[PATCH] arm64: fix the address of syscall in arch_syscall_addr if CFI is enabled

[Ben Dai <ben.dai9703@gmail.com>]

With CONFIG_CFI_CLANG, the addresses in sys_call_table[] actually point to
jump instructions like "b __arm64_sys_*", and if CONFIG_LTO_CLANG_FULL is
enabled, the compiler will not generate a symbol for each jump. It causes
syscall tracer can't get symbol name in find_syscall_meta() and fail to
initialize.

To fix this problem, implement an strong definition of arch_syscall_addr()
to get the actual addresses of system calls.

Signed-off-by: Ben Dai <ben.dai@unisoc.com>
---
 arch/arm64/kernel/syscall.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index 50a0f1a38e84..2b911603966b 100644
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -12,6 +12,8 @@
 #include <asm/debug-monitors.h>
 #include <asm/exception.h>
 #include <asm/fpsimd.h>
+#include <asm/insn.h>
+#include <asm/patching.h>
 #include <asm/syscall.h>
 #include <asm/thread_info.h>
 #include <asm/unistd.h>
@@ -19,6 +21,25 @@
 long compat_arm_syscall(struct pt_regs *regs, int scno);
 long sys_ni_syscall(void);
 
+#ifdef CONFIG_CFI_CLANG
+unsigned long __init arch_syscall_addr(int nr)
+{
+	u32 insn;
+	unsigned long addr = (unsigned long)sys_call_table[nr];
+
+	/*
+	 * Clang's CFI will replace the address of each system call function
+	 * with the address of a jump table entry. In this case, the jump
+	 * target address is the actual address of the system call.
+	 */
+	aarch64_insn_read((void *)addr, &insn);
+	if (likely(aarch64_insn_is_b(insn)))
+		addr += aarch64_get_branch_offset(insn);
+
+	return addr;
+}
+#endif
+
 static long do_ni_syscall(struct pt_regs *regs, int scno)
 {
 #ifdef CONFIG_COMPAT
-- 
2.17.0