Commit d19e789f authored by Ingo Molnar's avatar Ingo Molnar
Browse files

compiler.h: Move instrumentation_begin()/end() to new <linux/instrumentation.h> header 



Linus pointed out that compiler.h - which is a key header that gets included in every
single one of the 28,000+ kernel files during a kernel build - was bloated in:

  65538966: ("vmlinux.lds.h: Create section for protection against instrumentation")

Linus noted:

 > I have pulled this, but do we really want to add this to a header file
 > that is _so_ core that it gets included for basically every single
 > file built?
 >
 > I don't even see those instrumentation_begin/end() things used
 > anywhere right now.
 >
 > It seems excessive. That 53 lines is maybe not a lot, but it pushed
 > that header file to over 12kB, and while it's mostly comments, it's
 > extra IO and parsing basically for _every_ single file compiled in the
 > kernel.
 >
 > For what appears to be absolutely zero upside right now, and I really
 > don't see why this should be in such a core header file!

Move these primitives into a new header: <linux/instrumentation.h>, and include that
header in the headers that make use of it.

Unfortunately one of these headers is asm-generic/bug.h, which does get included
in a lot of places, similarly to compiler.h. So the de-bloating effect isn't as
good as we'd like it to be - but at least the interfaces are defined separately.

No change to functionality intended.

Reported-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200604071921.GA1361070@gmail.com
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
parent f37e99ac

arch/x86/include/asm/bug.h

+1 −0
Original line number Diff line number Diff line
@@ -3,6 +3,7 @@ 
#define _ASM_X86_BUG_H



#include <linux/stringify.h>

#include <linux/instrumentation.h>



/*

 * Despite that some emulators terminate on UD2, we use it for WARN().

include/asm-generic/bug.h

+1 −0
Original line number Diff line number Diff line
@@ -3,6 +3,7 @@ 
#define _ASM_GENERIC_BUG_H



#include <linux/compiler.h>

#include <linux/instrumentation.h>



#define CUT_HERE		"------------[ cut here ]------------\n"

include/linux/compiler.h

+0 −53
Original line number Diff line number Diff line
@@ -120,65 +120,12 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val, 
/* Annotate a C jump table to allow objtool to follow the code flow */

#define __annotate_jump_table __section(.rodata..c_jump_table)



#ifdef CONFIG_DEBUG_ENTRY

/* Begin/end of an instrumentation safe region */

#define instrumentation_begin() ({					\

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

		     ".pushsection .discard.instr_begin\n\t"		\

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

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

})



/*

 * Because instrumentation_{begin,end}() can nest, objtool validation considers

 * _begin() a +1 and _end() a -1 and computes a sum over the instructions.

 * When the value is greater than 0, we consider instrumentation allowed.

 *

 * There is a problem with code like:

 *

 * noinstr void foo()

 * {

 *	instrumentation_begin();

 *	...

 *	if (cond) {

 *		instrumentation_begin();

 *		...

 *		instrumentation_end();

 *	}

 *	bar();

 *	instrumentation_end();

 * }

 *

 * If instrumentation_end() would be an empty label, like all the other

 * annotations, the inner _end(), which is at the end of a conditional block,

 * would land on the instruction after the block.

 *

 * If we then consider the sum of the !cond path, we'll see that the call to

 * bar() is with a 0-value, even though, we meant it to happen with a positive

 * value.

 *

 * 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"					\

		     ".pushsection .discard.instr_end\n\t"		\

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

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

})

#endif /* CONFIG_DEBUG_ENTRY */



#else

#define annotate_reachable()

#define annotate_unreachable()

#define __annotate_jump_table

#endif



#ifndef instrumentation_begin

#define instrumentation_begin()		do { } while(0)

#define instrumentation_end()		do { } while(0)

#endif



#ifndef ASM_UNREACHABLE

# define ASM_UNREACHABLE

#endif

include/linux/context_tracking.h

+2 −0
Original line number Diff line number Diff line
@@ -5,6 +5,8 @@ 
#include <linux/sched.h>

#include <linux/vtime.h>

#include <linux/context_tracking_state.h>

#include <linux/instrumentation.h>



#include <asm/ptrace.h>

include/linux/instrumentation.h

0 → 100644
+57 −0
Original line number Diff line number Diff line 
/* SPDX-License-Identifier: GPL-2.0 */

#ifndef __LINUX_INSTRUMENTATION_H

#define __LINUX_INSTRUMENTATION_H



#if defined(CONFIG_DEBUG_ENTRY) && defined(CONFIG_STACK_VALIDATION)



/* Begin/end of an instrumentation safe region */

#define instrumentation_begin() ({					\

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

		     ".pushsection .discard.instr_begin\n\t"		\

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

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

})



/*

 * Because instrumentation_{begin,end}() can nest, objtool validation considers

 * _begin() a +1 and _end() a -1 and computes a sum over the instructions.

 * When the value is greater than 0, we consider instrumentation allowed.

 *

 * There is a problem with code like:

 *

 * noinstr void foo()

 * {

 *	instrumentation_begin();

 *	...

 *	if (cond) {

 *		instrumentation_begin();

 *		...

 *		instrumentation_end();

 *	}

 *	bar();

 *	instrumentation_end();

 * }

 *

 * If instrumentation_end() would be an empty label, like all the other

 * annotations, the inner _end(), which is at the end of a conditional block,

 * would land on the instruction after the block.

 *

 * If we then consider the sum of the !cond path, we'll see that the call to

 * bar() is with a 0-value, even though, we meant it to happen with a positive

 * value.

 *

 * 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"					\

		     ".pushsection .discard.instr_end\n\t"		\

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

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

})

#else

# define instrumentation_begin()	do { } while(0)

# define instrumentation_end()		do { } while(0)

#endif



#endif /* __LINUX_INSTRUMENTATION_H */

CRA Git | Maintained and supported by SUSTech CRA and CCSE