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

From: Linus Torvalds
Date: Fri Dec 21 2018 - 17:58:49 EST

On Fri, Dec 21, 2018 at 2:48 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> > Your patch actually had them, but in the body of your email you did
> >
> > > #define have_prefix(str, prefix) ({ \
> > > const char *__pfx = (const char *)prefix; \
> >
> > which is just completely wrong.
> >
> > Considering your _old_ patch had the exact same bug, I really think
> > you need to start internalizing the whole "macro arguments *have* to
> > be properly protected" thing.
> Well, there's less with assignments that can go wrong than with other
> code. That is, there's little that can happen with "int x = arg;" where
> arg is the macro paramater to cause something really nasty.

What's wrong, Steven?

The assignment is entirely irrelevant.

The problem is the cast.

A type cast has a very high priority, and so if you do

(const char *)prefix

it breaks completely if you might have something like"a+6" as the argument.

Think about what if "a" is of type "unsigned long", for example?