bpf, arm64: Fix BTI type used for freplace attached functions

When running an freplace attached bpf program on an arm64 system w were
seeing the following issue:
  Unhandled 64-bit el1h sync exception on CPU47, ESR 0x0000000036000003 -- BTI

After a bit of work to track it down I determined that what appeared to be
happening is that the 'bti c' at the start of the program was somehow being
reached after a 'br' instruction. Further digging pointed me toward the
fact that the function was attached via freplace. This in turn led me to
build_plt which I believe is invoking the long jump which is triggering
this error.

To resolve it we can replace the 'bti c' with 'bti jc' and add a comment
explaining why this has to be modified as such.

Fixes: b2ad54e1 ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Acked-by: Xu Kuohai <xukuohai@huawei.com>
Link: https://lore.kernel.org/r/168926677665.316237.9953845318337455525.stgit@ahduyck-xeon-server.home.arpaSigned-off-by: Alexei Starovoitov <ast@kernel.org>

arch/arm64/net/bpf_jit_comp.c

@@ -322,7 +322,13 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
 	 *
 	 */
 
-	emit_bti(A64_BTI_C, ctx);
+	/* bpf function may be invoked by 3 instruction types:
+	 * 1. bl, attached via freplace to bpf prog via short jump
+	 * 2. br, attached via freplace to bpf prog via long jump
+	 * 3. blr, working as a function pointer, used by emit_call.
+	 * So BTI_JC should used here to support both br and blr.
+	 */
+	emit_bti(A64_BTI_JC, ctx);
 
 	emit(A64_MOV(1, A64_R(9), A64_LR), ctx);
 	emit(A64_NOP, ctx);