x86: Set FSGSBASE to active if enabled by kernel

Linux kernel uses AT_HWCAP2 to indicate if FSGSBASE instructions are
enabled.  If the HWCAP2_FSGSBASE bit in AT_HWCAP2 is set, FSGSBASE
instructions can be used in user space.  Define dl_check_hwcap2 to set
the FSGSBASE feature to active on Linux when the HWCAP2_FSGSBASE bit is
set.

Add a test to verify that FSGSBASE is active on current kernels.
NB: This test will fail if the kernel doesn't set the HWCAP2_FSGSBASE
bit in AT_HWCAP2 while fsgsbase shows up in /proc/cpuinfo.
Reviewed-by: Florian Weimer <fweimer@redhat.com>

1 files changed, 27 insertions, 0 deletions

diff --git a/sysdeps/unix/sysv/linux/x86/dl-hwcap2.h b/sysdeps/unix/sysv/linux/x86/dl-hwcap2.h
new file mode 100644
index 0000000..28f38ea
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/x86/dl-hwcap2.h
@@ -0,0 +1,27 @@
+/* Handle AT_HWCAP2.  Linux/x86 version.
+   Copyright (C) 2023 Free Software Foundation, Inc.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* The following must match the kernel's <asm/hwcap2.h>.  */
+#define HWCAP2_RING3MWAIT	(1 << 0)
+#define HWCAP2_FSGSBASE		(1 << 1)
+
+static inline void
+dl_check_hwcap2 (struct cpu_features *cpu_features)
+{
+  if ((GLRO(dl_hwcap2) & HWCAP2_FSGSBASE) != 0)
+    CPU_FEATURE_SET_ACTIVE (cpu_features, FSGSBASE);
+}