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[92.26.98.202]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-5c88240a8e9sm5125834a12.36.2024.09.30.15.23.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 30 Sep 2024 15:23:38 -0700 (PDT) Message-ID: <54d392d3-32b3-4832-89e1-d2ada1af22a8@citrix.com> Date: Mon, 30 Sep 2024 23:23:38 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 13/14] x86: BHI stubs To: Josh Poimboeuf , Peter Zijlstra Cc: x86@kernel.org, linux-kernel@vger.kernel.org, alyssa.milburn@intel.com, scott.d.constable@intel.com, joao@overdrivepizza.com, jose.marchesi@oracle.com, hjl.tools@gmail.com, ndesaulniers@google.com, samitolvanen@google.com, nathan@kernel.org, ojeda@kernel.org, kees@kernel.org, alexei.starovoitov@gmail.com References: <20240927194856.096003183@infradead.org> <20240927194925.707462984@infradead.org> <20240930213030.ixbsyzziy6frh62f@treble> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 30/09/2024 10:30 pm, Josh Poimboeuf wrote: > On Fri, Sep 27, 2024 at 09:49:09PM +0200, Peter Zijlstra wrote: >> +/* >> + * At the function start, launder function arguments that are a pointer through >> + * CMOVcc, this will create a write dependency in the speculation flow. >> + * >> + * Notably, the CFI preambles calling these will have ZF set and r10 zero. >> + */ >> + >> +.pushsection .noinstr.text, "ax" >> + >> + .align 8 >> +SYM_CODE_START(__bhi_args_6c1) >> + ANNOTATE_NOENDBR >> + .align 8 >> +SYM_INNER_LABEL(__bhi_args_0, SYM_L_LOCAL) >> + UNWIND_HINT_FUNC >> + cmovne %r10, %rdi > IIUC, this works because if the "jz" in the CFI preamble mispredicts to > the __bhi_args_* code, "cmovne" will zero out the speculative value of > rdi. > > Why use %r10 instead of a literal $0? Also how do you know %r10 is 0? There's no encoding for CMOVcc which takes an $imm. %r10 is guaranteed zero after the FineIBT prologue, but I don't see anything in patch 11 which makes this true in the !FineIBT case. ~Andrew