From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lf1-f66.google.com ([209.85.167.66]:34600 "EHLO mail-lf1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728075AbgFJPri (ORCPT ); Wed, 10 Jun 2020 11:47:38 -0400 Reply-To: alex.popov@linux.com Subject: Re: [PATCH 2/5] gcc-plugins/stackleak: Use asm instrumentation to avoid useless register saving References: <20200604134957.505389-1-alex.popov@linux.com> <20200604134957.505389-3-alex.popov@linux.com> <202006091143.AD1A662@keescook> From: Alexander Popov Message-ID: <757cbafb-1e13-8989-e30d-33c557d33cc4@linux.com> Date: Wed, 10 Jun 2020 18:47:14 +0300 MIME-Version: 1.0 In-Reply-To: <202006091143.AD1A662@keescook> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kbuild-owner@vger.kernel.org List-ID: To: Kees Cook Cc: Emese Revfy , Miguel Ojeda , Masahiro Yamada , Michal Marek , Andrew Morton , Masahiro Yamada , Thiago Jung Bauermann , Luis Chamberlain , Jessica Yu , Sven Schnelle , Iurii Zaikin , Catalin Marinas , Will Deacon , Vincenzo Frascino , Thomas Gleixner , Peter Collingbourne , Naohiro Aota , Alexander Monakov , Mathias Krause , PaX Team , Brad Spengler , Laura Abbott , Florian Weimer , kernel-hardening@lists.openwall.com, linux-kbuild@vger.kernel.org, x86@kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, gcc@gcc.gnu.org, notify@kernel.org On 09.06.2020 21:46, Kees Cook wrote: > On Thu, Jun 04, 2020 at 04:49:54PM +0300, Alexander Popov wrote: >> Let's improve the instrumentation to avoid this: >> >> 1. Make stackleak_track_stack() save all register that it works with. >> Use no_caller_saved_registers attribute for that function. This attribute >> is available for x86_64 and i386 starting from gcc-7. >> >> 2. Insert calling stackleak_track_stack() in asm: >> asm volatile("call stackleak_track_stack" :: "r" (current_stack_pointer)) >> Here we use ASM_CALL_CONSTRAINT trick from arch/x86/include/asm/asm.h. >> The input constraint is taken into account during gcc shrink-wrapping >> optimization. It is needed to be sure that stackleak_track_stack() call is >> inserted after the prologue of the containing function, when the stack >> frame is prepared. > > Very cool; nice work! > >> +static void add_stack_tracking(gimple_stmt_iterator *gsi) >> +{ >> + /* >> + * The 'no_caller_saved_registers' attribute is used for >> + * stackleak_track_stack(). If the compiler supports this attribute for >> + * the target arch, we can add calling stackleak_track_stack() in asm. >> + * That improves performance: we avoid useless operations with the >> + * caller-saved registers in the functions from which we will remove >> + * stackleak_track_stack() call during the stackleak_cleanup pass. >> + */ >> + if (lookup_attribute_spec(get_identifier("no_caller_saved_registers"))) >> + add_stack_tracking_gasm(gsi); >> + else >> + add_stack_tracking_gcall(gsi); >> +} > > The build_for_x86 flag is only ever used as an assert() test against > no_caller_saved_registers, but we're able to test for that separately. > Why does the architecture need to be tested? (i.e. when this flag > becomes supported o other architectures, why must it still be x86-only?) The inline asm statement that is used for instrumentation is arch-specific. Trying to add asm volatile("call stackleak_track_stack") in gcc plugin on aarch64 makes gcc break spectacularly. I pass the target arch name to the plugin and check it explicitly to avoid that. Moreover, I'm going to create a gcc enhancement request for supporting no_caller_saved_registers attribute on aarch64. Best regards, Alexander