On Wed, Aug 25, 2021 at 7:56 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Wed, Aug 25, 2021 at 03:05:56PM -0700, Nick Desaulniers wrote:
> > On Sun, Aug 22, 2021 at 12:57 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> > >
> > > diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
> > > index a3cb1d9aacce..e232a63fd826 100644
> > > --- a/include/linux/fortify-string.h
> > > +++ b/include/linux/fortify-string.h
> > > @@ -10,6 +10,18 @@ void __read_overflow(void) __compiletime_error("detected read beyond size of obj
> > > void __read_overflow2(void) __compiletime_error("detected read beyond size of object (2nd parameter)");
> > > void __write_overflow(void) __compiletime_error("detected write beyond size of object (1st parameter)");
> > >
> > > +#define __compiletime_strlen(p) ({ \
> > > + size_t ret = (size_t)-1; \
> > > + size_t p_size = __builtin_object_size(p, 1); \
> > > + if (p_size != (size_t)-1) { \
> > > + size_t p_len = p_size - 1; \
> > > + if (__builtin_constant_p(p[p_len]) && \
> > > + p[p_len] == '\0') \
> > > + ret = __builtin_strlen(p); \
> > > + } \
> > > + ret; \
> > > +})
> >
> > Can this be a `static inline` function that accepts a `const char *`
> > and returns a `size_t` rather than a statement expression?
>
> No because both __builtin_object_size() and __builtin_strlen() may not
> work. See:
> https://lore.kernel.org/lkml/20210818060533.3569517-64-keescook@xxxxxxxxxxxx/
Ah right, then consider adding a comment to encourage others not to
rewrite it as such.
>
> Regardless, it will always collapse to a const value of either -1 or
> the length of the string.
--
Thanks,
~Nick Desaulniers
