[RESEND PATCH] arm64: fix alternatives with LLVM's integrated assembler

[RESEND PATCH] arm64: fix alternatives with LLVM's integrated assembler

[Sami Tolvanen]

LLVM's integrated assembler fails with the following error when
building KVM:

  <inline asm>:12:6: error: expected absolute expression
   .if kvm_update_va_mask == 0
       ^
  <inline asm>:21:6: error: expected absolute expression
   .if kvm_update_va_mask == 0
       ^
  <inline asm>:24:2: error: unrecognized instruction mnemonic
          NOT_AN_INSTRUCTION
          ^
  LLVM ERROR: Error parsing inline asm

These errors come from ALTERNATIVE_CB and __ALTERNATIVE_CFG,
which test for the existence of the callback parameter in inline
assembly using the following expression:

  " .if " __stringify(cb) " == 0\n"

This works with GNU as, but isn't supported by LLVM. This change
splits __ALTERNATIVE_CFG and ALTINSTR_ENTRY into separate macros
to fix the LLVM build.

Link: https://github.com/ClangBuiltLinux/linux/issues/472
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
---
 arch/arm64/include/asm/alternative.h | 32 ++++++++++++++++++----------
 1 file changed, 21 insertions(+), 11 deletions(-)

diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h
index b9f8d787eea9..324e7d5ab37e 100644
--- a/arch/arm64/include/asm/alternative.h
+++ b/arch/arm64/include/asm/alternative.h
@@ -35,13 +35,16 @@ void apply_alternatives_module(void *start, size_t length);
 static inline void apply_alternatives_module(void *start, size_t length) { }
 #endif
 
-#define ALTINSTR_ENTRY(feature,cb)					      \
+#define ALTINSTR_ENTRY(feature)					              \
 	" .word 661b - .\n"				/* label           */ \
-	" .if " __stringify(cb) " == 0\n"				      \
 	" .word 663f - .\n"				/* new instruction */ \
-	" .else\n"							      \
+	" .hword " __stringify(feature) "\n"		/* feature bit     */ \
+	" .byte 662b-661b\n"				/* source len      */ \
+	" .byte 664f-663f\n"				/* replacement len */
+
+#define ALTINSTR_ENTRY_CB(feature, cb)					      \
+	" .word 661b - .\n"				/* label           */ \
 	" .word " __stringify(cb) "- .\n"		/* callback */	      \
-	" .endif\n"							      \
 	" .hword " __stringify(feature) "\n"		/* feature bit     */ \
 	" .byte 662b-661b\n"				/* source len      */ \
 	" .byte 664f-663f\n"				/* replacement len */
@@ -62,15 +65,14 @@ static inline void apply_alternatives_module(void *start, size_t length) { }
  *
  * Alternatives with callbacks do not generate replacement instructions.
  */
-#define __ALTERNATIVE_CFG(oldinstr, newinstr, feature, cfg_enabled, cb)	\
+#define __ALTERNATIVE_CFG(oldinstr, newinstr, feature, cfg_enabled)	\
 	".if "__stringify(cfg_enabled)" == 1\n"				\
 	"661:\n\t"							\
 	oldinstr "\n"							\
 	"662:\n"							\
 	".pushsection .altinstructions,\"a\"\n"				\
-	ALTINSTR_ENTRY(feature,cb)					\
+	ALTINSTR_ENTRY(feature)						\
 	".popsection\n"							\
-	" .if " __stringify(cb) " == 0\n"				\
 	".pushsection .altinstr_replacement, \"a\"\n"			\
 	"663:\n\t"							\
 	newinstr "\n"							\
@@ -78,17 +80,25 @@ static inline void apply_alternatives_module(void *start, size_t length) { }
 	".popsection\n\t"						\
 	".org	. - (664b-663b) + (662b-661b)\n\t"			\
 	".org	. - (662b-661b) + (664b-663b)\n"			\
-	".else\n\t"							\
+	".endif\n"
+
+#define __ALTERNATIVE_CFG_CB(oldinstr, feature, cfg_enabled, cb)	\
+	".if "__stringify(cfg_enabled)" == 1\n"				\
+	"661:\n\t"							\
+	oldinstr "\n"							\
+	"662:\n"							\
+	".pushsection .altinstructions,\"a\"\n"				\
+	ALTINSTR_ENTRY_CB(feature, cb)					\
+	".popsection\n"							\
 	"663:\n\t"							\
 	"664:\n\t"							\
-	".endif\n"							\
 	".endif\n"
 
 #define _ALTERNATIVE_CFG(oldinstr, newinstr, feature, cfg, ...)	\
-	__ALTERNATIVE_CFG(oldinstr, newinstr, feature, IS_ENABLED(cfg), 0)
+	__ALTERNATIVE_CFG(oldinstr, newinstr, feature, IS_ENABLED(cfg))
 
 #define ALTERNATIVE_CB(oldinstr, cb) \
-	__ALTERNATIVE_CFG(oldinstr, "NOT_AN_INSTRUCTION", ARM64_CB_PATCH, 1, cb)
+	__ALTERNATIVE_CFG_CB(oldinstr, ARM64_CB_PATCH, 1, cb)
 #else
 
 #include <asm/assembler.h>
-- 
2.24.0.rc0.303.g954a862665-goog

Re: [RESEND PATCH] arm64: fix alternatives with LLVM's integrated assembler

[Kees Cook]

On Thu, Oct 31, 2019 at 12:46:52PM -0700, Sami Tolvanen wrote:
> LLVM's integrated assembler fails with the following error when
> building KVM:
> 
>   <inline asm>:12:6: error: expected absolute expression
>    .if kvm_update_va_mask == 0
>        ^
>   <inline asm>:21:6: error: expected absolute expression
>    .if kvm_update_va_mask == 0
>        ^
>   <inline asm>:24:2: error: unrecognized instruction mnemonic
>           NOT_AN_INSTRUCTION
>           ^
>   LLVM ERROR: Error parsing inline asm
> 
> These errors come from ALTERNATIVE_CB and __ALTERNATIVE_CFG,
> which test for the existence of the callback parameter in inline
> assembly using the following expression:
> 
>   " .if " __stringify(cb) " == 0\n"
> 
> This works with GNU as, but isn't supported by LLVM. This change
> splits __ALTERNATIVE_CFG and ALTINSTR_ENTRY into separate macros
> to fix the LLVM build.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/472
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
>  arch/arm64/include/asm/alternative.h | 32 ++++++++++++++++++----------
>  1 file changed, 21 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h
> index b9f8d787eea9..324e7d5ab37e 100644
> --- a/arch/arm64/include/asm/alternative.h
> +++ b/arch/arm64/include/asm/alternative.h
> @@ -35,13 +35,16 @@ void apply_alternatives_module(void *start, size_t length);
>  static inline void apply_alternatives_module(void *start, size_t length) { }
>  #endif
>  
> -#define ALTINSTR_ENTRY(feature,cb)					      \
> +#define ALTINSTR_ENTRY(feature)					              \
>  	" .word 661b - .\n"				/* label           */ \
> -	" .if " __stringify(cb) " == 0\n"				      \
>  	" .word 663f - .\n"				/* new instruction */ \
> -	" .else\n"							      \
> +	" .hword " __stringify(feature) "\n"		/* feature bit     */ \
> +	" .byte 662b-661b\n"				/* source len      */ \
> +	" .byte 664f-663f\n"				/* replacement len */
> +
> +#define ALTINSTR_ENTRY_CB(feature, cb)					      \
> +	" .word 661b - .\n"				/* label           */ \
>  	" .word " __stringify(cb) "- .\n"		/* callback */	      \
> -	" .endif\n"							      \
>  	" .hword " __stringify(feature) "\n"		/* feature bit     */ \
>  	" .byte 662b-661b\n"				/* source len      */ \
>  	" .byte 664f-663f\n"				/* replacement len */
> @@ -62,15 +65,14 @@ static inline void apply_alternatives_module(void *start, size_t length) { }
>   *
>   * Alternatives with callbacks do not generate replacement instructions.
>   */
> -#define __ALTERNATIVE_CFG(oldinstr, newinstr, feature, cfg_enabled, cb)	\
> +#define __ALTERNATIVE_CFG(oldinstr, newinstr, feature, cfg_enabled)	\
>  	".if "__stringify(cfg_enabled)" == 1\n"				\
>  	"661:\n\t"							\
>  	oldinstr "\n"							\
>  	"662:\n"							\
>  	".pushsection .altinstructions,\"a\"\n"				\
> -	ALTINSTR_ENTRY(feature,cb)					\
> +	ALTINSTR_ENTRY(feature)						\
>  	".popsection\n"							\
> -	" .if " __stringify(cb) " == 0\n"				\
>  	".pushsection .altinstr_replacement, \"a\"\n"			\
>  	"663:\n\t"							\
>  	newinstr "\n"							\
> @@ -78,17 +80,25 @@ static inline void apply_alternatives_module(void *start, size_t length) { }
>  	".popsection\n\t"						\
>  	".org	. - (664b-663b) + (662b-661b)\n\t"			\
>  	".org	. - (662b-661b) + (664b-663b)\n"			\
> -	".else\n\t"							\
> +	".endif\n"
> +
> +#define __ALTERNATIVE_CFG_CB(oldinstr, feature, cfg_enabled, cb)	\
> +	".if "__stringify(cfg_enabled)" == 1\n"				\
> +	"661:\n\t"							\
> +	oldinstr "\n"							\
> +	"662:\n"							\
> +	".pushsection .altinstructions,\"a\"\n"				\
> +	ALTINSTR_ENTRY_CB(feature, cb)					\
> +	".popsection\n"							\
>  	"663:\n\t"							\
>  	"664:\n\t"							\
> -	".endif\n"							\
>  	".endif\n"
>  
>  #define _ALTERNATIVE_CFG(oldinstr, newinstr, feature, cfg, ...)	\
> -	__ALTERNATIVE_CFG(oldinstr, newinstr, feature, IS_ENABLED(cfg), 0)
> +	__ALTERNATIVE_CFG(oldinstr, newinstr, feature, IS_ENABLED(cfg))
>  
>  #define ALTERNATIVE_CB(oldinstr, cb) \
> -	__ALTERNATIVE_CFG(oldinstr, "NOT_AN_INSTRUCTION", ARM64_CB_PATCH, 1, cb)
> +	__ALTERNATIVE_CFG_CB(oldinstr, ARM64_CB_PATCH, 1, cb)
>  #else
>  
>  #include <asm/assembler.h>
> -- 
> 2.24.0.rc0.303.g954a862665-goog
> 

-- 
Kees Cook

Re: [RESEND PATCH] arm64: fix alternatives with LLVM's integrated assembler

[Will Deacon]

Hi Sami,

Sorry -- I thought I'd already replied to this, but it had actually
slipped through the cracks.

On Thu, Oct 31, 2019 at 12:46:52PM -0700, Sami Tolvanen wrote:
> LLVM's integrated assembler fails with the following error when
> building KVM:
> 
>   <inline asm>:12:6: error: expected absolute expression
>    .if kvm_update_va_mask == 0
>        ^
>   <inline asm>:21:6: error: expected absolute expression
>    .if kvm_update_va_mask == 0
>        ^
>   <inline asm>:24:2: error: unrecognized instruction mnemonic
>           NOT_AN_INSTRUCTION
>           ^
>   LLVM ERROR: Error parsing inline asm
> 
> These errors come from ALTERNATIVE_CB and __ALTERNATIVE_CFG,
> which test for the existence of the callback parameter in inline
> assembly using the following expression:
> 
>   " .if " __stringify(cb) " == 0\n"
> 
> This works with GNU as, but isn't supported by LLVM. This change
> splits __ALTERNATIVE_CFG and ALTINSTR_ENTRY into separate macros
> to fix the LLVM build.

Please could you explain a bit more about the failure and why LLVM's
integrated assembler rejects this? Could we use something like .ifb or
.ifeqs instead?

Thanks,

Will

Re: [RESEND PATCH] arm64: fix alternatives with LLVM's integrated assembler

[Nick Desaulniers]

On Thu, Nov 14, 2019 at 8:55 AM Will Deacon <will@kernel.org> wrote:
>
> Hi Sami,
>
> Sorry -- I thought I'd already replied to this, but it had actually
> slipped through the cracks.
>
> On Thu, Oct 31, 2019 at 12:46:52PM -0700, Sami Tolvanen wrote:
> > LLVM's integrated assembler fails with the following error when
> > building KVM:
> >
> >   <inline asm>:12:6: error: expected absolute expression
> >    .if kvm_update_va_mask == 0
> >        ^
> >   <inline asm>:21:6: error: expected absolute expression
> >    .if kvm_update_va_mask == 0
> >        ^
> >   <inline asm>:24:2: error: unrecognized instruction mnemonic
> >           NOT_AN_INSTRUCTION
> >           ^
> >   LLVM ERROR: Error parsing inline asm
> >
> > These errors come from ALTERNATIVE_CB and __ALTERNATIVE_CFG,
> > which test for the existence of the callback parameter in inline
> > assembly using the following expression:
> >
> >   " .if " __stringify(cb) " == 0\n"
> >
> > This works with GNU as, but isn't supported by LLVM. This change
> > splits __ALTERNATIVE_CFG and ALTINSTR_ENTRY into separate macros
> > to fix the LLVM build.
>
> Please could you explain a bit more about the failure and why LLVM's
> integrated assembler rejects this?

There are currently more than one issue with `.if` assembler
directives we're tracking against Clang's integrated assembler
currently, particularly around the handling of special cases related
to "fragments."
Recommended reading:
https://eli.thegreenplace.net/2013/01/03/assembler-relaxation
This particular case looks like the error is related to referring to
section before it has been seen.  My current understanding is that
Clang's integrated assembler is one pass, unlike GAS, so it chokes on
references to symbols it has not yet seen.

> Could we use something like .ifb or
> .ifeqs instead?
>
> Thanks,
>
> Will



-- 
Thanks,
~Nick Desaulniers