Seems that I sent this as my daughter. I'm not home now but will resend again as myself when I am.
-- Steve On December 19, 2018 4:54:57 PM EST, Joe Perches <j...@perches.com> wrote: >On Wed, 2018-12-19 at 16:32 -0500, Bryana Rostedt wrote: >> 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: >> >> #define strncmp_const(str, const) \ >> 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. >> >> I plan on only applying this patch and updating the tracing hooks for >> this merge window. And perhaps use it to fix some of the bugs that >were >> found. >> >> I was going to just use: >> >> #define strncmp_prefix(str, prefix) \ >> strncmp(str, prefix, strlen(prefix)) >> >> but then realized that "prefix" is used twice, and will break if >> someone does something like: >> >> strncmp_prefix(str, ptr++); >> >> So instead I check with __builtin_constant_p() to see if the second >> parameter is truly a constant, which I use sizeof() anyway (why >bother >> gcc to optimize it, if we already know it's a constant), and copy the >> parameter into a local variable and use that local variable for the >non >> constant part (with strlen). >> >> Link: >http://lkml.kernel.org/r/e3e754f2bd18e56eaa8baf79bee619316ebf4cfc.1545161087.git.tom.zanu...@linux.intel.com >> >> Signed-off-by: Steven Rostedt (VMware) <rost...@goodmis.org> >> --- >> diff --git a/include/linux/string.h b/include/linux/string.h >> index 27d0482e5e05..3dc743e3a0ba 100644 >> --- a/include/linux/string.h >> +++ b/include/linux/string.h >> @@ -14,6 +14,27 @@ extern void *memdup_user(const void __user *, >size_t); >> extern void *vmemdup_user(const void __user *, size_t); >> extern void *memdup_user_nul(const void __user *, size_t); >> >> +/* >> + * 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 strncmp_prefix(). >> + */ >> +#define strncmp_prefix(str, prefix) \e >> + ({ \ >> + int ____strcmp_prefix_ret____; \ >> + char *____strcmp_prefix____ = prefix; \ > >This creates a warning and discards any const from prefix when used >like > > static const char foo[] = "bar"; > > strncmp_prefix(str, foo); > >> + if (__builtin_constant_p(&prefix)) \ >> + ____strcmp_prefix_ret____ = \ >> + strncmp(str, prefix, sizeof(prefix) - 1); \ >> + else \ >> + ____strcmp_prefix_ret____ = \ >> + strncmp(str, ____strcmp_prefix____, \ >> + strlen(____strcmp_prefix____)); \ >> + ____strcmp_prefix_ret____; \ >> + }) >> + > >Perhaps > >#define strncmp_prefix(str, prefix) \ >({ \ > typeof(prefix) p = (prefix); \ > strncmp(str, p, strlen(p)); \ >}) -- Sent from my Android device with K-9 Mail. Please excuse my brevity and top posting.