Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20191018161033.261971-1-samitolvanen@google.com>
 <20191024225132.13410-1-samitolvanen@google.com> <20191024225132.13410-10-samitolvanen@google.com>
 <20191025110313.GE40270@lakrids.cambridge.arm.com>
In-Reply-To: <20191025110313.GE40270@lakrids.cambridge.arm.com>
From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Tue, 29 Oct 2019 10:45:22 -0700
Message-ID: <CABCJKud1xYEx_GVgfBHUuwNGKMxX+uVaE5TR6DEqo7CoSJJnNA@mail.gmail.com>
Subject: Re: [PATCH v2 09/17] arm64: disable function graph tracing with SCS
To:     Mark Rutland <mark.rutland@arm.com>,
        Nick Desaulniers <ndesaulniers@google.com>
Cc:     Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Dave Martin <Dave.Martin@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Laura Abbott <labbott@redhat.com>,
        Jann Horn <jannh@google.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Oct 25, 2019 at 4:03 AM Mark Rutland <mark.rutland@arm.com> wrote:
> I'm guessing it's difficult to always figure out the SCS slot for an
> instrumented callsite unless we pass this explicitly from the ftrace
> entry code, so we'd probably have to change some common infrastructure
> for that.
>
> We have a similar issue with pointer authentication, and we're solving
> that with -fpatchable-function-entry, which allows us to hook the
> callsite before it does anything with the return address. IIUC we could
> use the same mechanism here (and avoid introducing a third).
>
> Are there plans to implement -fpatchable-function-entry on the clang
> side?

I'm not sure if there are plans at the moment, but if this feature is
needed for PAC, adding it to clang shouldn't be a problem. Nick, did
you have any thoughts on this?

Sami