linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Kees Cook <keescook@chromium.org>
To: "Gustavo A . R . Silva" <gustavoars@kernel.org>
Cc: Kees Cook <keescook@chromium.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Keith Busch <kbusch@kernel.org>,
	linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: [PATCH] overflow: Provide constant expression struct_size
Date: Wed,  9 Feb 2022 16:43:26 -0800	[thread overview]
Message-ID: <20220210004326.776574-1-keescook@chromium.org> (raw)

There have been cases where struct_size() (or flex_array_size()) needs
to be calculated for an initializer, which requires it be a constant
expression. This is possible when the "count" argument is a constant
expression, so provide this ability for the helpers.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/linux/overflow.h | 10 +++++++---
 lib/test_overflow.c      |  7 +++++++
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 59d7228104d0..f1221d11f8e5 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -4,6 +4,7 @@
 
 #include <linux/compiler.h>
 #include <linux/limits.h>
+#include <linux/const.h>
 
 /*
  * We need to compute the minimum and maximum values representable in a given
@@ -221,8 +222,9 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
  * Return: number of bytes needed or SIZE_MAX on overflow.
  */
 #define flex_array_size(p, member, count)				\
-	size_mul(count,							\
-		 sizeof(*(p)->member) + __must_be_array((p)->member))
+	__builtin_choose_expr(__is_constexpr(count),			\
+		(count) * sizeof(*(p)->member) + __must_be_array((p)->member),	\
+		size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member)))
 
 /**
  * struct_size() - Calculate size of structure with trailing flexible array.
@@ -237,6 +239,8 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
  * Return: number of bytes needed or SIZE_MAX on overflow.
  */
 #define struct_size(p, member, count)					\
-	size_add(sizeof(*(p)), flex_array_size(p, member, count))
+	__builtin_choose_expr(__is_constexpr(count),			\
+		sizeof(*(p)) + flex_array_size(p, member, count),	\
+		size_add(sizeof(*(p)), flex_array_size(p, member, count)))
 
 #endif /* __LINUX_OVERFLOW_H */
diff --git a/lib/test_overflow.c b/lib/test_overflow.c
index 712fb2351c27..d488ffaf56be 100644
--- a/lib/test_overflow.c
+++ b/lib/test_overflow.c
@@ -602,11 +602,18 @@ struct __test_flex_array {
 
 static int __init test_overflow_size_helpers(void)
 {
+	/* Make sure struct_size() can be used in a constant expression. */
+	u8 ce_array[struct_size((struct __test_flex_array *)0, data, 55)];
 	struct __test_flex_array *obj;
 	int count = 0;
 	int err = 0;
 	int var;
 
+	/* Verify constant expression against runtime version. */
+	var = 55;
+	OPTIMIZER_HIDE_VAR(var);
+	err |= sizeof(ce_array) != struct_size(obj, data, var);
+
 #define check_one_size_helper(expected, func, args...)	({	\
 	bool __failure = false;					\
 	size_t _r;						\
-- 
2.30.2


             reply	other threads:[~2022-02-10  1:58 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-02-10  0:43 Kees Cook [this message]
2022-02-10  1:04 ` [PATCH] overflow: Provide constant expression struct_size Gustavo A. R. Silva

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20220210004326.776574-1-keescook@chromium.org \
    --to=keescook@chromium.org \
    --cc=gustavoars@kernel.org \
    --cc=kbusch@kernel.org \
    --cc=linux-hardening@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux@rasmusvillemoes.dk \
    --cc=nathan@kernel.org \
    --cc=ndesaulniers@google.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).