[PATCH 06/12] aarch64: fix RTLD_START for BTI

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

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From 1e8662264c07e69d807761882e8d77f0916ae562 Mon Sep 17 00:00:00 2001
From: Szabolcs Nagy <szabolcs.nagy@arm.com>
Date: Tue, 31 Mar 2020 17:32:14 +0100
Subject: [PATCH 06/12] aarch64: fix RTLD_START for BTI

Tailcalls must use x16 or x17 for the indirect branch instruction
to be compatible with code that uses BTI c at function entries.
(Other forms of indirect branches can only land on BTI j.)

Also added a BTI c at the ELF entry point of rtld, this is not
strictly necessary since the kernel does not use indirect branch
to get there, but it seems safest once building glibc itself with
BTI is supported.
---
 sysdeps/aarch64/dl-machine.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/sysdeps/aarch64/dl-machine.h b/sysdeps/aarch64/dl-machine.h
index db3335e5ad..70b9ed3925 100644
--- a/sysdeps/aarch64/dl-machine.h
+++ b/sysdeps/aarch64/dl-machine.h
@@ -125,6 +125,8 @@ elf_machine_runtime_setup (struct link_map *l, int lazy, int profile)
 .globl _dl_start_user							\n\
 .type _dl_start_user, %function						\n\
 _start:									\n\
+	// bti c							\n\
+	hint	34							\n\
 	mov	" PTR "0, " PTR_SP "					\n\
 	bl	_dl_start						\n\
 	// returns user entry point in x0				\n\
@@ -178,7 +180,8 @@ _dl_start_user:								\n\
 	adrp	x0, _dl_fini						\n\
 	add	" PTR "0, " PTR "0, #:lo12:_dl_fini			\n\
 	// jump to the user_s entry point				\n\
-	br      x21							\n\
+	mov     x16, x21						\n\
+	br      x16							\n\
 ");
 
 #define elf_machine_type_class(type)					\
-- 
2.17.1