aarch64: Add sysv specific enabling code for memory tagging

Add various defines and stubs for enabling MTE on AArch64 sysv-like
systems such as Linux.  The HWCAP feature bit is copied over in the
same way as other feature bits.  Similarly we add a new wrapper header
for mman.h to define the PROT_MTE flag that can be used with mmap and
related functions.

We add a new field to struct cpu_features that can be used, for
example, to check whether or not certain ifunc'd routines should be
bound to MTE-safe versions.

Finally, if we detect that MTE should be enabled (ie via the glibc
tunable); we enable MTE during startup as required.

Support in the Linux kernel was added in version 5.10.

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>

4 files changed, 34 insertions, 0 deletions

diff --git a/sysdeps/unix/sysv/linux/aarch64/bits/hwcap.h b/sysdeps/unix/sysv/linux/aarch64/bits/hwcap.h
index af90d8a..389852f 100644
--- a/sysdeps/unix/sysv/linux/aarch64/bits/hwcap.h
+++ b/sysdeps/unix/sysv/linux/aarch64/bits/hwcap.h
@@ -73,3 +73,4 @@
 #define HWCAP2_DGH		(1 << 15)
 #define HWCAP2_RNG		(1 << 16)
 #define HWCAP2_BTI		(1 << 17)
+#define HWCAP2_MTE		(1 << 18)
diff --git a/sysdeps/unix/sysv/linux/aarch64/bits/mman.h b/sysdeps/unix/sysv/linux/aarch64/bits/mman.h
index ecae046..c5ec0aa 100644
--- a/sysdeps/unix/sysv/linux/aarch64/bits/mman.h
+++ b/sysdeps/unix/sysv/linux/aarch64/bits/mman.h
@@ -24,6 +24,7 @@
    arch/arm64/include/uapi/asm/mman.h.  */
 
 #define PROT_BTI	0x10
+#define PROT_MTE	0x20
 
 #include <bits/mman-map-flags-generic.h>
 
diff --git a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
index b9ab827..bd899c4 100644
--- a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
+++ b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
@@ -19,10 +19,17 @@
 #include <cpu-features.h>
 #include <sys/auxv.h>
 #include <elf/dl-hwcaps.h>
+#include <sys/prctl.h>
 
 #define DCZID_DZP_MASK (1 << 4)
 #define DCZID_BS_MASK (0xf)
 
+/* The maximal set of permitted tags that the MTE random tag generation
+   instruction may use.  We exclude tag 0 because a) we want to reserve
+   that for the libc heap structures and b) because it makes it easier
+   to see when pointer have been correctly tagged.  */
+#define MTE_ALLOWED_TAGS (0xfffe << PR_MTE_TAG_SHIFT)
+
 #if HAVE_TUNABLES
 struct cpu_list
 {
@@ -86,4 +93,27 @@ init_cpu_features (struct cpu_features *cpu_features)
 
   /* Check if BTI is supported.  */
   cpu_features->bti = GLRO (dl_hwcap2) & HWCAP2_BTI;
+
+  /* Setup memory tagging support if the HW and kernel support it, and if
+     the user has requested it.  */
+  cpu_features->mte_state = 0;
+
+#ifdef USE_MTAG
+# if HAVE_TUNABLES
+  int mte_state = TUNABLE_GET (glibc, mem, tagging, unsigned, 0);
+  cpu_features->mte_state = (GLRO (dl_hwcap2) & HWCAP2_MTE) ? mte_state : 0;
+  /* If we lack the MTE feature, disable the tunable, since it will
+     otherwise cause instructions that won't run on this CPU to be used.  */
+  TUNABLE_SET (glibc, mem, tagging, unsigned, cpu_features->mte_state);
+# endif
+
+  if (cpu_features->mte_state & 2)
+    __prctl (PR_SET_TAGGED_ADDR_CTRL,
+	     (PR_TAGGED_ADDR_ENABLE | PR_MTE_TCF_SYNC | MTE_ALLOWED_TAGS),
+	     0, 0, 0);
+  else if (cpu_features->mte_state)
+    __prctl (PR_SET_TAGGED_ADDR_CTRL,
+	     (PR_TAGGED_ADDR_ENABLE | PR_MTE_TCF_ASYNC | MTE_ALLOWED_TAGS),
+	     0, 0, 0);
+#endif
 }
diff --git a/sysdeps/unix/sysv/linux/aarch64/cpu-features.h b/sysdeps/unix/sysv/linux/aarch64/cpu-features.h
index 00a4d0c..bebf321 100644
--- a/sysdeps/unix/sysv/linux/aarch64/cpu-features.h
+++ b/sysdeps/unix/sysv/linux/aarch64/cpu-features.h
@@ -70,6 +70,8 @@ struct cpu_features
   uint64_t midr_el1;
   unsigned zva_size;
   bool bti;
+  /* Currently, the GLIBC memory tagging tunable only defines 8 bits.  */
+  uint8_t mte_state;
 };
 
 #endif /* _CPU_FEATURES_AARCH64_H  */