[RFC][PATCH] string.h: Add strncmp_prefix() helper macro

[Bryana Rostedt]

A discussion came up in the trace triggers thread about converting a
bunch of:

 strncmp(str, "const", sizeof("const") - 1)

use cases into a helper macro. It started with:

#define strncmp_const(str, const) \
	strncmp(str, const, sizeof(const) - 1)

But then Joe Perches mentioned that if a const is not used, the
sizeof() will be the size of a pointer, which can be bad. And that
gcc will optimize strlen("const") into "sizeof("const") - 1".

Thinking about this more, a quick grep in the kernel tree found several
(thousands!) of cases that use this construct. A quick grep also
revealed that there's probably several bugs in that use case. Some are
that people forgot the "- 1" (which I found) and others could be that
the constant for the sizeof is different than the constant (although, I
haven't found any of those, but I also didn't look hard).

I figured the best thing to do is to create a helper macro and place it
into include/linux/string.h. And go around and fix all the open coded
versions of it later.

I plan on only applying this patch and updating the tracing hooks for
this merge window. And perhaps use it to fix some of the bugs that were
found.

I was going to just use:

#define strncmp_prefix(str, prefix) \
	strncmp(str, prefix, strlen(prefix))

but then realized that "prefix" is used twice, and will break if
someone does something like:

	strncmp_prefix(str, ptr++);

So instead I check with __builtin_constant_p() to see if the second
parameter is truly a constant, which I use sizeof() anyway (why bother
gcc to optimize it, if we already know it's a constant), and copy the
parameter into a local variable and use that local variable for the non
constant part (with strlen).

Link: http://lkml.kernel.org/r/e3e754f2bd18e56eaa8baf79bee619316ebf4cfc.1545161087.git.tom.zanussi@xxxxxxxxxxxxxxx

Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
diff --git a/include/linux/string.h b/include/linux/string.h
index 27d0482e5e05..3dc743e3a0ba 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -14,6 +14,27 @@ extern void *memdup_user(const void __user *, size_t);
 extern void *vmemdup_user(const void __user *, size_t);
 extern void *memdup_user_nul(const void __user *, size_t);
 
+/*
+ * A common way to test a prefix of a string is to do:
+ *  strncmp(str, prefix, sizeof(prefix) - 1)
+ *
+ * But this can lead to bugs due to typos, or if prefix is a pointer
+ * and not a constant. Instead use strncmp_prefix().
+ */
+#define strncmp_prefix(str, prefix)					\
+	({								\
+		int ____strcmp_prefix_ret____;				\
+		char *____strcmp_prefix____ = prefix;			\
+		if (__builtin_constant_p(&prefix))			\
+			____strcmp_prefix_ret____ =			\
+				strncmp(str, prefix, sizeof(prefix) - 1); \
+		else							\
+			____strcmp_prefix_ret____ =			\
+				strncmp(str, ____strcmp_prefix____,	\
+					strlen(____strcmp_prefix____));	\
+		____strcmp_prefix_ret____;				\
+	})
+
 /*
  * Include machine specific inline routines
  */