[PATCH 0/2] objtool annotations: Avoid using inline asm operand modifiers

[PATCH 0/2] objtool annotations: Avoid using inline asm operand modifiers

[Vasily Gorbik]

Make objtool annotations arch assembler independent. Would be nice if this goes
via objtool tree. I have further patches which depend on that.

Vasily Gorbik (2):
  compiler.h: Avoid using inline asm operand modifiers
  instrumentation.h: Avoid using inline asm operand modifiers

 include/linux/compiler.h        | 22 ++++++++++++++--------
 include/linux/instrumentation.h | 20 ++++++++++++--------
 2 files changed, 26 insertions(+), 16 deletions(-)

-- 
2.25.4

[PATCH 1/2] compiler.h: Avoid using inline asm operand modifiers

[Vasily Gorbik]

The expansion of annotate_reachable/annotate_unreachable on s390 will
result in a compiler error if the __COUNTER__ value is high enough.
For example with "i" (154) the "%c0" operand of annotate_reachable
will be expanded to -102:

        -102:
        .pushsection .discard.reachable
        .long -102b - .
        .popsection

This is a quirk of the gcc backend for s390, it interprets the %c0
as a signed byte value. Avoid using operand modifiers in this case
by simply converting __COUNTER__ to string, with the same result,
but in an arch assembler independent way.

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
---
 include/linux/compiler.h | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index df5b405e6305..77047904cf70 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -115,18 +115,24 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
  * The __COUNTER__ based labels are a hack to make each instance of the macros
  * unique, to convince GCC not to merge duplicate inline asm statements.
  */
-#define annotate_reachable() ({						\
-	asm volatile("%c0:\n\t"						\
+#define __stringify_label(n) #n
+
+#define __annotate_reachable(c) ({					\
+	asm volatile(__stringify_label(c) ":\n\t"			\
 		     ".pushsection .discard.reachable\n\t"		\
-		     ".long %c0b - .\n\t"				\
-		     ".popsection\n\t" : : "i" (__COUNTER__));		\
+		     ".long " __stringify_label(c) "b - .\n\t"		\
+		     ".popsection\n\t");				\
 })
-#define annotate_unreachable() ({					\
-	asm volatile("%c0:\n\t"						\
+#define annotate_reachable() __annotate_reachable(__COUNTER__)
+
+#define __annotate_unreachable(c) ({					\
+	asm volatile(__stringify_label(c) ":\n\t"			\
 		     ".pushsection .discard.unreachable\n\t"		\
-		     ".long %c0b - .\n\t"				\
-		     ".popsection\n\t" : : "i" (__COUNTER__));		\
+		     ".long " __stringify_label(c) "b - .\n\t"		\
+		     ".popsection\n\t");				\
 })
+#define annotate_unreachable() __annotate_unreachable(__COUNTER__)
+
 #define ASM_UNREACHABLE							\
 	"999:\n\t"							\
 	".pushsection .discard.unreachable\n\t"				\
-- 
2.25.4

Re: [PATCH 1/2] compiler.h: Avoid using inline asm operand modifiers

[Josh Poimboeuf]

On Wed, May 19, 2021 at 03:03:08PM +0200, Vasily Gorbik wrote:
> The expansion of annotate_reachable/annotate_unreachable on s390 will
> result in a compiler error if the __COUNTER__ value is high enough.
> For example with "i" (154) the "%c0" operand of annotate_reachable
> will be expanded to -102:
> 
>         -102:
>         .pushsection .discard.reachable
>         .long -102b - .
>         .popsection
> 
> This is a quirk of the gcc backend for s390, it interprets the %c0
> as a signed byte value. Avoid using operand modifiers in this case
> by simply converting __COUNTER__ to string, with the same result,
> but in an arch assembler independent way.
> 
> Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

Hi Vasily,

This patch causes these macros to break with Clang and
CONFIG_TRACE_BRANCH_PROFILING.

I get a lot of warnings like

  arch/x86/kernel/traps.o: warning: objtool: handle_xfd_event()+0x134: unreachable instruction

Without an asm input, 'volatile' is ignored for some reason and Clang
feels free to move the reachable() annotation away from its intended
location.

I wonder if we could go back to the original approach of providing
__COUNTER__ as an input to the asm, but then mask it to < 128.

Does this work on s390?

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 3d5af56337bd..42935500a712 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -115,24 +115,18 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
  * The __COUNTER__ based labels are a hack to make each instance of the macros
  * unique, to convince GCC not to merge duplicate inline asm statements.
  */
-#define __stringify_label(n) #n
-
-#define __annotate_reachable(c) ({					\
-	asm volatile(__stringify_label(c) ":\n\t"			\
+#define annotate_reachable() ({						\
+	asm volatile("%c0:\n\t"						\
 		     ".pushsection .discard.reachable\n\t"		\
-		     ".long " __stringify_label(c) "b - .\n\t"		\
-		     ".popsection\n\t");				\
+		     ".long %c0b - .\n\t"				\
+		     ".popsection\n\t" : : "i" (__COUNTER__ & 0x7f));	\
 })
-#define annotate_reachable() __annotate_reachable(__COUNTER__)
-
-#define __annotate_unreachable(c) ({					\
-	asm volatile(__stringify_label(c) ":\n\t"			\
+#define annotate_unreachable() ({					\
+	asm volatile("%c0:\n\t"						\
 		     ".pushsection .discard.unreachable\n\t"		\
-		     ".long " __stringify_label(c) "b - .\n\t"		\
-		     ".popsection\n\t");				\
+		     ".long %c0b - .\n\t"				\
+		     ".popsection\n\t" : : "i" (__COUNTER__ & 0x7f));	\
 })
-#define annotate_unreachable() __annotate_unreachable(__COUNTER__)
-
 #define ASM_UNREACHABLE							\
 	"999:\n\t"							\
 	".pushsection .discard.unreachable\n\t"				\

Re: [PATCH 1/2] compiler.h: Avoid using inline asm operand modifiers

[Vasily Gorbik]

On Fri, Nov 05, 2021 at 09:54:18AM -0700, Josh Poimboeuf wrote:
> On Wed, May 19, 2021 at 03:03:08PM +0200, Vasily Gorbik wrote:
> > The expansion of annotate_reachable/annotate_unreachable on s390 will
> > result in a compiler error if the __COUNTER__ value is high enough.
> > For example with "i" (154) the "%c0" operand of annotate_reachable
> > will be expanded to -102:
> > 
> >         -102:
> >         .pushsection .discard.reachable
> >         .long -102b - .
> >         .popsection
> > 
> > This is a quirk of the gcc backend for s390, it interprets the %c0
> > as a signed byte value. Avoid using operand modifiers in this case
> > by simply converting __COUNTER__ to string, with the same result,
> > but in an arch assembler independent way.
> > 
> > Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
> 
> This patch causes these macros to break with Clang and
> CONFIG_TRACE_BRANCH_PROFILING.
> 
> I get a lot of warnings like
> 
>   arch/x86/kernel/traps.o: warning: objtool: handle_xfd_event()+0x134: unreachable instruction
> 
> Without an asm input, 'volatile' is ignored for some reason and Clang
> feels free to move the reachable() annotation away from its intended
> location.
> 
> I wonder if we could go back to the original approach of providing
> __COUNTER__ as an input to the asm, but then mask it to < 128.
> 
> Does this work on s390?
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 3d5af56337bd..42935500a712 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -115,24 +115,18 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>   * The __COUNTER__ based labels are a hack to make each instance of the macros
>   * unique, to convince GCC not to merge duplicate inline asm statements.
>   */
> -#define __stringify_label(n) #n
> -
> -#define __annotate_reachable(c) ({					\
> -	asm volatile(__stringify_label(c) ":\n\t"			\
> +#define annotate_reachable() ({						\
> +	asm volatile("%c0:\n\t"						\
>  		     ".pushsection .discard.reachable\n\t"		\
> -		     ".long " __stringify_label(c) "b - .\n\t"		\
> -		     ".popsection\n\t");				\
> +		     ".long %c0b - .\n\t"				\
> +		     ".popsection\n\t" : : "i" (__COUNTER__ & 0x7f));	\
>  })

hm, could we just add asm input back and not use it? and keep
__stringify_label(c) as is? would that work as well?

Re: [PATCH 1/2] compiler.h: Avoid using inline asm operand modifiers

[Josh Poimboeuf]

On Mon, Nov 08, 2021 at 04:40:38PM +0100, Vasily Gorbik wrote:
> > Does this work on s390?
> > 
> > diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> > index 3d5af56337bd..42935500a712 100644
> > --- a/include/linux/compiler.h
> > +++ b/include/linux/compiler.h
> > @@ -115,24 +115,18 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
> >   * The __COUNTER__ based labels are a hack to make each instance of the macros
> >   * unique, to convince GCC not to merge duplicate inline asm statements.
> >   */
> > -#define __stringify_label(n) #n
> > -
> > -#define __annotate_reachable(c) ({					\
> > -	asm volatile(__stringify_label(c) ":\n\t"			\
> > +#define annotate_reachable() ({						\
> > +	asm volatile("%c0:\n\t"						\
> >  		     ".pushsection .discard.reachable\n\t"		\
> > -		     ".long " __stringify_label(c) "b - .\n\t"		\
> > -		     ".popsection\n\t");				\
> > +		     ".long %c0b - .\n\t"				\
> > +		     ".popsection\n\t" : : "i" (__COUNTER__ & 0x7f));	\
> >  })
> 
> hm, could we just add asm input back and not use it? and keep
> __stringify_label(c) as is? would that work as well?

Yeah, that seems to work:

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 3d5af56337bd..429dcebe2b99 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -121,7 +121,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 	asm volatile(__stringify_label(c) ":\n\t"			\
 		     ".pushsection .discard.reachable\n\t"		\
 		     ".long " __stringify_label(c) "b - .\n\t"		\
-		     ".popsection\n\t");				\
+		     ".popsection\n\t" : : "i" (c));			\
 })
 #define annotate_reachable() __annotate_reachable(__COUNTER__)
 
@@ -129,7 +129,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 	asm volatile(__stringify_label(c) ":\n\t"			\
 		     ".pushsection .discard.unreachable\n\t"		\
 		     ".long " __stringify_label(c) "b - .\n\t"		\
-		     ".popsection\n\t");				\
+		     ".popsection\n\t" : : "i" (c));			\
 })
 #define annotate_unreachable() __annotate_unreachable(__COUNTER__)
 
diff --git a/include/linux/instrumentation.h b/include/linux/instrumentation.h
index fa2cd8c63dcc..24359b4a9605 100644
--- a/include/linux/instrumentation.h
+++ b/include/linux/instrumentation.h
@@ -11,7 +11,7 @@
 	asm volatile(__stringify(c) ": nop\n\t"				\
 		     ".pushsection .discard.instr_begin\n\t"		\
 		     ".long " __stringify(c) "b - .\n\t"		\
-		     ".popsection\n\t");				\
+		     ".popsection\n\t" : : "i" (c));			\
 })
 #define instrumentation_begin() __instrumentation_begin(__COUNTER__)
 
@@ -50,7 +50,7 @@
 	asm volatile(__stringify(c) ": nop\n\t"				\
 		     ".pushsection .discard.instr_end\n\t"		\
 		     ".long " __stringify(c) "b - .\n\t"		\
-		     ".popsection\n\t");				\
+		     ".popsection\n\t" : : "i" (c));			\
 })
 #define instrumentation_end() __instrumentation_end(__COUNTER__)
 #else

RE: [PATCH 1/2] compiler.h: Avoid using inline asm operand modifiers

[David Laight]

From: Vasily Gorbik
> Sent: 08 November 2021 15:41
> 
> On Fri, Nov 05, 2021 at 09:54:18AM -0700, Josh Poimboeuf wrote:
> > On Wed, May 19, 2021 at 03:03:08PM +0200, Vasily Gorbik wrote:
> > > The expansion of annotate_reachable/annotate_unreachable on s390 will
> > > result in a compiler error if the __COUNTER__ value is high enough.
...
> > I wonder if we could go back to the original approach of providing
> > __COUNTER__ as an input to the asm, but then mask it to < 128.
...
> >   * The __COUNTER__ based labels are a hack to make each instance of the macros
> >   * unique, to convince GCC not to merge duplicate inline asm statements.

I've used asm comments get the same effect.
Annoyingly there isn't a standard comment delimiter.

I think you can just use a .equ (or .set) directive (to assign a value)
rather than putting a value into a 'to be discarded' section.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

[PATCH 2/2] instrumentation.h: Avoid using inline asm operand modifiers

[Vasily Gorbik]

The expansion of instrumentation_begin/instrumentation_end on s390 will
result in a compiler error if the __COUNTER__ value is high enough.
For example with "i" (154) the "%c0" operand of annotate_reachable
will be expanded to -102:

        -102:
        .pushsection .discard.instr_begin
        .long -102b - .
        .popsection

This is a quirk of the gcc backend for s390, it interprets the %c0
as a signed byte value. Avoid using operand modifiers in this case
by simply converting __COUNTER__ to string, with the same result,
but in an arch assembler independent way.

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
---
 include/linux/instrumentation.h | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/include/linux/instrumentation.h b/include/linux/instrumentation.h
index 93e2ad67fc10..fa2cd8c63dcc 100644
--- a/include/linux/instrumentation.h
+++ b/include/linux/instrumentation.h
@@ -4,13 +4,16 @@
 
 #if defined(CONFIG_DEBUG_ENTRY) && defined(CONFIG_STACK_VALIDATION)
 
+#include <linux/stringify.h>
+
 /* Begin/end of an instrumentation safe region */
-#define instrumentation_begin() ({					\
-	asm volatile("%c0: nop\n\t"						\
+#define __instrumentation_begin(c) ({					\
+	asm volatile(__stringify(c) ": nop\n\t"				\
 		     ".pushsection .discard.instr_begin\n\t"		\
-		     ".long %c0b - .\n\t"				\
-		     ".popsection\n\t" : : "i" (__COUNTER__));		\
+		     ".long " __stringify(c) "b - .\n\t"		\
+		     ".popsection\n\t");				\
 })
+#define instrumentation_begin() __instrumentation_begin(__COUNTER__)
 
 /*
  * Because instrumentation_{begin,end}() can nest, objtool validation considers
@@ -43,12 +46,13 @@
  * To avoid this, have _end() be a NOP instruction, this ensures it will be
  * part of the condition block and does not escape.
  */
-#define instrumentation_end() ({					\
-	asm volatile("%c0: nop\n\t"					\
+#define __instrumentation_end(c) ({					\
+	asm volatile(__stringify(c) ": nop\n\t"				\
 		     ".pushsection .discard.instr_end\n\t"		\
-		     ".long %c0b - .\n\t"				\
-		     ".popsection\n\t" : : "i" (__COUNTER__));		\
+		     ".long " __stringify(c) "b - .\n\t"		\
+		     ".popsection\n\t");				\
 })
+#define instrumentation_end() __instrumentation_end(__COUNTER__)
 #else
 # define instrumentation_begin()	do { } while(0)
 # define instrumentation_end()		do { } while(0)
-- 
2.25.4

Re: [PATCH 0/2] objtool annotations: Avoid using inline asm operand modifiers

[Josh Poimboeuf]

On Wed, May 19, 2021 at 03:03:02PM +0200, Vasily Gorbik wrote:
> Make objtool annotations arch assembler independent. Would be nice if this goes
> via objtool tree. I have further patches which depend on that.
> 
> Vasily Gorbik (2):
>   compiler.h: Avoid using inline asm operand modifiers
>   instrumentation.h: Avoid using inline asm operand modifiers
> 
>  include/linux/compiler.h        | 22 ++++++++++++++--------
>  include/linux/instrumentation.h | 20 ++++++++++++--------
>  2 files changed, 26 insertions(+), 16 deletions(-)

Thanks Vasily, looks good to me.  Running it through some testing.

-- 
Josh