linux-kernel - [PATCH] x86/alternatives, kvm: Fix a couple of CALLs without a frame pointer

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Message-ID: <20240618200539.GHZnHok3nr60R4BXSK@fat_crate.local>
Date: Tue, 18 Jun 2024 22:05:39 +0200
From: Borislav Petkov <bp@...en8.de>
To: Peter Zijlstra <peterz@...radead.org>
Cc: kernel test robot <lkp@...el.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>, oe-kbuild-all@...ts.linux.dev,
	linux-kernel@...r.kernel.org, x86@...nel.org
Subject: [PATCH] x86/alternatives, kvm: Fix a couple of CALLs without a frame
 pointer

Ok, I was able to scratch up this from the whole shebang:

---
From: "Borislav Petkov (AMD)" <bp@...en8.de>
Date: Tue, 18 Jun 2024 21:57:27 +0200

objtool complains:

  arch/x86/kvm/kvm.o: warning: objtool: .altinstr_replacement+0xc5: call without frame pointer save/setup
  vmlinux.o: warning: objtool: .altinstr_replacement+0x2eb: call without frame pointer save/setup

Make sure rSP is an output operand to the respective asm() statements.

The test_cc() hunk courtesy of peterz. Also from him add some helpful
debugging info to the documentation.

Reported-by: kernel test robot <lkp@...el.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202406141648.jO9qNGLa-lkp@intel.com/
Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
---
 arch/x86/include/asm/alternative.h      |  2 +-
 arch/x86/kernel/alternative.c           |  2 +-
 arch/x86/kvm/emulate.c                  |  2 +-
 tools/objtool/Documentation/objtool.txt | 19 +++++++++++++++++++
 4 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 89fa50d27a08..8cff4626478b 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -248,7 +248,7 @@ static inline int alternatives_text_reserved(void *start, void *end)
  */
 #define alternative_call(oldfunc, newfunc, ft_flags, output, input...)	\
 	asm_inline volatile(ALTERNATIVE("call %c[old]", "call %c[new]", ft_flags) \
-		: output : [old] "i" (oldfunc), [new] "i" (newfunc), ## input)
+		: output, ASM_CALL_CONSTRAINT : [old] "i" (oldfunc), [new] "i" (newfunc), ## input)
 
 /*
  * Like alternative_call, but there are two features and respective functions.
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 37596a417094..333b16181357 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1657,7 +1657,7 @@ static noinline void __init alt_reloc_selftest(void)
 	 */
 	asm_inline volatile (
 		ALTERNATIVE("", "lea %[mem], %%" _ASM_ARG1 "; call __alt_reloc_selftest;", X86_FEATURE_ALWAYS)
-		: /* output */
+		: ASM_CALL_CONSTRAINT
 		: [mem] "m" (__alt_reloc_selftest_addr)
 		: _ASM_ARG1
 	);
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 5d4c86133453..c8cc578646d0 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -1069,7 +1069,7 @@ static __always_inline u8 test_cc(unsigned int condition, unsigned long flags)
 
 	flags = (flags & EFLAGS_MASK) | X86_EFLAGS_IF;
 	asm("push %[flags]; popf; " CALL_NOSPEC
-	    : "=a"(rc) : [thunk_target]"r"(fop), [flags]"r"(flags));
+	    : "=a"(rc), ASM_CALL_CONSTRAINT : [thunk_target]"r"(fop), [flags]"r"(flags));
 	return rc;
 }
 
diff --git a/tools/objtool/Documentation/objtool.txt b/tools/objtool/Documentation/objtool.txt
index fe39c2a8ef0d..7c3ee959b63c 100644
--- a/tools/objtool/Documentation/objtool.txt
+++ b/tools/objtool/Documentation/objtool.txt
@@ -284,6 +284,25 @@ the objtool maintainers.
 
    Otherwise the stack frame may not get created before the call.
 
+   objtool can help with pinpointing the exact function where it happens:
+
+   $ OBJTOOL_ARGS="--verbose" make arch/x86/kvm/
+
+   arch/x86/kvm/kvm.o: warning: objtool: .altinstr_replacement+0xc5: call without frame pointer save/setup
+   arch/x86/kvm/kvm.o: warning: objtool:   em_loop.part.0+0x29: (alt)
+   arch/x86/kvm/kvm.o: warning: objtool:   em_loop.part.0+0x0: <=== (sym)
+    LD [M]  arch/x86/kvm/kvm-intel.o
+   0000 0000000000028220 <em_loop.part.0>:
+   0000    28220:  0f b6 47 61             movzbl 0x61(%rdi),%eax
+   0004    28224:  3c e2                   cmp    $0xe2,%al
+   0006    28226:  74 2c                   je     28254 <em_loop.part.0+0x34>
+   0008    28228:  48 8b 57 10             mov    0x10(%rdi),%rdx
+   000c    2822c:  83 f0 05                xor    $0x5,%eax
+   000f    2822f:  48 c1 e0 04             shl    $0x4,%rax
+   0013    28233:  25 f0 00 00 00          and    $0xf0,%eax
+   0018    28238:  81 e2 d5 08 00 00       and    $0x8d5,%edx
+   001e    2823e:  80 ce 02                or     $0x2,%dh
+   ...
 
 2. file.o: warning: objtool: .text+0x53: unreachable instruction
 
-- 
2.43.0

-- 
Regards/Gruss,
    Boris.

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