From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Google-Smtp-Source: AIpwx4+n93SS0ALj+ey2jxJoykrpm5Uv1eqh0/FBpw5jtIclMg5gIA5LkFa7U3lRiXTjP1JGIDfB ARC-Seal: i=1; a=rsa-sha256; t=1522461179; cv=none; d=google.com; s=arc-20160816; b=v7DDrN3EA3aUDJ8e3wB2VvnSKEdQj2Sqcg9J/xP/K+LvzH4l9HK7V6VZfuNagTl6cJ u35OrtiJuxWIsOxF1AVKYS1ytr/yBgH9R7k8mUNeClBbZsYSfrFabDbbC52RhXvs5AlQ S1gCe4nPcYptLLuJWiZV4ZWe4YQdT7vWY69C9cxSl1q6yucmKEAaB9jYeutkW2DpoQSK 6GsIjB+CtXiTqoLxPClTFJw9t6+CTIk7tg+jWYyM6YmeO8Gi/Cq4sXuRj3AMlQKv9iTJ nGFOeo94ViempdNZyuHCB8XfY4/MCs7MSA483Sq6DUy7dwTYZV2UZpyMSZLkhTIv5xF0 9agw== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=content-transfer-encoding:content-disposition:mime-version :message-id:subject:cc:to:from:date:dkim-signature:delivered-to :list-id:list-subscribe:list-unsubscribe:list-help:list-post :precedence:mailing-list:arc-authentication-results; bh=pU4bnVT94aB2863bO/FEp94lh2qIMfiybQq0IjDRgr4=; b=ZFEB3wJeD6Ybz01pNq7DBgFl6uSfoMIlfHgiy0mjE3WEOGIc+WjpPuUOoEKAcGUBT3 QKd8y57OaNRQ8As/8VLn2R4RNcLOKIbOpdFwW9dj3WmM07ew9VTS++OqMOl+UFySvp5z xFh4Tai1d6fufEKg18IRjbY5lqHGeBdeHaEMMDxdjPC+OSfJVYcUL2vYLbBnr3RunL7n APm3emxoAZjXSViIkxhNE3tetMUS3UrXUL7YQQhpu4i2JpJCeOheyQa4B5ewdo/qQsiE 7Rr7CpodY6G/hTb6HMldNWqHaHMKFfGKytaDQ6QYrVIiKjLCjCX1B7jZBp/G+4eEZj+k URZg== ARC-Authentication-Results: i=1; mx.google.com; dkim=pass header.i=@chromium.org header.s=google header.b=aIJfVgkh; spf=pass (google.com: domain of kernel-hardening-return-12836-gregkh=linuxfoundation.org@lists.openwall.com designates 195.42.179.200 as permitted sender) smtp.mailfrom=kernel-hardening-return-12836-gregkh=linuxfoundation.org@lists.openwall.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=chromium.org Authentication-Results: mx.google.com; dkim=pass header.i=@chromium.org header.s=google header.b=aIJfVgkh; spf=pass (google.com: domain of kernel-hardening-return-12836-gregkh=linuxfoundation.org@lists.openwall.com designates 195.42.179.200 as permitted sender) smtp.mailfrom=kernel-hardening-return-12836-gregkh=linuxfoundation.org@lists.openwall.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=chromium.org Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm List-Post: List-Help: List-Unsubscribe: List-Subscribe: Date: Fri, 30 Mar 2018 18:52:36 -0700 From: Kees Cook To: Andrew Morton Cc: Linus Torvalds , Martin Uecker , Josh Poimboeuf , Rasmus Villemoes , Randy Dunlap , Miguel Ojeda , Ingo Molnar , David Laight , Ian Abbott , linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com Subject: [PATCH v7] kernel.h: Retain constant expression output for max()/min() Message-ID: <20180331015236.GA3852@beast> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: =?utf-8?q?1596416253887758070?= X-GMAIL-MSGID: =?utf-8?q?1596416253887758070?= X-Mailing-List: linux-kernel@vger.kernel.org List-ID: In the effort to remove all VLAs from the kernel[1], it is desirable to build with -Wvla. However, this warning is overly pessimistic, in that it is only happy with stack array sizes that are declared as constant expressions, and not constant values. One case of this is the evaluation of the max() macro which, due to its construction, ends up converting constant expression arguments into a constant value result. All attempts to rewrite this macro with __builtin_constant_p() failed with older compilers (e.g. gcc 4.4)[2]. However, Martin Uecker, constructed[3] a mind-shattering solution that works everywhere. Cthulhu fhtagn! This patch updates the min()/max() macros to evaluate to a constant expression when called on constant expression arguments. This removes several false-positive stack VLA warnings from an x86 allmodconfig build when -Wvla is added: $ diff -u before.txt after.txt | grep ^- -drivers/input/touchscreen/cyttsp4_core.c:871:2: warning: ISO C90 forbids variable length array ‘ids’ [-Wvla] -fs/btrfs/tree-checker.c:344:4: warning: ISO C90 forbids variable length array ‘namebuf’ [-Wvla] -lib/vsprintf.c:747:2: warning: ISO C90 forbids variable length array ‘sym’ [-Wvla] -net/ipv4/proc.c:403:2: warning: ISO C90 forbids variable length array ‘buff’ [-Wvla] -net/ipv6/proc.c:198:2: warning: ISO C90 forbids variable length array ‘buff’ [-Wvla] -net/ipv6/proc.c:218:2: warning: ISO C90 forbids variable length array ‘buff64’ [-Wvla] This also updates the one case where different enums were being compared and explicitly casts them to int (which matches the old side-effect of the single-evaluation code). [1] https://lkml.org/lkml/2018/3/7/621 [2] https://lkml.org/lkml/2018/3/10/170 [3] https://lkml.org/lkml/2018/3/20/845 Co-Developed-by: Linus Torvalds Co-Developed-by: Martin Uecker Signed-off-by: Kees Cook Acked-by: Ingo Molnar Acked-by: Miguel Ojeda --- v7: - __is_constant() renamed to __is_constexpr() (Miguel Ojeda) - adjust memory offset from 1 to 8 (David Laight) - min_t()/max_t() "t" renamed back to "type" (0-day bot) - add Acks --- drivers/char/tpm/tpm_tis_core.h | 8 ++--- include/linux/kernel.h | 71 ++++++++++++++++++++++++----------------- 2 files changed, 45 insertions(+), 34 deletions(-) diff --git a/drivers/char/tpm/tpm_tis_core.h b/drivers/char/tpm/tpm_tis_core.h index d5c6a2e952b3..f6e1dbe212a7 100644 --- a/drivers/char/tpm/tpm_tis_core.h +++ b/drivers/char/tpm/tpm_tis_core.h @@ -62,10 +62,10 @@ enum tis_defaults { /* Some timeout values are needed before it is known whether the chip is * TPM 1.0 or TPM 2.0. */ -#define TIS_TIMEOUT_A_MAX max(TIS_SHORT_TIMEOUT, TPM2_TIMEOUT_A) -#define TIS_TIMEOUT_B_MAX max(TIS_LONG_TIMEOUT, TPM2_TIMEOUT_B) -#define TIS_TIMEOUT_C_MAX max(TIS_SHORT_TIMEOUT, TPM2_TIMEOUT_C) -#define TIS_TIMEOUT_D_MAX max(TIS_SHORT_TIMEOUT, TPM2_TIMEOUT_D) +#define TIS_TIMEOUT_A_MAX max_t(int, TIS_SHORT_TIMEOUT, TPM2_TIMEOUT_A) +#define TIS_TIMEOUT_B_MAX max_t(int, TIS_LONG_TIMEOUT, TPM2_TIMEOUT_B) +#define TIS_TIMEOUT_C_MAX max_t(int, TIS_SHORT_TIMEOUT, TPM2_TIMEOUT_C) +#define TIS_TIMEOUT_D_MAX max_t(int, TIS_SHORT_TIMEOUT, TPM2_TIMEOUT_D) #define TPM_ACCESS(l) (0x0000 | ((l) << 12)) #define TPM_INT_ENABLE(l) (0x0008 | ((l) << 12)) diff --git a/include/linux/kernel.h b/include/linux/kernel.h index 3fd291503576..87acb8b58ae9 100644 --- a/include/linux/kernel.h +++ b/include/linux/kernel.h @@ -783,41 +783,58 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { } #endif /* CONFIG_TRACING */ /* - * min()/max()/clamp() macros that also do - * strict type-checking.. See the - * "unnecessary" pointer comparison. + * min()/max()/clamp() macros must accomplish three things: + * + * - avoid multiple evaluations of the arguments (so side-effects like + * "x++" happen only once) when non-constant. + * - perform strict type-checking (to generate warnings instead of + * nasty runtime surprises). See the "unnecessary" pointer comparison + * in __typecheck(). + * - retain result as a constant expressions when called with only + * constant expressions (to avoid tripping VLA warnings in stack + * allocation usage). + */ +#define __typecheck(x, y) \ + (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1))) + +/* + * This returns a constant expression while determining if an argument is + * a constant expression, most importantly without evaluating the argument. + * Glory to Martin Uecker */ -#define __min(t1, t2, min1, min2, x, y) ({ \ - t1 min1 = (x); \ - t2 min2 = (y); \ - (void) (&min1 == &min2); \ - min1 < min2 ? min1 : min2; }) +#define __is_constexpr(x) \ + (sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8))) + +#define __no_side_effects(x, y) \ + (__is_constexpr(x) && __is_constexpr(y)) + +#define __safe_cmp(x, y) \ + (__typecheck(x, y) && __no_side_effects(x, y)) + +#define __cmp(x, y, op) ((x) op (y) ? (x) : (y)) + +#define __cmp_once(x, y, op) ({ \ + typeof(x) __x = (x); \ + typeof(y) __y = (y); \ + __cmp(__x, __y, op); }) + +#define __careful_cmp(x, y, op) \ + __builtin_choose_expr(__safe_cmp(x, y), \ + __cmp(x, y, op), __cmp_once(x, y, op)) /** * min - return minimum of two values of the same or compatible types * @x: first value * @y: second value */ -#define min(x, y) \ - __min(typeof(x), typeof(y), \ - __UNIQUE_ID(min1_), __UNIQUE_ID(min2_), \ - x, y) - -#define __max(t1, t2, max1, max2, x, y) ({ \ - t1 max1 = (x); \ - t2 max2 = (y); \ - (void) (&max1 == &max2); \ - max1 > max2 ? max1 : max2; }) +#define min(x, y) __careful_cmp(x, y, <) /** * max - return maximum of two values of the same or compatible types * @x: first value * @y: second value */ -#define max(x, y) \ - __max(typeof(x), typeof(y), \ - __UNIQUE_ID(max1_), __UNIQUE_ID(max2_), \ - x, y) +#define max(x, y) __careful_cmp(x, y, >) /** * min3 - return minimum of three values @@ -869,10 +886,7 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { } * @x: first value * @y: second value */ -#define min_t(type, x, y) \ - __min(type, type, \ - __UNIQUE_ID(min1_), __UNIQUE_ID(min2_), \ - x, y) +#define min_t(type, x, y) __careful_cmp((type)(x), (type)(y), <) /** * max_t - return maximum of two values, using the specified type @@ -880,10 +894,7 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { } * @x: first value * @y: second value */ -#define max_t(type, x, y) \ - __max(type, type, \ - __UNIQUE_ID(min1_), __UNIQUE_ID(min2_), \ - x, y) +#define max_t(type, x, y) __careful_cmp((type)(x), (type)(y), >) /** * clamp_t - return a value clamped to a given range using a given type -- 2.7.4 -- Kees Cook Pixel Security