Re: [for-next][PATCH 23/24] string.h: Add strncmp_prefix() helper macro

Steven Rostedt
2018-12-21T20:55:18Z

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
> > */
> >