linux-kernel - RE: [PATCH v3] tools/nolibc: fix up size inflate regression

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Message-ID: <f51e54bcf470451ea36f24640f000e61@AcuMS.aculab.com>
Date:   Mon, 7 Aug 2023 08:40:27 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Zhangjin Wu' <falcon@...ylab.org>, "w@....eu" <w@....eu>
CC:     "arnd@...db.de" <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "thomas@...ch.de" <thomas@...ch.de>
Subject: RE: [PATCH v3] tools/nolibc: fix up size inflate regression

From: Zhangjin Wu
> Sent: 07 August 2023 06:58
...
> +/* __auto_type is used instead of __typeof__ to workaround the build error
> + * 'error: assignment of read-only variable' when the argument has 'const' in
> + * the type, but __auto_type is a new feature from newer gcc version and it
> + * only works with 'const' from gcc 11.0 (__GXX_ABI_VERSION = 1016)
> + * https://gcc.gnu.org/legacy-ml/gcc-patches/2013-11/msg01378.html
> + */

You can use typeof((x) + 0) to lose the 'const' flag.
The only downside is that char/short become int.

> +
> +#if __GXX_ABI_VERSION >= 1016
> +#define __GXX_HAS_AUTO_TYPE_WITH_CONST_SUPPORT
> +#endif
> +
> +#ifdef __GXX_HAS_AUTO_TYPE_WITH_CONST_SUPPORT
> +#define __sysret(arg)                                                    \
> +({                                                                       \
> +	__auto_type __ret = (arg);                                       \
> +	if (__is_signed_type(__typeof__(arg))) {                         \
> +		if (__ret < 0) {                                         \
> +			SET_ERRNO(-(long)__ret);                         \
> +			__ret = (__typeof__(arg))(-1L);                  \
> +		}                                                        \
> +	} else {                                                         \
> +		if ((unsigned long)__ret >= (unsigned long)-MAX_ERRNO) { \
> +			SET_ERRNO(-(long)__ret);                         \
> +			__ret = (__typeof__(arg))(-1L);                  \
> +		}                                                        \
> +	}                                                                \
> +	__ret;                                                           \
> +})
> +
> +#else  /* ! __GXX_HAS_AUTO_TYPE_WITH_CONST_SUPPORT */
> +#define __sysret(arg)                                                    \
> +({                                                                       \
> +	long __ret = (long)(arg);                                        \
> +	if (__is_signed_type(__typeof__(arg))) {                         \
> +		if (__ret < 0) {                                         \
> +			SET_ERRNO(-__ret);                               \
> +			__ret = -1L;                                     \
> +		}                                                        \
> +	} else {                                                         \
> +		if ((unsigned long)__ret >= (unsigned long)-MAX_ERRNO) { \
> +			SET_ERRNO(-__ret);                               \
> +			__ret = -1L;                                     \
> +		}                                                        \
> +	}                                                                \
> +	(__typeof__(arg))__ret;                                          \
> +})
> +#endif /* ! __GXX_HAS_AUTO_TYPE_WITH_CONST_SUPPORT */

with (retyped so it may be wrong):
#define is_constexpr(x) sizeof(*(0 ? (void *)((long)(x) * 0) : (int *)0)) == 1)
and (because even (void *)0 isn't completely constant):
#define is_pointer(x) (!is_constexpr((typeof(x))0))

You can probably do:
#define __sysret(arg) \
({ \
	typeof((arg) + 0) __ret = arg; \
	if (__built_choose_expr(is_pointer(arg), (unsigned long)-(MAX_ERRNO+1), __ret) \
			< (__built_choose_expr(is_pointer(arg), (unsigned long)__ret, 0)) { \
		SET_ERRNO(-__ret); \
		__reg = typeof(ret)-1L; \
	} \
	__ret; \
})

Apart from the annoyance of having to reverse the conditional
that only has one copy of the check.

Using two __builtin_choose_expr() saves you having to write two
comparisons that are valid for both pointer and integer.

	David

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