linux-kernel - RE: [PATCH v4 00/45] x86: Kernel IBT

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Date:   Thu, 10 Mar 2022 09:22:59 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
CC:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "joao@...rdrivepizza.com" <joao@...rdrivepizza.com>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "samitolvanen@...gle.com" <samitolvanen@...gle.com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "alyssa.milburn@...el.com" <alyssa.milburn@...el.com>,
        "mbenes@...e.cz" <mbenes@...e.cz>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "mhiramat@...nel.org" <mhiramat@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "andrii@...nel.org" <andrii@...nel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>
Subject: RE: [PATCH v4 00/45] x86: Kernel IBT

From: Peter Zijlstra
> Sent: 10 March 2022 09:05
> 
> On Wed, Mar 09, 2022 at 04:30:28PM -0800, Nick Desaulniers wrote:
> 
> > I observed the following error when building with
> > CONFIG_LTO_CLANG_FULL=y enabled:
> >
> > ld.lld: error: ld-temp.o <inline asm>:7:2: symbol 'ibt_selftest_ip' is
> > already defined
> >         ibt_selftest_ip:
> >         ^
> >
> > Seems to come from
> > commit a802350ba65a ("x86/ibt: Add IBT feature, MSR and #CP handling")
> >
> > Commenting out the label in the inline asm, I then observed:
> > vmlinux.o: warning: objtool: identify_cpu()+0x6d0: sibling call from
> > callable instruction with modified stack frame
> > vmlinux.o: warning: objtool: identify_cpu()+0x6e0: stack state
> > mismatch: cfa1=4+64 cfa2=4+8
> > These seemed to disappear when I kept CONFIG_LTO_CLANG_FULL=y but then
> > disabled CONFIG_X86_KERNEL_IBT. (perhaps due to the way I hacked out
> > the ibt_selftest_ip label).
> 
> Urgh.. I'm thikning this is a clang bug :/
> 
> The code in question is:
> 
> 
> void ibt_selftest_ip(void); /* code label defined in asm below */
> 
> DEFINE_IDTENTRY_ERRORCODE(exc_control_protection)
> {
> 	/* ... */
> 
> 	if (unlikely(regs->ip == (unsigned long)ibt_selftest_ip)) {
> 		regs->ax = 0;
> 		return;
> 	}
> 
> 	/* ... */
> }
> 
> bool ibt_selftest(void)
> {
> 	unsigned long ret;
> 
> 	asm ("	lea ibt_selftest_ip(%%rip), %%rax\n\t"
> 	     ANNOTATE_RETPOLINE_SAFE
> 	     "	jmp *%%rax\n\t"
> 	     "ibt_selftest_ip:\n\t"
> 	     UNWIND_HINT_FUNC
> 	     ANNOTATE_NOENDBR
> 	     "	nop\n\t"
> 
> 	     : "=a" (ret) : : "memory");
> 
> 	return !ret;
> }
> 
> There is only a single definition of that symbol, the one in the asm.
> The other is a declaration, which is used in the exception handler to
> compare against regs->ip.

LTO has probably inlined it twice.

	David

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