Re: [PATCH v4] string.h: Add str_has_prefix() helper function

[Namhyung Kim <namhyung@kernel.org>]

On Fri, Dec 21, 2018 at 11:19:24PM -0500, Steven Rostedt wrote:
> 
> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> 
> 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:
> 
> 	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.
> 
> Note, gcc appears to optimize this when we make it into an always_inline
> static function, which removes a lot of issues that a macro produces.
> 
> Link: http://lkml.kernel.org/r/e3e754f2bd18e56eaa8baf79bee619316ebf4cfc.1545161087.git.tom.zanussi@linux.intel.com
> Link: http://lkml.kernel.org/r/20181219211615.2298e781@gandalf.local.home
> Link: http://lkml.kernel.org/r/CAHk-=wg_sR-UEC1ggmkZpypOUYanL5CMX4R7ceuaV4QMf5jBtg@mail.gmail.com
> 
> Cc: Tom Zanussi <zanussi@kernel.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Suggestions-by: Joe Perches <joe@perches.com>
> Suggestions-by: Andreas Schwab <schwab@linux-m68k.org>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
> 
> Since I haven't heard anything since Linus said he preferred the len
> to be the return value, I'm posting this last version before running
> it through my tests.
> 
> Changes since v3:
> 
>   - Use size_t instead of int (Joe Perches)
> 
>  include/linux/string.h | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/include/linux/string.h b/include/linux/string.h
> index 27d0482e5e05..7927b875f80c 100644
> --- a/include/linux/string.h
> +++ b/include/linux/string.h
> @@ -456,4 +456,24 @@ static inline void memcpy_and_pad(void *dest, size_t dest_len,
>  		memcpy(dest, src, dest_len);
>  }
>  
> +/**
> + * str_has_prefix - Test if a string has a given prefix
> + * @str: The string to test
> + * @prefix: The string to see if @str starts with
> + *
> + * 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 str_has_prefix().
> + *
> + * Returns: 0 if @str does not start with @prefix
> +         strlen(@prefix) if @str does start with @prefix
> + */
> +static __always_inline size_t str_has_prefix(const char *str, const char *prefix)
> +{
> +	size_t len = strlen(prefix);
> +	return strncmp(str, prefix, len) == 0 ? len : 0;

As it already knows the length (and it needs to use it for return
value), isn't it (slightly) better using memcmp() instead?

Thanks,
Namhyung


> +}
> +
>  #endif /* _LINUX_STRING_H_ */
> -- 
> 2.19.1
>