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Message-ID: <202209212034.16D9025882@keescook> Date: Wed, 21 Sep 2022 20:46:47 -0700 From: Kees Cook <keescook@...omium.org> To: Siddhesh Poyarekar <siddhesh@...plt.org> Cc: linux-hardening@...r.kernel.org, Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, Arnd Bergmann <arnd@...db.de>, Juergen Gross <jgross@...e.com>, Boris Ostrovsky <boris.ostrovsky@...cle.com>, Tom Rix <trix@...hat.com>, Miguel Ojeda <ojeda@...nel.org>, linux-kernel@...r.kernel.org, llvm@...ts.linux.dev Subject: Re: [PATCH 2/4] fortify: Explicitly check bounds are compile-time constants On Wed, Sep 21, 2022 at 07:48:44AM -0400, Siddhesh Poyarekar wrote: > On 2022-09-20 15:22, Kees Cook wrote: > > In preparation for replacing __builtin_object_size() with > > __builtin_dynamic_object_size(), all the compile-time size checks need > > to check that the bounds variables are, in fact, known at compile-time. > > Enforce what was guaranteed with __bos(). In other words, since all uses > > of __bos() were constant expressions, it was not required to test for > > this. When these change to __bdos(), they _may_ be constant expressions, > > and the checks are only valid when the prior condition holds. This > > results in no binary differences. > > > > Signed-off-by: Kees Cook <keescook@...omium.org> > > --- > > include/linux/fortify-string.h | 50 +++++++++++++++++++++------------- > > 1 file changed, 31 insertions(+), 19 deletions(-) > > > > diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h > > index ff879efe94ed..71c0a432c638 100644 > > --- a/include/linux/fortify-string.h > > +++ b/include/linux/fortify-string.h > > @@ -80,6 +80,12 @@ extern char *__underlying_strncpy(char *p, const char *q, __kernel_size_t size) > > #define POS __pass_object_size(1) > > #define POS0 __pass_object_size(0) > > +#define __compiletime_lessthan(bounds, length) ( \ > > + __builtin_constant_p(length) && \ > > + __builtin_constant_p(bounds) && \ > > + bounds < length \ > > +) > > So with the gcc ranger, the compiler has lately been quite successful at > computing a constant `bounds < length` even though bounds and length are not > constant. So perhaps this: > > #define __compiletime_lessthan (bounds, length) ( \ > __builtin_constant (bounds < length) && \ > bounds < length \ > ) > > might succeed in a few more cases. Oh, interesting! That's very cool -- I never considered tossing a full expression into __bcp. Yeah, that seems to work just fine: https://godbolt.org/z/xrchErEx1 -- Kees Cook
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