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Date: Tue, 25 Jun 2024 13:20:56 +0200
From: Borislav Petkov <bp@...en8.de>
To: linux-kernel@...r.kernel.org
Cc: linux-tip-commits@...r.kernel.org, kernel test robot <lkp@...el.com>,
Sean Christopherson <seanjc@...gle.com>, x86@...nel.org,
Michael Matz <matz@...e.de>
Subject: [PATCH -v2] x86/alternatives, kvm: Fix a couple of CALLs without a
frame pointer
On Thu, Jun 20, 2024 at 10:48:53AM +0200, Borislav Petkov wrote:
> On Wed, Jun 19, 2024 at 08:39:52AM -0000, tip-bot2 for Borislav Petkov (AMD) wrote:
> > The following commit has been merged into the x86/alternatives branch of tip:
> >
> > Commit-ID: 93f78dadee5e56ae48aff567583d503868aa3bf2
> > Gitweb: https://git.kernel.org/tip/93f78dadee5e56ae48aff567583d503868aa3bf2
> > Author: Borislav Petkov (AMD) <bp@...en8.de>
> > AuthorDate: Tue, 18 Jun 2024 21:57:27 +02:00
> > Committer: Borislav Petkov (AMD) <bp@...en8.de>
> > CommitterDate: Wed, 19 Jun 2024 10:33:25 +02:00
> >
> > x86/alternatives, kvm: Fix a couple of CALLs without a frame pointer
> >
> > 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>
> > Acked-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> > 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 89fa50d..8cff462 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)
>
> Yeah, this doesn't fly currently:
>
> https://lore.kernel.org/r/202406200507.AXxJ6Bmw-lkp@intel.com
>
> because those atomic64_32.h macros do
>
> alternative_atomic64(set, /* no output */,
> "S" (v), "b" (low), "c" (high)
>
> so without an output, it ends up becoming:
>
> asm __inline volatile("# ALT: oldinstr\n" ... ".popsection\n" : , "+r" (current_stack_pointer) : [old] "i" ...
>
> note the preceding ",".
>
> And I can't do "output..." macro argument with ellipsis and paste with "##
> output" because "input..." already does that. :-\
>
> So I am not sure what to do here. Removing the ASM_CALL_CONSTRAINT works,
> let's see whether it passes build tests.
>
> Or add dummy output arguments to the three atomic macros which have no
> output?
Ok, after a lot of back'n'forth, here's v2:
---
From: "Borislav Petkov (AMD)" <bp@...en8.de>
Date: Tue, 18 Jun 2024 21:57:27 +0200
Subject: [PATCH] x86/alternatives, kvm: Fix a couple of CALLs without a frame pointer
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 and ALT_OUTPUT_SP() courtesy of peterz. Also from him
add some helpful debugging info to the documentation.
Now on to the explanations:
tl;dr: The alternatives macros are pretty fragile.
If I do ALT_OUTPUT_SP(output) in order to be able to package in a %rsp
reference for objtool so that a stack frame gets properly generated, the
inline asm input operand with positional argument 0 in clear_page():
"0" (page)
gets "renumbered" due to the added
: "+r" (current_stack_pointer), "=D" (page)
and then gcc says:
./arch/x86/include/asm/page_64.h:53:9: error: inconsistent operand constraints in an ‘asm’
The fix is to use an explicit "D" constraint which points to a singleton
register class (gcc terminology) which ends up doing what is expected
here: the page pointer - input and output - should be in the same %rdi
register.
Other register classes have more than one register in them - example:
"r" and "=r" or "A":
‘A’
The ‘a’ and ‘d’ registers. This class is used for
instructions that return double word results in the ‘ax:dx’
register pair. Single word values will be allocated either in
‘ax’ or ‘dx’.
so using "D" and "=D" just works in this particular case.
And yes, one would say, sure, why don't you do "+D" but then:
: "+r" (current_stack_pointer), "+D" (page)
: [old] "i" (clear_page_orig), [new1] "i" (clear_page_rep), [new2] "i" (clear_page_erms),
: "cc", "memory", "rax", "rcx")
now find the Waldo^Wcomma which throws a wrench into all this.
Because that silly macro has an "input..." consume-all last macro arg
and in it, one is supposed to supply input *and* clobbers, leading to
silly syntax snafus.
Yap, they need to be cleaned up, one fine day...
Cc: Michael Matz <matz@...e.de>
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 | 11 +++++++----
arch/x86/include/asm/page_64.h | 2 +-
arch/x86/kernel/alternative.c | 2 +-
arch/x86/kvm/emulate.c | 2 +-
tools/objtool/Documentation/objtool.txt | 19 +++++++++++++++++++
5 files changed, 29 insertions(+), 7 deletions(-)
diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 89fa50d27a08..ca9ae606aab9 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -246,9 +246,10 @@ static inline int alternatives_text_reserved(void *start, void *end)
* references: i.e., if used for a function, it would add the PLT
* suffix.
*/
-#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)
+#define alternative_call(oldfunc, newfunc, ft_flags, output, input...) \
+ asm_inline volatile(ALTERNATIVE("call %c[old]", "call %c[new]", ft_flags) \
+ : ALT_OUTPUT_SP(output) \
+ : [old] "i" (oldfunc), [new] "i" (newfunc), ## input)
/*
* Like alternative_call, but there are two features and respective functions.
@@ -260,7 +261,7 @@ static inline int alternatives_text_reserved(void *start, void *end)
output, input...) \
asm_inline volatile(ALTERNATIVE_2("call %c[old]", "call %c[new1]", ft_flags1, \
"call %c[new2]", ft_flags2) \
- : output, ASM_CALL_CONSTRAINT \
+ : ALT_OUTPUT_SP(output) \
: [old] "i" (oldfunc), [new1] "i" (newfunc1), \
[new2] "i" (newfunc2), ## input)
@@ -276,6 +277,8 @@ static inline int alternatives_text_reserved(void *start, void *end)
*/
#define ASM_NO_INPUT_CLOBBER(clbr...) "i" (0) : clbr
+#define ALT_OUTPUT_SP(...) ASM_CALL_CONSTRAINT, ## __VA_ARGS__
+
/* Macro for creating assembler functions avoiding any C magic. */
#define DEFINE_ASM_FUNC(func, instr, sec) \
asm (".pushsection " #sec ", \"ax\"\n" \
diff --git a/arch/x86/include/asm/page_64.h b/arch/x86/include/asm/page_64.h
index cc6b8e087192..af4302d79b59 100644
--- a/arch/x86/include/asm/page_64.h
+++ b/arch/x86/include/asm/page_64.h
@@ -54,7 +54,7 @@ static inline void clear_page(void *page)
clear_page_rep, X86_FEATURE_REP_GOOD,
clear_page_erms, X86_FEATURE_ERMS,
"=D" (page),
- "0" (page)
+ "D" (page)
: "cc", "memory", "rax", "rcx");
}
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.
https://people.kernel.org/tglx/notes-about-netiquette
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