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>
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