[PATCH v7 17/18] x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2

[Chang S. Bae]

From: Andi Kleen <ak@xxxxxxxxxxxxxxx>

The kernel needs to explicitly enable FSGSBASE. So, the application needs
to know if it can safely use these instructions. Just looking at the CPUID
bit is not enough because it may be running in a kernel that does not
enable the instructions.

One way for the application would be to just try and catch the SIGILL.
But that is difficult to do in libraries which may not want to overwrite
the signal handlers of the main application.

So we need to provide a way for the application to discover the kernel
capability.

I used AT_HWCAP2 in the ELF aux vector which is already used by PPC for
similar things. We define a new Linux defined bitmap returned in AT_HWCAP.
Next to MONITOR/MWAIT, bit 1 is reserved for FSGSBASE capability checks.

The application can then access it manually or using the getauxval()
function in newer glibc.

Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
---
 arch/x86/include/uapi/asm/hwcap2.h | 3 +++
 arch/x86/kernel/cpu/common.c       | 4 +++-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/uapi/asm/hwcap2.h b/arch/x86/include/uapi/asm/hwcap2.h
index 6ebaae9..c5ce54e 100644
--- a/arch/x86/include/uapi/asm/hwcap2.h
+++ b/arch/x86/include/uapi/asm/hwcap2.h
@@ -5,4 +5,7 @@
 /* MONITOR/MWAIT enabled in Ring 3 */
 #define HWCAP2_RING3MWAIT		(1 << 0)
 
+/* Kernel allows FSGSBASE instructions available in Ring 3 */
+#define HWCAP2_FSGSBASE			BIT(1)
+
 #endif
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index fd7ef49..f8b1d7a 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1361,8 +1361,10 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	setup_umip(c);
 
 	/* Enable FSGSBASE instructions if available. */
-	if (cpu_has(c, X86_FEATURE_FSGSBASE))
+	if (cpu_has(c, X86_FEATURE_FSGSBASE)) {
 		cr4_set_bits(X86_CR4_FSGSBASE);
+		elf_hwcap2 |= HWCAP2_FSGSBASE;
+	}
 
 	/*
 	 * The vendor-specific functions might have changed features.
-- 
2.7.4