[PATCH bpf-next v3 2/2] bpf: Add arm64 JIT support for bpf_addr_space_cast instruction.

[Puranjay Mohan]

LLVM generates bpf_addr_space_cast instruction while translating
pointers between native (zero) address space and
__attribute__((address_space(N))). The addr_space=1 is reserved as
bpf_arena address space.

rY = addr_space_cast(rX, 0, 1) is processed by the verifier and
converted to normal 32-bit move: wX = wY

rY = addr_space_cast(rX, 1, 0) has to be converted by JIT:

Here I explain using symbolic language what the JIT is supposed to do:
We have:
	src = [src_upper32][src_lower32] // 64 bit src kernel pointer
	uvm = [uvm_upper32][uvm_lower32] // 64 bit user_vm_start

The JIT has to make the dst reg like following
	dst = [uvm_upper32][src_lower32] // if src_lower32 != 0
	dst = [00000000000][00000000000] // if src_lower32 == 0

Signed-off-by: Puranjay Mohan <puranjay12@xxxxxxxxx>
---
 arch/arm64/net/bpf_jit_comp.c                | 17 +++++++++++++++++
 tools/testing/selftests/bpf/DENYLIST.aarch64 |  2 --
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index b9b5febe64f0..cd418c07f806 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -82,6 +82,7 @@ struct jit_ctx {
 	__le32 *ro_image;
 	u32 stack_size;
 	int fpb_offset;
+	u64 user_vm_start;
 };
 
 struct bpf_plt {
@@ -868,6 +869,16 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
 	/* dst = src */
 	case BPF_ALU | BPF_MOV | BPF_X:
 	case BPF_ALU64 | BPF_MOV | BPF_X:
+		if (insn->off == BPF_ADDR_SPACE_CAST && insn->imm == 1U << 16 &&
+		    BPF_CLASS(code) == BPF_ALU64) {
+			emit(A64_MOV(0, tmp, src), ctx); // 32-bit mov clears the upper 32 bits
+			emit_a64_mov_i(0, dst, ctx->user_vm_start >> 32, ctx);
+			emit(A64_LSL(1, dst, dst, 32), ctx);
+			emit(A64_CBZ(1, tmp, 2), ctx);
+			emit(A64_ORR(1, tmp, dst, tmp), ctx);
+			emit(A64_MOV(1, dst, tmp), ctx);
+			break;
+		}
 		switch (insn->off) {
 		case 0:
 			emit(A64_MOV(is64, dst, src), ctx);
@@ -1690,6 +1701,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 	}
 
 	ctx.fpb_offset = find_fpb_offset(prog);
+	ctx.user_vm_start = bpf_arena_get_user_vm_start(prog->aux->arena);
 
 	/*
 	 * 1. Initial fake pass to compute ctx->idx and ctx->offset.
@@ -2511,6 +2523,11 @@ bool bpf_jit_supports_exceptions(void)
 	return true;
 }
 
+bool bpf_jit_supports_arena(void)
+{
+	return true;
+}
+
 void bpf_jit_free(struct bpf_prog *prog)
 {
 	if (prog->jited) {
diff --git a/tools/testing/selftests/bpf/DENYLIST.aarch64 b/tools/testing/selftests/bpf/DENYLIST.aarch64
index d8ade15e2789..0445ac38bc07 100644
--- a/tools/testing/selftests/bpf/DENYLIST.aarch64
+++ b/tools/testing/selftests/bpf/DENYLIST.aarch64
@@ -10,5 +10,3 @@ fill_link_info/kprobe_multi_link_info            # bpf_program__attach_kprobe_mu
 fill_link_info/kretprobe_multi_link_info         # bpf_program__attach_kprobe_multi_opts unexpected error: -95
 fill_link_info/kprobe_multi_invalid_ubuff        # bpf_program__attach_kprobe_multi_opts unexpected error: -95
 missed/kprobe_recursion                          # missed_kprobe_recursion__attach unexpected error: -95 (errno 95)
-verifier_arena                                   # JIT does not support arena
-arena_htab                                       # JIT does not support arena
-- 
2.40.1