[glibc] aarch64: Use future HWCAP2_MTE in ifunc resolver

[glibc] aarch64: Use future HWCAP2_MTE in ifunc resolver

[Szabolcs Nagy]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=2dc33b928b389f50e7fd8cadd952b79112a071ab

commit 2dc33b928b389f50e7fd8cadd952b79112a071ab
Author: Szabolcs Nagy <szabolcs.nagy@arm.com>
Date:   Fri Jul 24 12:37:23 2020 +0100

    aarch64: Use future HWCAP2_MTE in ifunc resolver
    
    Make glibc MTE-safe on systems where MTE is available. This allows
    using heap tagging with an LD_PRELOADed malloc implementation that
    enables MTE. We don't document this as guaranteed contract yet, so
    glibc may not be MTE safe when HWCAP2_MTE is set (older glibcs
    certainly aren't). This is mainly for testing and debugging.
    
    The HWCAP flag is not exposed in public headers until Linux adds it
    to its uapi. The HWCAP value reservation will be in Linux 5.9.

Diff:
---
 sysdeps/aarch64/multiarch/strlen.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/sysdeps/aarch64/multiarch/strlen.c b/sysdeps/aarch64/multiarch/strlen.c
index 7c0352dd87..9440decf75 100644
--- a/sysdeps/aarch64/multiarch/strlen.c
+++ b/sysdeps/aarch64/multiarch/strlen.c
@@ -26,8 +26,14 @@
 # include <string.h>
 # include <init-arch.h>
 
-/* This should check HWCAP_MTE when it is available.  */
-#define MTE_ENABLED() (false)
+/* This should check HWCAP2_MTE when it is available: current
+   linux kernel does not expose it, but its value is reserved.
+   This is needed to make glibc MTE-safe on future systems in
+   case user code enables MTE. The ABI contract for enabling
+   MTE is not yet specified, but it can be useful for at least
+   debugging which does not need a contract.  */
+#define FUTURE_HWCAP2_MTE (1 << 18)
+#define MTE_ENABLED() (GLRO(dl_hwcap2) & FUTURE_HWCAP2_MTE)
 
 extern __typeof (__redirect_strlen) __strlen;