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>

3 files changed, 31 insertions, 0 deletions

diff --git a/sysdeps/x86/cpu-features.c b/sysdeps/x86/cpu-features.c
index 978eb29f72..95ad48b501 100644
--- a/sysdeps/x86/cpu-features.c
+++ b/sysdeps/x86/cpu-features.c
@@ -22,6 +22,7 @@
 #include <cacheinfo.h>
 #include <dl-cacheinfo.h>
 #include <dl-minsigstacksize.h>
+#include <dl-hwcap2.h>
 
 extern void TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *)
   attribute_hidden;
@@ -299,6 +300,8 @@ update_active (struct cpu_features *cpu_features)
       CPU_FEATURE_SET_ACTIVE (cpu_features, WIDE_KL);
     }
 
+  dl_check_hwcap2 (cpu_features);
+
   cpu_features->isa_1 = get_isa_level (cpu_features);
 }
 
diff --git a/sysdeps/x86/dl-hwcap2.h b/sysdeps/x86/dl-hwcap2.h
new file mode 100644
index 0000000000..73cd6cc37d
--- /dev/null
+++ b/sysdeps/x86/dl-hwcap2.h
@@ -0,0 +1,21 @@
+/* Handle AT_HWCAP2.  Generic 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/>.  */
+
+static inline void
+dl_check_hwcap2 (struct cpu_features *cpu_features)
+{
+}
diff --git a/sysdeps/x86/tst-cpu-features-cpuinfo.c b/sysdeps/x86/tst-cpu-features-cpuinfo.c
index e963592c4b..18d64375ca 100644
--- a/sysdeps/x86/tst-cpu-features-cpuinfo.c
+++ b/sysdeps/x86/tst-cpu-features-cpuinfo.c
@@ -96,6 +96,11 @@ check_proc (const char *proc_name, const char *search_name, int flag,
 	      CPU_FEATURE_USABLE (name), \
 	      "HAS_CPU_FEATURE (" #name ")")
 
+#define CHECK_PROC_ACTIVE(str, name) \
+  check_proc (#str, " "#str" ", CPU_FEATURE_USABLE (name), \
+	      CPU_FEATURE_USABLE (name), \
+	      "CPU_FEATURE_USABLE (" #name ")")
+
 static int
 do_test (int argc, char **argv)
 {
@@ -298,6 +303,8 @@ do_test (int argc, char **argv)
   fails += CHECK_PROC (xsaves, XSAVES);
   fails += CHECK_PROC (xtpr, XTPRUPDCTRL);
 
+  fails += CHECK_PROC_ACTIVE (fsgsbase, FSGSBASE);
+
   printf ("%d differences between /proc/cpuinfo and glibc code.\n", fails);
 
   return (fails != 0);