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

[Joe Perches]

On Wed, 2018-12-19 at 16:32 -0500, Bryana Rostedt wrote:
> 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)					\e
> +	({								\
> +		int ____strcmp_prefix_ret____;				\
> +		char *____strcmp_prefix____ = prefix;			\

This creates a warning and discards any const from prefix when used like

	static const char foo[] = "bar";

	strncmp_prefix(str, foo);

> +		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____;				\
> +	})
> +

Perhaps

#define strncmp_prefix(str, prefix)					\
({									\
	typeof(prefix) p = (prefix);					\
	strncmp(str, p, strlen(p));					\
})