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[81.140.166.164]) by smtp.gmail.com with ESMTPSA id z3sm2609010wrs.94.2020.01.23.03.45.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 23 Jan 2020 03:46:00 -0800 (PST) Subject: Re: [RFC v5 11/57] objtool: Abstract alternative special case handling To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, jpoimboe@redhat.com, raphael.gault@arm.com, catalin.marinas@arm.com, will@kernel.org References: <20200109160300.26150-1-jthierry@redhat.com> <20200109160300.26150-12-jthierry@redhat.com> <20200120145401.GB14897@hirez.programming.kicks-ass.net> From: Julien Thierry Message-ID: Date: Thu, 23 Jan 2020 11:45:58 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0 MIME-Version: 1.0 In-Reply-To: <20200120145401.GB14897@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 1/20/20 2:54 PM, Peter Zijlstra wrote: > On Thu, Jan 09, 2020 at 04:02:14PM +0000, Julien Thierry wrote: >> diff --git a/tools/objtool/arch/x86/arch_special.c b/tools/objtool/arch/x86/arch_special.c >> new file mode 100644 >> index 000000000000..6dba31f419d0 >> --- /dev/null >> +++ b/tools/objtool/arch/x86/arch_special.c >> @@ -0,0 +1,34 @@ >> +// SPDX-License-Identifier: GPL-2.0-or-later >> +#include "../../special.h" >> +#include "../../builtin.h" >> + >> +void arch_handle_alternative(unsigned short feature, struct special_alt *alt) >> +{ >> + /* >> + * If UACCESS validation is enabled; force that alternative; >> + * otherwise force it the other way. >> + * >> + * What we want to avoid is having both the original and the >> + * alternative code flow at the same time, in that case we can >> + * find paths that see the STAC but take the NOP instead of >> + * CLAC and the other way around. >> + */ > > That comment ^, > >> + switch (feature) { >> + case X86_FEATURE_SMAP: > > goes here > > Good catch, I'll fix that. >> + if (uaccess) >> + alt->skip_orig = true; >> + else >> + alt->skip_alt = true; >> + break; > >> + case X86_FEATURE_POPCNT: >> + /* >> + * It has been requested that we don't validate the !POPCNT >> + * feature path which is a "very very small percentage of >> + * machines". >> + */ >> + alt->skip_orig = true; >> + break; >> + default: >> + break; >> + } >> +} > Thanks, -- Julien Thierry