Messages in this thread | | | Date | Fri, 21 Dec 2018 15:55:13 -0500 | From | Steven Rostedt <> | Subject | Re: [for-next][PATCH 23/24] string.h: Add strncmp_prefix() helper macro |
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On Fri, 21 Dec 2018 12:41:17 -0800 Linus Torvalds <torvalds@linux-foundation.org> wrote:
> Parentheses....
?
-- Steve
> > On Fri, Dec 21, 2018, 12:35 Steven Rostedt <rostedt@goodmis.org wrote: > > > On Fri, 21 Dec 2018 12:01:48 -0800 > > Linus Torvalds <torvalds@linux-foundation.org> wrote: > > > > > On Fri, Dec 21, 2018 at 11:40 AM Steven Rostedt <rostedt@goodmis.org> > > wrote: > > > > > > > > OK, what about if we just use strlen() and say that this macro is not > > > > safe for parameters with side effects. > > > > > > I think gcc follows simple assignments just fine, and does the > > > optimized strlen() for them too. > > > > > > So why not just do > > > > > > #define have_prefix(str,prefix) ({ \ > > > const char *__pfx = prefix; \ > > > size_t __pfxlen = strlen(__pfx); \ > > > strncmp(str, __pfx, __pfxlen) ? 0 : __pfxlen); }) > > > > > > and be done with it safely? > > > > > > The above is ENTIRELY untested. > > > > > > > At first I thought this would have issues, but with a slight change... > > > > #define have_prefix(str, prefix) ({ \ > > const char *__pfx = (const char *)prefix; \ > > > > > > And the rest the same, it appears to work. > > > > Need the cast because if for some reason someone passed in something > > like "const unsigned char" then it wouldn't work. But that's just a nit. > > > > So something like this then? > > > > -- Steve > > > > diff --git a/include/linux/string.h b/include/linux/string.h > > index 27d0482e5e05..4586fee60194 100644 > > --- a/include/linux/string.h > > +++ b/include/linux/string.h > > @@ -14,6 +14,28 @@ 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); > > > > +/** > > + * have_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 has_prefix(). > > + * > > + * Returns: 0 if @str does not start with @prefix > > + strlen(@prefix) if @str does start with @prefix > > + */ > > +#define has_prefix(str, prefix) > > \ > > + ({ \ > > + const char *____prefix____ = (const char *)(prefix); \ > > + int ____len____ = strlen(____prefix____); \ > > + strncmp(str, ____prefix____, ____len____) == 0 ? \ > > + ____len____ : 0; \ > > + }) > > + > > /* > > * Include machine specific inline routines > > */ > >
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