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x86: Sanity-check static_cpu_has usage

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static_cpu_has may be used only after alternatives have run. Before that
it always returns false if constant folding with __builtin_constant_p()
doesn't happen. And you don't want that.

This patch is the result of me debugging an issue where I overzealously
put static_cpu_has in code which executed before alternatives have run
and had to spend some time with scratching head and cursing at the
monitor.

So add a jump to a warning which screams loudly when we use this
function too early. The alternatives patch that check away in
conjunction with patching the rest of the kernel image.

[ hpa: factored this into its own configuration option.  If we want to
  have an overarching option, it should be an option which selects
  other options, not as a group option in the source code. ]

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1370772454-6106-4-git-send-email-bp@alien8.de
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
This commit is contained in:
Borislav Petkov 2013-06-09 12:07:32 +02:00 committed by H. Peter Anvin
parent c3b83598c1
commit 5700f743b5
3 changed files with 46 additions and 2 deletions
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10
arch/x86/Kconfig.debug
View File

@ -304,4 +304,14 @@ config DEBUG_NMI_SELFTEST
If unsure, say N.
config X86_DEBUG_STATIC_CPU_HAS
bool "Debug alternatives"
depends on DEBUG_KERNEL
---help---
This option causes additional code to be generated which
fails if static_cpu_has() is used before alternatives have
run.
If unsure, say N.
endmenu

30
arch/x86/include/asm/cpufeature.h
View File

@ -356,15 +356,35 @@ extern const char * const x86_power_flags[32];
#endif /* CONFIG_X86_64 */
#if __GNUC__ >= 4
extern void warn_pre_alternatives(void);
/*
* Static testing of CPU features. Used the same as boot_cpu_has().
* These are only valid after alternatives have run, but will statically
* patch the target code for additional performance.
*
*/
static __always_inline __pure bool __static_cpu_has(u16 bit)
{
#if __GNUC__ > 4 || __GNUC_MINOR__ >= 5
#ifdef CONFIG_X86_DEBUG_STATIC_CPU_HAS
/*
* Catch too early usage of this before alternatives
* have run.
*/
asm goto("1: jmp %l[t_warn]\n"
"2:\n"
".section .altinstructions,\"a\"\n"
" .long 1b - .\n"
" .long 0\n" /* no replacement */
" .word %P0\n" /* 1: do replace */
" .byte 2b - 1b\n" /* source len */
" .byte 0\n" /* replacement len */
".previous\n"
/* skipping size check since replacement size = 0 */
: : "i" (X86_FEATURE_ALWAYS) : : t_warn);
#endif
asm goto("1: jmp %l[t_no]\n"
"2:\n"
".section .altinstructions,\"a\"\n"
@ -379,7 +399,13 @@ static __always_inline __pure bool __static_cpu_has(u16 bit)
return true;
t_no:
return false;
#else
#ifdef CONFIG_X86_DEBUG_STATIC_CPU_HAS
t_warn:
warn_pre_alternatives();
return false;
#endif
#else /* GCC_VERSION >= 40500 */
u8 flag;
/* Open-coded due to __stringify() in ALTERNATIVE() */
asm volatile("1: movb $0,%0\n"

8
arch/x86/kernel/cpu/common.c
View File

@ -1364,3 +1364,11 @@ void __cpuinit cpu_init(void)
fpu_init();
}
#endif
#ifdef CONFIG_X86_DEBUG_STATIC_CPU_HAS
void warn_pre_alternatives(void)
{
WARN(1, "You're using static_cpu_has before alternatives have run!\n");
}
EXPORT_SYMBOL_GPL(warn_pre_alternatives);
#endif