[PATCH v1 4/4] arm64/unwind_user/sframe: Enable sframe unwinding on arm64

[Jens Remus]

Add arm64 support for unwinding of user space using SFrame.

This leverages the unwind user (sframe) support for s390 which
enables architectures that pass the return address in a register,
may not necessarily save the return address on the stack (for
instance in leaf functions), and have SP at call site equal
SP at entry.

For this purpose provide arm64-specific unwind_user_get_ra_reg() and
unwind_user_get_reg() implementations, which return the value of the
link register (LR) or an arbitrary register in the topmost user space
frame.  Define the arm64 SP and FP DWARF register numbers.

Signed-off-by: Jens Remus <jremus at linux.ibm.com>
---

Notes (jremus):
    Note:  An arm64 implementation of unwind_user_get_reg() is strictly
    only needed, if SFrame V3 flexible FDE would get generated for aarch64,
    which is currently not the case in GNU Binutils 2.46.

 arch/arm64/Kconfig                          |  1 +
 arch/arm64/include/asm/unwind_user.h        | 23 +++++++++++++++++++++
 arch/arm64/include/asm/unwind_user_sframe.h |  8 +++++++
 3 files changed, 32 insertions(+)
 create mode 100644 arch/arm64/include/asm/unwind_user_sframe.h

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 994fd5162a1d..641a3a5fe5c9 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -254,6 +254,7 @@ config ARM64
 	select HAVE_STACKPROTECTOR
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_UNWIND_USER_FP
+	select HAVE_UNWIND_USER_SFRAME
 	select HAVE_KPROBES
 	select HAVE_KRETPROBES
 	select HAVE_GENERIC_VDSO
diff --git a/arch/arm64/include/asm/unwind_user.h b/arch/arm64/include/asm/unwind_user.h
index 0641d4d97b0f..3c7fd8c4ba5b 100644
--- a/arch/arm64/include/asm/unwind_user.h
+++ b/arch/arm64/include/asm/unwind_user.h
@@ -4,6 +4,7 @@
 
 #include <linux/sched/task_stack.h>
 #include <linux/types.h>
+#include <asm/insn.h>
 
 #ifdef CONFIG_UNWIND_USER
 
@@ -16,6 +17,28 @@ static inline int unwind_user_word_size(struct pt_regs *regs)
 	return sizeof(long);
 }
 
+static inline int unwind_user_get_ra_reg(unsigned long *val)
+{
+	struct pt_regs *regs = task_pt_regs(current);
+	*val = regs->regs[AARCH64_INSN_REG_LR];
+	return 0;
+}
+#define unwind_user_get_ra_reg unwind_user_get_ra_reg
+
+static inline int unwind_user_get_reg(unsigned long *val, unsigned int regnum)
+{
+	const struct pt_regs *regs = task_pt_regs(current);
+
+	if (regnum <= 30)
+		/* DWARF register numbers 0..15 */
+		*val = regs->regs[regnum];
+	else
+		return -EINVAL;
+
+	return 0;
+}
+#define unwind_user_get_reg unwind_user_get_reg
+
 #endif /* CONFIG_UNWIND_USER */
 
 #ifdef CONFIG_HAVE_UNWIND_USER_FP
diff --git a/arch/arm64/include/asm/unwind_user_sframe.h b/arch/arm64/include/asm/unwind_user_sframe.h
new file mode 100644
index 000000000000..65c0a6b6c835
--- /dev/null
+++ b/arch/arm64/include/asm/unwind_user_sframe.h
@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_ARM64_UNWIND_USER_SFRAME_H
+#define _ASM_ARM64_UNWIND_USER_SFRAME_H
+
+#define SFRAME_REG_SP	31
+#define SFRAME_REG_FP	29
+
+#endif /* _ASM_ARM64_UNWIND_USER_SFRAME_H */
-- 
2.51.0