[PATCH v16 18/18] bpf: Add get_entry_ip() for arm64

[Masami Hiramatsu (Google)]

From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

Add get_entry_ip() implementation for arm64. This is based on the
information in ftrace_call_adjust() on arm64. Basically function entry
address = ftrace call entry_ip - 4, but when there is a BTI at the first
instruction, we need one more instruction back (entry_ip - 8.)

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
---
 kernel/trace/bpf_trace.c |   64 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 64 insertions(+)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index edd577297dc2..a7827a0a6d81 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -1042,6 +1042,70 @@ static unsigned long get_entry_ip(unsigned long fentry_ip)
 		fentry_ip -= ENDBR_INSN_SIZE;
 	return fentry_ip;
 }
+#elif defined(CONFIG_ARM64)
+#include <asm/insn.h>
+
+static unsigned long get_entry_ip(unsigned long fentry_ip)
+{
+	u32 insn;
+
+	/*
+	 * When using patchable-function-entry without pre-function NOPS, ftrace
+	 * entry is the address of the first NOP after the function entry point.
+	 *
+	 * The compiler has either generated:
+	 *
+	 * func+00:	func:	NOP		// To be patched to MOV X9, LR
+	 * func+04:		NOP		// To be patched to BL <caller>
+	 *
+	 * Or:
+	 *
+	 * func-04:		BTI	C
+	 * func+00:	func:	NOP		// To be patched to MOV X9, LR
+	 * func+04:		NOP		// To be patched to BL <caller>
+	 *
+	 * The fentry_ip is the address of `BL <caller>` which is at `func + 4`
+	 * bytes in either case.
+	 */
+	if (!IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS))
+		return fentry_ip - AARCH64_INSN_SIZE;
+
+	/*
+	 * When using patchable-function-entry with pre-function NOPs, BTI is
+	 * a bit different.
+	 *
+	 * func+00:	func:	NOP		// To be patched to MOV X9, LR
+	 * func+04:		NOP		// To be patched to BL <caller>
+	 *
+	 * Or:
+	 *
+	 * func+00:	func:	BTI	C
+	 * func+04:		NOP		// To be patched to MOV X9, LR
+	 * func+08:		NOP		// To be patched to BL <caller>
+	 *
+	 * The fentry_ip is the address of `BL <caller>` which is at either
+	 * `func + 4` or `func + 8` depends on whether there is a BTI.
+	 */
+
+	/* If there is no BTI, the func address should be one instruction before. */
+	if (!IS_ENABLED(CONFIG_ARM64_BTI_KERNEL))
+		return fentry_ip - AARCH64_INSN_SIZE;
+
+	/* We want to be extra safe in case entry ip is on the page edge,
+	 * but otherwise we need to avoid get_kernel_nofault()'s overhead.
+	 */
+	if ((fentry_ip & ~PAGE_MASK) < AARCH64_INSN_SIZE * 2) {
+		if (get_kernel_nofault(insn, (u32 *)(fentry_ip - AARCH64_INSN_SIZE * 2)))
+			return fentry_ip - AARCH64_INSN_SIZE;
+	} else {
+		insn = *(u32 *)(fentry_ip - AARCH64_INSN_SIZE * 2);
+	}
+
+	if (aarch64_insn_is_bti(le32_to_cpu((__le32)insn)))
+		return fentry_ip - AARCH64_INSN_SIZE * 2;
+
+	return fentry_ip - AARCH64_INSN_SIZE;
+}
 #else
 #define get_entry_ip(fentry_ip) fentry_ip
 #endif