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x86/alternatives, kvm: Fix a couple of CALLs without a frame pointer

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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...

Closes: https://lore.kernel.org/oe-kbuild-all/202406141648.jO9qNGLa-lkp@intel.com/
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Sean Christopherson <seanjc@google.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240625112056.GDZnqoGDXgYuWBDUwu@fat_crate.local
This commit is contained in:
Borislav Petkov (AMD) 2024-06-18 21:57:27 +02:00
parent f776e41fdc
commit 0d3db1f14a
5 changed files with 29 additions and 7 deletions
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11
arch/x86/include/asm/alternative.h
View File

@ -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" \

2
arch/x86/include/asm/page_64.h
View File

@ -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");
}

2
arch/x86/kernel/alternative.c
View File

@ -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
);

2
arch/x86/kvm/emulate.c
View File

@ -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;
}

19
tools/objtool/Documentation/objtool.txt
View File

@ -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