From: Phillip Wood Date: Sat, 05 Nov 2022 16:36:19 GMT Subject: Re: [PATCH 09/13] parse-options API: don't restrict OPT_SUBCOMMAND() to one *_fn type Message-ID: <1b158749-44b1-34e0-5b52-1d3bfad9bc9a@dunelm.org.uk> 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