From 72921427d46bf9731a1ab7864adc64c43dfae29f Mon Sep 17 00:00:00 2001 From: "Steven Rostedt (VMware)" Date: Fri, 21 Dec 2018 18:10:14 -0500 Subject: [PATCH] string.h: Add str_has_prefix() helper function 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 Cc: Greg Kroah-Hartman Acked-by: Namhyung Kim Suggestions-by: Linus Torvalds Suggestions-by: Joe Perches Suggestions-by: Andreas Schwab Signed-off-by: Steven Rostedt (VMware) --- include/linux/string.h | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/include/linux/string.h b/include/linux/string.h index 27d0482..7927b87 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; +} + #endif /* _LINUX_STRING_H_ */ -- 2.7.4