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Fri, 29 May 2020 16:05:18 +0000 (UTC) Received: from treble (ovpn-116-170.rdu2.redhat.com [10.10.116.170]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 86B287A8D3; Fri, 29 May 2020 16:05:16 +0000 (UTC) Date: Fri, 29 May 2020 11:05:14 -0500 From: Josh Poimboeuf To: Peter Zijlstra Cc: Christoph Hellwig , Randy Dunlap , Andrew Morton , broonie@kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-next@vger.kernel.org, mhocko@suse.cz, mm-commits@vger.kernel.org, sfr@canb.auug.org.au, Linus Torvalds , viro@zeniv.linux.org.uk, x86@kernel.org, Steven Rostedt Subject: Re: mmotm 2020-05-13-20-30 uploaded (objtool warnings) Message-ID: <20200529160514.cyaytn33thphb3tz@treble> References: <20200514033104.kRFL_ctMQ%akpm@linux-foundation.org> <611fa14d-8d31-796f-b909-686d9ebf84a9@infradead.org> <20200528172005.GP2483@worktop.programming.kicks-ass.net> <20200529135750.GA1580@lst.de> <20200529143556.GE706478@hirez.programming.kicks-ass.net> <20200529145325.GB706518@hirez.programming.kicks-ass.net> <20200529153336.GC706518@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200529153336.GC706518@hirez.programming.kicks-ass.net> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Rspamd-Queue-Id: 90AB4180938D4 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam05 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, May 29, 2020 at 05:33:36PM +0200, Peter Zijlstra wrote: > On Fri, May 29, 2020 at 04:53:25PM +0200, Peter Zijlstra wrote: > > On Fri, May 29, 2020 at 04:35:56PM +0200, Peter Zijlstra wrote: > > > *groan*, this is one of those CONFIG_PROFILE_ALL_BRANCHES builds. If I > > disable that it goes away. > > > > Still trying to untangle the mess it generated, but on first go it > > looks like objtool is right, but I'm not sure what went wrong. > > $ tools/objtool/objtool check -fab arch/x86/lib/csum-wrappers_64.o > arch/x86/lib/csum-wrappers_64.o: warning: objtool: csum_and_copy_from_user()+0x29f: call to memset() with UACCESS enabled > arch/x86/lib/csum-wrappers_64.o: warning: objtool: csum_and_copy_from_user()+0x283: (branch) > arch/x86/lib/csum-wrappers_64.o: warning: objtool: csum_and_copy_from_user()+0x113: (branch) > arch/x86/lib/csum-wrappers_64.o: warning: objtool: .altinstr_replacement+0xffffffffffffffff: (branch) > arch/x86/lib/csum-wrappers_64.o: warning: objtool: csum_and_copy_from_user()+0xea: (alt) > arch/x86/lib/csum-wrappers_64.o: warning: objtool: .altinstr_replacement+0xffffffffffffffff: (branch) > arch/x86/lib/csum-wrappers_64.o: warning: objtool: csum_and_copy_from_user()+0xe7: (alt) > arch/x86/lib/csum-wrappers_64.o: warning: objtool: csum_and_copy_from_user()+0xd2: (branch) > arch/x86/lib/csum-wrappers_64.o: warning: objtool: csum_and_copy_from_user()+0x7e: (branch) > arch/x86/lib/csum-wrappers_64.o: warning: objtool: csum_and_copy_from_user()+0x43: (branch) > arch/x86/lib/csum-wrappers_64.o: warning: objtool: csum_and_copy_from_user()+0x0: <=== (sym) > > The problem is with the +0x113 branch, which is at 0x1d1. > > That looks to be: > > if (!likely(user_access_begin(src, len))) > goto out_err; > > Except that the brach profiling stuff confused GCC enough to leak STAC > into the error path or something. It looks to me like GCC is doing the right thing. That likely() translates to: # define likely(x) (__branch_check__(x, 1, __builtin_constant_p(x))) which becomes: #define __branch_check__(x, expect, is_constant) ({ \ long ______r; \ static struct ftrace_likely_data \ __aligned(4) \ __section(_ftrace_annotated_branch) \ ______f = { \ .data.func = __func__, \ .data.file = __FILE__, \ .data.line = __LINE__, \ }; \ ______r = __builtin_expect(!!(x), expect); \ ftrace_likely_update(&______f, ______r, \ expect, is_constant); \ ______r; \ }) Here 'x' is the call to user_access_begin(). It evaluates 'x' -- and thus calls user_access_begin() -- before the call to ftrace_likely_update(). So it's working as designed, right? The likely() just needs to be changed to likely_notrace(). -- Josh