:51       ` Ævar Arnfjörð Bjarmason
2022-11-10 16:36     ` [PATCH v3 3/3] bisect--helper: parse subcommand with OPT_SUBCOMMAND Đoàn Trần Công Danh
2022-11-10 16:36     ` [PATCH v2 00/11] Turn git-bisect to be builtin Đoàn Trần Công Danh
2022-11-10 16:36       ` [PATCH v2 01/11] bisect tests: test for v2.30.0 "bisect run" regressions Đoàn Trần Công Danh
2022-11-10 16:36       ` [PATCH v2 02/11] bisect: refactor bisect_run() to match CodingGuidelines Đoàn Trần Công Danh
2022-11-10 16:36       ` [PATCH v2 03/11] bisect: fix output regressions in v2.30.0 Đoàn Trần Công Danh
2022-11-10 16:36       ` [PATCH v2 04/11] bisect run: keep some of the post-v2.30.0 output Đoàn Trần Công Danh
2022-11-10 16:36       ` [PATCH v2 05/11] bisect-run: verify_good: account for non-negative exit status Đoàn Trần Công Danh
2022-11-10 16:36       ` [PATCH v2 06/11] bisect--helper: identify as bisect when report error Đoàn Trần Công Danh
2022-11-10 16:36       ` [PATCH v2 07/11] bisect test: test exit codes on bad usage Đoàn Trần Công Danh
2022-11-10 16:36       ` [PATCH v2 08/11] bisect--helper: emit usage for "git bisect" Đoàn Trần Công Danh
2022-11-10 16:36       ` [PATCH v2 09/11] bisect--helper: handle states directly Đoàn Trần Công Danh
2022-11-10 16:36       ` [PATCH v2 10/11] bisect--helper: log: allow arbitrary number of arguments Đoàn Trần Công Danh
2022-11-11 14:01         ` Ævar Arnfjörð Bjarmason
2022-11-10 16:36       ` [PATCH v2 11/11] Turn `git bisect` into a full built-in Đoàn Trần Công Danh
2022-11-11 13:53         ` Ævar Arnfjörð Bjarmason
2022-11-11 15:37           ` Jeff King
2022-11-11 21:09             ` Ævar Arnfjörð Bjarmason
2022-11-11 22:07       ` [PATCH v2 00/11] Turn git-bisect to be builtin Taylor Blau
2022-11-15 19:18         ` Taylor Blau
2022-11-15 19:36           ` Jeff King
2022-11-15 19:40             ` Taylor Blau
2022-11-11 12:32     ` [PATCH v3 0/3] Convert git-bisect--helper to OPT_SUBCOMMAND Ævar Arnfjörð Bjarmason
2022-11-04 13:22 ` [PATCH 00/13] bisect: v2.30.0 "run" regressions + make it built-in Ævar Arnfjörð Bjarmason
2022-11-04 13:22   ` [PATCH 01/13] bisect tests: test for v2.30.0 "bisect run" regressions Ævar Arnfjörð Bjarmason
2022-11-04 13:22   ` [PATCH 02/13] bisect: refactor bisect_run() to match CodingGuidelines Ævar Arnfjörð Bjarmason
2022-11-04 13:22   ` [PATCH 03/13] bisect: fix output regressions in v2.30.0 Ævar Arnfjörð Bjarmason
2022-11-04 13:22   ` [PATCH 04/13] bisect run: fix "--log" eating regression " Ævar Arnfjörð Bjarmason
2022-11-04 13:22   ` [PATCH 05/13] bisect run: keep some of the post-v2.30.0 output Ævar Arnfjörð Bjarmason
2022-11-04 13:22   ` [PATCH 06/13] bisect test: test exit codes on bad usage Ævar Arnfjörð Bjarmason
2022-11-04 13:22   ` [PATCH 07/13] bisect--helper: emit usage for "git bisect" Ævar Arnfjörð Bjarmason
2022-11-04 13:22   ` [PATCH 08/13] bisect--helper: have all functions take state, argc, argv, prefix Ævar Arnfjörð Bjarmason
2022-11-04 13:22   ` [PATCH 09/13] parse-options API: don't restrict OPT_SUBCOMMAND() to one *_fn type Ævar Arnfjörð Bjarmason
2022-11-05  8:32     ` René Scharfe
2022-11-05 11:34       ` Đoàn Trần Công Danh
2022-11-05 21:32         ` Phillip Wood
2022-11-05 13:52       ` Ævar Arnfjörð Bjarmason
2022-11-05 16:36         ` Phillip Wood [this message]
2022-11-05 21:59           ` Ævar Arnfjörð Bjarmason
2022-11-05 17:26         ` René Scharfe
2022-11-05 22:33           ` Ævar Arnfjörð Bjarmason
2022-11-06  8:25             ` René Scharfe
2022-11-06 13:28               ` Ævar Arnfjörð Bjarmason
2022-11-12 10:42                 ` René Scharfe
2022-11-12 16:34                   ` Jeff King
2022-11-12 16:55                     ` Ævar Arnfjörð Bjarmason
2022-11-13 17:31                       ` René Scharfe
2022-11-04 13:22   ` [PATCH 10/13] bisect--helper: remove dead --bisect-{next-check,autostart} code Ævar Arnfjörð Bjarmason
2022-11-04 13:22   ` [PATCH 11/13] bisect--helper: convert to OPT_SUBCOMMAND_CB() Ævar Arnfjörð Bjarmason
2022-11-04 13:22   ` [PATCH 12/13] bisect--helper: make `state` optional Ævar Arnfjörð Bjarmason
2022-11-04 13:22   ` [PATCH 13/13] Turn `git bisect` into a full built-in Ævar Arnfjörð Bjarmason
2022-11-05  0:13   ` [PATCH 00/13] bisect: v2.30.0 "run" regressions + make it built-in Taylor Blau
2022-11-10 12:50   ` Johannes Schindelin

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