From: Borislav Petkov <b...@suse.de>
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.
Signed-off-by: Borislav Petkov <b...@suse.de>
---
arch/x86/include/asm/cpufeature.h | 30 ++++++++++++++++++++++++++++--
arch/x86/kernel/cpu/common.c | 8 ++++++++
2 files changed, 36 insertions(+), 2 deletions(-)
diff --git a/arch/x86/include/asm/cpufeature.h
b/arch/x86/include/asm/cpufeature.h
index 043d61e66efc..eba671c0e5b1 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -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_MISC_DEBUG_CHECKS
+ /*
+ * 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_MISC_DEBUG_CHECKS
+ 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"
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 6a0361d7132c..407510155f40 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1365,3 +1365,11 @@ void __cpuinit cpu_init(void)
fpu_init();
}
#endif
+
+#ifdef CONFIG_X86_MISC_DEBUG_CHECKS
+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
--
1.8.3
--
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