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From: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
To: phillip.wood@dunelm.org.uk
Cc: "René Scharfe" <l.s.r@web.de>,
	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, 05 Nov 2022 22:59:07 +0100	[thread overview]
Message-ID: <221105.86sfixvxqk.gmgdl@evledraar.gmail.com> (raw)
In-Reply-To: <1b158749-44b1-34e0-5b52-1d3bfad9bc9a@dunelm.org.uk>


On Sat, Nov 05 2022, Phillip Wood wrote:

> 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?

I'm saying it bridges the gap in that explanation, i.e. reinforces that
the main standard body isn't referring to function pointer casts to void
* and intptr_t.

>> 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?

I'm just repeating the terminology the standard uses: "The following
extensions are widely used in many systems, but are not portable to all
implementations".

>> 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. 

I was saying, among other things, that it's standardized by POSIX, so
it's in the same category as ssize_t, not some hypothetical "happens to
work" undefined behavior.

But we also make use of it already, so that gives us some real-world
data on potential issues.

  reply	other threads:[~2022-11-05 22:34 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
2022-11-05 21:59           ` Ævar Arnfjörð Bjarmason [this message]
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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