bpf, arm64: fix stack_depth tracking in combination with tail calls

Using dynamic stack_depth tracking in arm64 JIT is currently broken in
combination with tail calls. In prologue, we cache ctx->stack_size and
adjust SP reg for setting up function call stack, and tearing it down
again in epilogue. Problem is that when doing a tail call, the cached
ctx->stack_size might not be the same.

One way to fix the problem with minimal overhead is to re-adjust SP in
emit_bpf_tail_call() and properly adjust it to the current program's
ctx->stack_size. Tested on Cavium ThunderX ARMv8.

Fixes: f1c9eed7 ("bpf, arm64: take advantage of stack_depth tracking")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

arch/arm64/net/bpf_jit_comp.c

@@ -148,7 +148,8 @@ static inline int epilogue_offset(const struct jit_ctx *ctx)
 /* Stack must be multiples of 16B */
 #define STACK_ALIGN(sz) (((sz) + 15) & ~15)
 
-#define PROLOGUE_OFFSET 8
+/* Tail call offset to jump into */
+#define PROLOGUE_OFFSET 7
 
 static int build_prologue(struct jit_ctx *ctx)
 {
@@ -200,19 +201,19 @@ static int build_prologue(struct jit_ctx *ctx)
 	/* Initialize tail_call_cnt */
 	emit(A64_MOVZ(1, tcc, 0, 0), ctx);
 
-	/* 4 byte extra for skb_copy_bits buffer */
-	ctx->stack_size = prog->aux->stack_depth + 4;
-	ctx->stack_size = STACK_ALIGN(ctx->stack_size);
-
-	/* Set up function call stack */
-	emit(A64_SUB_I(1, A64_SP, A64_SP, ctx->stack_size), ctx);
-
 	cur_offset = ctx->idx - idx0;
 	if (cur_offset != PROLOGUE_OFFSET) {
 		pr_err_once("PROLOGUE_OFFSET = %d, expected %d!\n",
 			    cur_offset, PROLOGUE_OFFSET);
 		return -1;
 	}
+
+	/* 4 byte extra for skb_copy_bits buffer */
+	ctx->stack_size = prog->aux->stack_depth + 4;
+	ctx->stack_size = STACK_ALIGN(ctx->stack_size);
+
+	/* Set up function call stack */
+	emit(A64_SUB_I(1, A64_SP, A64_SP, ctx->stack_size), ctx);
 	return 0;
 }
 
@@ -260,11 +261,12 @@ static int emit_bpf_tail_call(struct jit_ctx *ctx)
 	emit(A64_LDR64(prg, tmp, prg), ctx);
 	emit(A64_CBZ(1, prg, jmp_offset), ctx);
 
-	/* goto *(prog->bpf_func + prologue_size); */
+	/* goto *(prog->bpf_func + prologue_offset); */
 	off = offsetof(struct bpf_prog, bpf_func);
 	emit_a64_mov_i64(tmp, off, ctx);
 	emit(A64_LDR64(tmp, prg, tmp), ctx);
 	emit(A64_ADD_I(1, tmp, tmp, sizeof(u32) * PROLOGUE_OFFSET), ctx);
+	emit(A64_ADD_I(1, A64_SP, A64_SP, ctx->stack_size), ctx);
 	emit(A64_BR(tmp), ctx);
 
 	/* out: */