x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2

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.

Enumerate the enabled FSGSBASE capability in bit 1 of AT_HWCAP2 in the ELF
aux vector. AT_HWCAP2 is already used by PPC for similar purposes.

The application can access it open coded or by using the getauxval()
function in newer versions of glibc.

[ tglx: Massaged changelog ]

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/1557309753-24073-18-git-send-email-chang.seok.bae@intel.com
Link: https://lkml.kernel.org/r/20200528201402.1708239-14-sashal@kernel.org

diff --git a/arch/x86/include/uapi/asm/hwcap2.h b/arch/x86/include/uapi/asm/hwcap2.h
index 8b2effe6efb82f0ac7e599a8ce8a1c5aa38bf6ec..5fdfcb47000f9bb5ac55345c92f8dc1655bac1f7 100644 (file)
--- 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 18857ceb6bcee0108bc33727108bc685a189f415..fca56129ddc7b1050c32f397576b12505ac6a02f 100644 (file)
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1512,8 +1512,10 @@ 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.