linux-kernel - [PATCH v2] add -Wpointer-arith sparse flag to toggle sizeof(void) warnings

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Date:   Sat, 7 Apr 2018 11:58:05 -1000
From:   Joey Pabalinas <joeypabalinas@...il.com>
To:     linux-sparse@...r.kernel.org
Cc:     Kees Cook <keescook@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Martin Uecker <Martin.Uecker@....uni-goettingen.de>,
        Al Viro <viro@...IV.linux.org.uk>,
        Christopher Li <sparse@...isli.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Joey Pabalinas <joeypabalinas@...il.com>
Subject: [PATCH v2] add -Wpointer-arith sparse flag to toggle sizeof(void)
 warnings

Recent changes to the min()/max() macros in include/linux/kernel.h
have added a lot of noise when compiling the kernel with Sparse checking
enabled. This mostly is due to the *huge* increase in the number of
sizeof(void) warnings, a larger number of which can safely be ignored.

Add the -Wpointer-arith flag to enable/disable these warnings (along
with the warning when applying sizeof to function types exactly like the
GCC -Wpointer-arith flag) on demand; the warning itself has been disabled
by default to reduce the large influx of noise which was inadvertently
added by commit 3c8ba0d61d04ced9f8 (kernel.h: Retain constant expression
output for max()/min()).

Update the manpage to document the new flag.

CC: Kees Cook <keescook@...omium.org>
CC: Linus Torvalds <torvalds@...ux-foundation.org>
CC: Martin Uecker <Martin.Uecker@....uni-goettingen.de>
CC: Al Viro <viro@...IV.linux.org.uk>
CC: Christopher Li <sparse@...isli.org>
CC: Joey Pabalinas <joeypabalinas@...il.com>
CC: Luc Van Oostenryck <luc.vanoostenryck@...il.com>
Signed-off-by: Joey Pabalinas <joeypabalinas@...il.com>
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@...il.com>

 4 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/evaluate.c b/evaluate.c
index 4e1dffe9c5416380df..f828da37df8e686623 100644
--- a/evaluate.c
+++ b/evaluate.c
@@ -2193,7 +2193,8 @@ static struct symbol *evaluate_sizeof(struct expression *expr)
 	size = type->bit_size;
 
 	if (size < 0 && is_void_type(type)) {
-		warning(expr->pos, "expression using sizeof(void)");
+		if (Wpointer_arith)
+			warning(expr->pos, "expression using sizeof(void)");
 		size = bits_in_char;
 	}
 
@@ -2204,7 +2205,8 @@ static struct symbol *evaluate_sizeof(struct expression *expr)
 	}
 
 	if (is_function(type->ctype.base_type)) {
-		warning(expr->pos, "expression using sizeof on a function");
+		if (Wpointer_arith)
+			warning(expr->pos, "expression using sizeof on a function");
 		size = bits_in_char;
 	}
 
diff --git a/lib.c b/lib.c
index 73d372c36626538bab..f7fdac96674aec4c24 100644
--- a/lib.c
+++ b/lib.c
@@ -242,6 +242,7 @@ int Woverride_init = 1;
 int Woverride_init_all = 0;
 int Woverride_init_whole_range = 0;
 int Wparen_string = 0;
+int Wpointer_arith = 0;
 int Wptr_subtraction_blows = 0;
 int Wreturn_void = 0;
 int Wshadow = 0;
@@ -654,6 +655,7 @@ static const struct flag warnings[] = {
 	{ "return-void", &Wreturn_void },
 	{ "shadow", &Wshadow },
 	{ "sizeof-bool", &Wsizeof_bool },
+	{ "pointer-arith", &Wpointer_arith },
 	{ "sparse-error", &Wsparse_error },
 	{ "tautological-compare", &Wtautological_compare },
 	{ "transparent-union", &Wtransparent_union },
diff --git a/lib.h b/lib.h
index 3050b5577ba4d42e97..e34bb9a02ebac03f52 100644
--- a/lib.h
+++ b/lib.h
@@ -151,6 +151,7 @@ extern int Woverride_init;
 extern int Woverride_init_all;
 extern int Woverride_init_whole_range;
 extern int Wparen_string;
+extern int Wpointer_arith;
 extern int Wptr_subtraction_blows;
 extern int Wreturn_void;
 extern int Wshadow;
diff --git a/sparse.1 b/sparse.1
index 88343f3170f195297a..4379406999c94b806e 100644
--- a/sparse.1
+++ b/sparse.1
@@ -288,6 +288,25 @@ Standard C syntax does not permit a parenthesized string as an array
 initializer.  GCC allows this syntax as an extension.  With
 \fB\-Wparen\-string\fR, Sparse will warn about this syntax.
 
+Sparse does not issue these warnings by default.
+.
+.TP
+.B \-Wpointer-arith
+Warn about anything that depends on the \fBsizeof\fR a function type or of void.
+
+C99 does not allow the \fBsizeof\fR operator to be applied to function types or to
+incomplete types such as void. GCC allows \fBsizeof\fR to be applied to these
+types as an extension and assigns these types a size of \fI1\fR.
+
+Although non-standard (and somewhat illogical), constructs such as \fBsizeof(void)\fR
+are often useful when the intent is to operate on an expression without evaluating
+it, e.g. in the following integer constant expression predicate:
+
+.nf
+#define __is_constexpr(x) \\
+	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
+.fi
+
 Sparse does not issue these warnings by default.
 .
 .TP
-- 
2.17.0.rc1.35.g90bbd502d54fe92035.dirty

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