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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id 135sm17293101pfu.207.2020.03.24.11.18.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 24 Mar 2020 11:18:08 -0700 (PDT) Date: Tue, 24 Mar 2020 11:18:07 -0700 From: Kees Cook To: Josh Poimboeuf Cc: Peter Zijlstra , Randy Dunlap , Stephen Rothwell , Linux Next Mailing List , Linux Kernel Mailing List Subject: Re: linux-next: Tree for Mar 18 (objtool) Message-ID: <202003241105.4707F983@keescook> References: <20200318182352.2dgwwl4ugbwndi4x@treble> <20200318200542.GK20730@hirez.programming.kicks-ass.net> <20200319173101.wufpymi7obhqgoqd@treble> <20200319173326.oj4qs24x4ly5lrgt@treble> <20200319174028.azzaisoj5gbss7zk@treble> <20200319174550.4wpx4j357nw67nzz@treble> <20200320082613.GA20696@hirez.programming.kicks-ass.net> <202003201131.9B688BC@keescook> <20200324164433.qusyu5h7ykx3f2bu@treble> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200324164433.qusyu5h7ykx3f2bu@treble> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 24, 2020 at 11:44:33AM -0500, Josh Poimboeuf wrote: > On Fri, Mar 20, 2020 at 11:35:37AM -0700, Kees Cook wrote: > > On Fri, Mar 20, 2020 at 09:26:13AM +0100, Peter Zijlstra wrote: > > > On Thu, Mar 19, 2020 at 12:45:50PM -0500, Josh Poimboeuf wrote: > > > > > On Thu, Mar 19, 2020 at 12:33:31PM -0500, Josh Poimboeuf wrote: > > > > > > > > > Actually I suspect it's the __builtin_unreachable() annotation which is > > > > > > making UBSAN add the __builtin_trap()... because I don't see any double > > > > > > UD2s for WARNs. > > > > > > > Actually, removing __builtin_unreachable() *does* make the extra UD2 go > > > > away -- I forgot I had some silly debug code. > > > > > > LOL, check this: > > > > > > "Built-in Function: void __builtin_unreachable (void) > > > > > > If control flow reaches the point of the __builtin_unreachable, the > > > program is undefined. It is useful in situations where the compiler > > > cannot deduce the unreachability of the code. " > > > > > > Which, I bet, is what makes UBSAN insert that __builtin_trap(). > > > > > > What a friggin mess :/ > > > > What I'd like is to be able to specify to UBSAN what function to call > > for the trap. I'd prefer to specify a well-defined exception handler, > > but at present, UBSAN just inserts __builtin_trap(). > > > > Can't objtool be told to ignore a ud2 that lacks an execution path to it? > > It can ignore unreachable UD2s, if we think that's the right fix. > > I was hoping we could find a way to get rid of the double UD2s, but I > couldn't figure out a way to do that when I looked at it last week. As far as I could tell, this needs patches to the UBSAN support in gcc and clang. I have opened bugs for each: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94307 https://bugs.llvm.org/show_bug.cgi?id=45295 -- Kees Cook