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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id bv22sm8729991pjb.31.2022.02.06.10.45.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 06 Feb 2022 10:45:16 -0800 (PST) Date: Sun, 6 Feb 2022 10:45:15 -0800 From: Kees Cook To: Sami Tolvanen Cc: Sean Christopherson , Peter Zijlstra , LKML , linux-arm-kernel , kvmarm , kvm@vger.kernel.org, Will McVicker Subject: Re: [PATCH v4 09/17] perf/core: Use static_call to optimize perf_guest_info_callbacks Message-ID: <202202061011.A255DE55B@keescook> References: <20211111020738.2512932-1-seanjc@google.com> <20211111020738.2512932-10-seanjc@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Fri, Feb 04, 2022 at 09:35:49AM -0800, Sami Tolvanen wrote: > On Wed, Feb 2, 2022 at 10:43 AM Sean Christopherson wrote: > > > +DEFINE_STATIC_CALL_RET0(__perf_guest_state, *perf_guest_cbs->state); > > > +DEFINE_STATIC_CALL_RET0(__perf_guest_get_ip, *perf_guest_cbs->get_ip); > > > +DEFINE_STATIC_CALL_RET0(__perf_guest_handle_intel_pt_intr, *perf_guest_cbs->handle_intel_pt_intr); > > > > Using __static_call_return0() makes clang's CFI sad on arm64 due to the resulting > > function prototype mistmatch, which IIUC, is verified by clang's __cfi_check() > > for indirect calls, i.e. architectures without CONFIG_HAVE_STATIC_CALL. > > > > We could fudge around the issue by using stubs, massaging prototypes, etc..., but > > that means doing that for every arch-agnostic user of __static_call_return0(). > > > > Any clever ideas? Can we do something like generate a unique function for every > > DEFINE_STATIC_CALL_RET0 for CONFIG_HAVE_STATIC_CALL=n, e.g. using typeof() to > > get the prototype? > > I'm not sure there's a clever fix for this. On architectures without > HAVE_STATIC_CALL, this is an indirect call to a function with a > mismatching type, which CFI is intended to catch. > > The obvious way to solve the problem would be to use a stub function > with the correct type, which I agree, isn't going to scale. You can > alternatively check if .func points to __static_call_return0 and not > make the indirect call if it does. If neither of these options are > feasible, you can disable CFI checking in the functions that have > these static calls using the __nocfi attribute. > > Kees, any thoughts? I'm digging through the macros to sort this out, but IIUC, an example of the problem is: perf_guest_cbs->handle_intel_pt_intr is: unsigned int (*handle_intel_pt_intr)(void); The declaration for this starts with: DECLARE_STATIC_CALL(__perf_guest_handle_intel_pt_intr, *perf_guest_cbs->handle_intel_pt_intr); which produces: extern struct static_call_key STATIC_CALL_KEY(__perf_guest_handle_intel_pt_intr); extern typeof(*perf_guest_cbs->handle_intel_pt_intr) STATIC_CALL_TRAMP(__perf_guest_handle_intel_pt_intr); and the last line becomes: extern unsigned int (*__SCT____perf_guest_handle_intel_pt_intr)(void); with !HAVE_STATIC_CALL, when perf_guest_handle_intel_pt_intr() does: return static_call(__perf_guest_handle_intel_pt_intr)(); it is resolving static_call() into: ((typeof(STATIC_CALL_TRAMP(name))*)(STATIC_CALL_KEY(name).func)) so the caller is expecting "unsigned int (*)(void)" but the prototype of __static_call_return0 is "long (*)(void)": long __static_call_return0(void); Could we simply declare a type-matched ret0 trampoline too? #define STATIC_CALL_TRAMP_RET0_PREFIX __SCT__ret0__ #define STATIC_CALL_TRAMP_RET0(name) __PASTE(STATIC_CALL_TRAMP_RET0_PREFIX, name) #define DEFINE_STATIC_CALL_RET0(name, _func) \ static typeof(_func) STATIC_CALL_TRAMP_RET0(name) { return 0; } \ __DEFINE_STATIC_CALL(name, _func, STATIC_CALL_TRAMP_RET0(name)); static_call_update(__perf_guest_handle_intel_pt_intr, (void *)&STATIC_CALL_TRAMP_RET0(__perf_guest_handle_intel_pt_intr)) -- Kees Cook