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Thu, 26 Aug 2021 11:02:50 -0700 (PDT) MIME-Version: 1.0 References: <20210822075122.864511-1-keescook@chromium.org> <20210822075122.864511-20-keescook@chromium.org> <202108251950.61F7A4CD@keescook> In-Reply-To: <202108251950.61F7A4CD@keescook> From: Nick Desaulniers Date: Thu, 26 Aug 2021 11:02:39 -0700 Message-ID: Subject: Re: [PATCH for-next 19/25] fortify: Allow strlen() and strnlen() to pass compile-time known lengths To: Kees Cook Cc: linux-kernel@vger.kernel.org, Rasmus Villemoes , Daniel Micay , Francis Laniel , Bart Van Assche , David Gow , linux-mm@kvack.org, clang-built-linux@googlegroups.com, linux-hardening@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Authentication-Results: imf06.hostedemail.com; dkim=pass header.d=google.com header.s=20161025 header.b=B2FAnkUl; spf=pass (imf06.hostedemail.com: domain of ndesaulniers@google.com designates 209.85.167.54 as permitted sender) smtp.mailfrom=ndesaulniers@google.com; dmarc=pass (policy=reject) header.from=google.com X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: B6C0C801AB03 X-Stat-Signature: aqrxdko7f6egti3dg9u5y7xsqzkq6kg8 X-HE-Tag: 1630000972-924307 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Aug 25, 2021 at 7:56 PM Kees Cook 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 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@chromium.org/ 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