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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id f16sm3752101pfj.220.2021.04.01.00.17.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 01 Apr 2021 00:17:05 -0700 (PDT) Date: Thu, 1 Apr 2021 00:17:04 -0700 From: Kees Cook To: Will Deacon Cc: Peter Zijlstra , Nick Desaulniers , linux-toolchains@vger.kernel.org, clang-built-linux , Steven Rostedt , "Jose E. Marchesi" , Florian Weimer , Christian Brauner , nick.alcock@oracle.com, Segher Boessenkool , Josh Poimboeuf , andrew.cooper3@citrix.com Subject: Re: Plumbers CF MCs Message-ID: <202104010015.B879F44@keescook> References: <20210330141312.GA6327@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210330141312.GA6327@willie-the-truck> Precedence: bulk List-ID: X-Mailing-List: linux-toolchains@vger.kernel.org On Tue, Mar 30, 2021 at 03:13:12PM +0100, Will Deacon wrote: > On Tue, Mar 23, 2021 at 09:35:10AM +0100, Peter Zijlstra wrote: > > On Mon, Mar 22, 2021 at 01:23:03PM -0700, Nick Desaulniers wrote: > > > Hi all, > > > I saw plumbers opened call for microconferences: > > > https://www.linuxplumbersconf.org/blog/2021/index.php/2021/03/18/cfp-open-microconferences/ > > > > > > I was going to put together a submission; do we want to do a combined > > > toolchain MC, or have distinct ones this year? > > > > > > I know in 2020 the GNU cauldron was co-located with Plumbers, as well > > > as a GNU Tools Track MC and LLVM MC. > > > > A combined MC focussed on kernel issues seems very interesting. We still > > have the control dependency (volatile-if?) thing pending. We had a bit > > of a discussion on that after last year, but I don't think anything > > really came of that, can we pick that up? Ideally a compiler person does > > an actual proposal for this year. > > > > If we can sort that, there's the rest of the dependencies Will outlined > > :-) > > > > Then there seemed to be people that thought __always_inline was a > > suggestion... I think we need to explore how that was ever possible. > > > > There's the endless UB debate... can we define more to reduce the UB? I > > mean, we're already bound by architecture ABI on the one hand, and > > actual use on the other. It would be so very nice to be able to get more > > -fwrapv and -fno-strict-aliasing knobs that define UBs away. > > > > There also is talk about straight line speculation mitigations. for x86 > > we should probably emit an INT3 after every JMP and RET. Although this > > might not be controversial and be sorted by the time Plumbers happens. > > > > There was some talk about how compilers could help objtool make sense of > > jump tables. > > > > GCC's status on asm-goto with outputs? > > > > Clang's getting asm-constraints wrong ("rm" and it always picks "m"). > > > > > > And I'm sure there was more.. > > One thing I'd like to add, and which I think is possibly relevant to the SLS > mitigation for arm64, is whether there is scope for allowing the compiler to > generate alternative instruction sequences (e.g. in a separate section), > which the kernel could then patch in during boot. We already do a tonne of > code patching on arm64 for things like CPU errata workarounds but also > for enabling support for optional architecture features, where the kernel > code would trap on CPUs without hardware support. > > Another use of this would be to enable stack-taggging with MTE, where the > instrumentation is generated by the compiler but may use instructions which > are undefined if the CPU doesn't support MTE. Or swapping out SCS and stack-protector for PAC when the hardware supports it... -- Kees Cook