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authorSteven Rostedt (VMware) <rostedt@goodmis.org>2018-12-22 02:10:14 +0300
committerSteven Rostedt (VMware) <rostedt@goodmis.org>2018-12-23 06:50:29 +0300
commit72921427d46bf9731a1ab7864adc64c43dfae29f (patch)
treeb41bbec2205c7bc8d0d24f442352eafd9bcd1a4e /include/linux/string.h
parent1cce377df1800154301525a8ee04100dd83c9633 (diff)
downloadlinux-72921427d46bf9731a1ab7864adc64c43dfae29f.tar.xz
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 <zanussi@kernel.org> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Acked-by: Namhyung Kim <namhyung@kernel.org> Suggestions-by: Linus Torvalds <torvalds@linux-foundation.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>
Diffstat (limited to 'include/linux/string.h')
-rw-r--r--include/linux/string.h20
1 files changed, 20 insertions, 0 deletions
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;
+}
+
#endif /* _LINUX_STRING_H_ */