Commit c06c4d80 authored by Nadav Amit's avatar Nadav Amit Committed by Ingo Molnar
Browse files

x86/objtool: Use asm macros to work around GCC inlining bugs 



As described in:

  77b0bf55: ("kbuild/Makefile: Prepare for using macros in inline assembly code to work around asm() related GCC inlining bugs")

GCC's inlining heuristics are broken with common asm() patterns used in
kernel code, resulting in the effective disabling of inlining.

In the case of objtool the resulting borkage can be significant, since all the
annotations of objtool are discarded during linkage and never inlined,
yet GCC bogusly considers most functions affected by objtool annotations
as 'too large'.

The workaround is to set an assembly macro and call it from the inline
assembly block. As a result GCC considers the inline assembly block as
a single instruction. (Which it isn't, but that's the best we can get.)

This increases the kernel size slightly:

      text     data     bss      dec     hex filename
  18140829 10224724 2957312 31322865 1ddf2f1 ./vmlinux before
  18140970 10225412 2957312 31323694 1ddf62e ./vmlinux after (+829)

The number of static text symbols (i.e. non-inlined functions) is reduced:

  Before:  40321
  After:   40302 (-19)

[ mingo: Rewrote the changelog. ]

Tested-by: default avatarKees Cook <keescook@chromium.org>
Signed-off-by: default avatarNadav Amit <namit@vmware.com>
Reviewed-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Christopher Li <sparse@chrisli.org>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-sparse@vger.kernel.org
Link: http://lkml.kernel.org/r/20181003213100.189959-4-namit@vmware.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 77b0bf55

arch/x86/kernel/macros.S

+2 −0
Original line number Original line Diff line number Diff line
@@ -5,3 +5,5 @@ 
 * commonly used. The macros are precompiled into assmebly file which is later

 * commonly used. The macros are precompiled into assmebly file which is later

 * assembled together with each compiled file.

 * assembled together with each compiled file.

 */

 */



#include <linux/compiler.h>

include/linux/compiler.h

+43 −13
Original line number Original line Diff line number Diff line
@@ -99,22 +99,13 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val, 
 * unique, to convince GCC not to merge duplicate inline asm statements.

 * unique, to convince GCC not to merge duplicate inline asm statements.

 */

 */

#define annotate_reachable() ({						\

#define annotate_reachable() ({						\

	asm volatile("%c0:\n\t"						\

	asm volatile("ANNOTATE_REACHABLE counter=%c0"			\

		     ".pushsection .discard.reachable\n\t"		\

		     : : "i" (__COUNTER__));				\

		     ".long %c0b - .\n\t"				\

		     ".popsection\n\t" : : "i" (__COUNTER__));		\

})

})

#define annotate_unreachable() ({					\

#define annotate_unreachable() ({					\

	asm volatile("%c0:\n\t"						\

	asm volatile("ANNOTATE_UNREACHABLE counter=%c0"			\

		     ".pushsection .discard.unreachable\n\t"		\

		     : : "i" (__COUNTER__));				\

		     ".long %c0b - .\n\t"				\

		     ".popsection\n\t" : : "i" (__COUNTER__));		\

})

})

#define ASM_UNREACHABLE							\

	"999:\n\t"							\

	".pushsection .discard.unreachable\n\t"				\

	".long 999b - .\n\t"						\

	".popsection\n\t"

#else

#else

#define annotate_reachable()

#define annotate_reachable()

#define annotate_unreachable()

#define annotate_unreachable()
@@ -299,6 +290,45 @@ static inline void *offset_to_ptr(const int *off) 
	return (void *)((unsigned long)off + *off);

	return (void *)((unsigned long)off + *off);

}

}





#else /* __ASSEMBLY__ */



#ifdef __KERNEL__

#ifndef LINKER_SCRIPT



#ifdef CONFIG_STACK_VALIDATION

.macro ANNOTATE_UNREACHABLE counter:req

\counter:

	.pushsection .discard.unreachable

	.long \counter\()b -.

	.popsection

.endm



.macro ANNOTATE_REACHABLE counter:req

\counter:

	.pushsection .discard.reachable

	.long \counter\()b -.

	.popsection

.endm



.macro ASM_UNREACHABLE

999:

	.pushsection .discard.unreachable

	.long 999b - .

	.popsection

.endm

#else /* CONFIG_STACK_VALIDATION */

.macro ANNOTATE_UNREACHABLE counter:req

.endm



.macro ANNOTATE_REACHABLE counter:req

.endm



.macro ASM_UNREACHABLE

.endm

#endif /* CONFIG_STACK_VALIDATION */



#endif /* LINKER_SCRIPT */

#endif /* __KERNEL__ */

#endif /* __ASSEMBLY__ */

#endif /* __ASSEMBLY__ */





#ifndef __optimize

#ifndef __optimize

CRA Git | Maintained and supported by SUSTech CRA and CCSE