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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:

  655389666643: ("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: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo 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>
This commit is contained in:
Ingo Molnar 2020-07-24 13:50:25 +02:00
parent f37e99aca0
commit d19e789f06
5 changed files with 61 additions and 53 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
arch/x86/include/asm/bug.h
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@ -3,6 +3,7 @@
#define _ASM_X86_BUG_H #define _ASM_X86_BUG_H
#include <linux/stringify.h> #include <linux/stringify.h>
#include <linux/instrumentation.h>
/* /*
* Despite that some emulators terminate on UD2, we use it for WARN(). * Despite that some emulators terminate on UD2, we use it for WARN().

1
include/asm-generic/bug.h
View File

@ -3,6 +3,7 @@
#define _ASM_GENERIC_BUG_H #define _ASM_GENERIC_BUG_H
#include <linux/compiler.h> #include <linux/compiler.h>
#include <linux/instrumentation.h>
#define CUT_HERE "------------[ cut here ]------------\n" #define CUT_HERE "------------[ cut here ]------------\n"

53
include/linux/compiler.h
View File

@ -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 */ /* Annotate a C jump table to allow objtool to follow the code flow */
#define __annotate_jump_table __section(.rodata..c_jump_table) #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 #else
#define annotate_reachable() #define annotate_reachable()
#define annotate_unreachable() #define annotate_unreachable()
#define __annotate_jump_table #define __annotate_jump_table
#endif #endif
#ifndef instrumentation_begin
#define instrumentation_begin() do { } while(0)
#define instrumentation_end() do { } while(0)
#endif
#ifndef ASM_UNREACHABLE #ifndef ASM_UNREACHABLE
# define ASM_UNREACHABLE # define ASM_UNREACHABLE
#endif #endif

2
include/linux/context_tracking.h
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@ -5,6 +5,8 @@
#include <linux/sched.h> #include <linux/sched.h>
#include <linux/vtime.h> #include <linux/vtime.h>
#include <linux/context_tracking_state.h> #include <linux/context_tracking_state.h>
#include <linux/instrumentation.h>
#include <asm/ptrace.h> #include <asm/ptrace.h>

57
include/linux/instrumentation.h Normal file
View File

@ -0,0 +1,57 @@
/* 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 */