4.19-stable review patch. If anyone has any objections, please let me know.
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From: "Steven Rostedt (VMware)" rostedt@goodmis.org
commit 72921427d46bf9731a1ab7864adc64c43dfae29f upstream.
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... Link: http://lkml.kernel.org/r/20181219211615.2298e781@gandalf.local.home Link: http://lkml.kernel.org/r/CAHk-=wg_sR-UEC1ggmkZpypOUYanL5CMX4R7ceuaV4QMf5jBtg...
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 Signed-off-by: George Guo guodongtai@kylinos.cn Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- include/linux/string.h | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+)
--- a/include/linux/string.h +++ b/include/linux/string.h @@ -492,4 +492,24 @@ static inline void memcpy_and_pad(void * 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_ */