bpf: Add x86-64 JIT support for bpf_addr_space_cast instruction.

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:

aux_reg = upper_32_bits of arena->user_vm_start
aux_reg <<= 32
wX = wY // clear upper 32 bits of dst register
if (wX) // if not zero add upper bits of user_vm_start
  wX |= aux_reg

JIT can do it more efficiently:

mov dst_reg32, src_reg32  // 32-bit move
shl dst_reg, 32
or dst_reg, user_vm_start
rol dst_reg, 32
xor r11, r11
test dst_reg32, dst_reg32 // check if lower 32-bit are zero
cmove r11, dst_reg	  // if so, set dst_reg to zero
			  // Intel swapped src/dst register encoding in CMOVcc

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20240308010812.89848-5-alexei.starovoitov@gmail.com

Author: Alexei Starovoitov

arch/x86/net/bpf_jit_comp.c

@@ -1276,13 +1276,14 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image
 	bool tail_call_seen = false;
 	bool seen_exit = false;
 	u8 temp[BPF_MAX_INSN_SIZE + BPF_INSN_SAFETY];
-	u64 arena_vm_start;
+	u64 arena_vm_start, user_vm_start;
 	int i, excnt = 0;
 	int ilen, proglen = 0;
 	u8 *prog = temp;
 	int err;
 
 	arena_vm_start = bpf_arena_get_kern_vm_start(bpf_prog->aux->arena);
+	user_vm_start = bpf_arena_get_user_vm_start(bpf_prog->aux->arena);
 
 	detect_reg_usage(insn, insn_cnt, callee_regs_used,
 			 &tail_call_seen);
@@ -1350,6 +1351,40 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image
 			break;
 
 		case BPF_ALU64 | BPF_MOV | BPF_X:
+			if (insn->off == BPF_ADDR_SPACE_CAST &&
+			    insn->imm == 1U << 16) {
+				if (dst_reg != src_reg)
+					/* 32-bit mov */
+					emit_mov_reg(&prog, false, dst_reg, src_reg);
+				/* shl dst_reg, 32 */
+				maybe_emit_1mod(&prog, dst_reg, true);
+				EMIT3(0xC1, add_1reg(0xE0, dst_reg), 32);
+
+				/* or dst_reg, user_vm_start */
+				maybe_emit_1mod(&prog, dst_reg, true);
+				if (is_axreg(dst_reg))
+					EMIT1_off32(0x0D,  user_vm_start >> 32);
+				else
+					EMIT2_off32(0x81, add_1reg(0xC8, dst_reg),  user_vm_start >> 32);
+
+				/* rol dst_reg, 32 */
+				maybe_emit_1mod(&prog, dst_reg, true);
+				EMIT3(0xC1, add_1reg(0xC0, dst_reg), 32);
+
+				/* xor r11, r11 */
+				EMIT3(0x4D, 0x31, 0xDB);
+
+				/* test dst_reg32, dst_reg32; check if lower 32-bit are zero */
+				maybe_emit_mod(&prog, dst_reg, dst_reg, false);
+				EMIT2(0x85, add_2reg(0xC0, dst_reg, dst_reg));
+
+				/* cmove r11, dst_reg; if so, set dst_reg to zero */
+				/* WARNING: Intel swapped src/dst register encoding in CMOVcc !!! */
+				maybe_emit_mod(&prog, AUX_REG, dst_reg, true);
+				EMIT3(0x0F, 0x44, add_2reg(0xC0, AUX_REG, dst_reg));
+				break;
+			}
+			fallthrough;
 		case BPF_ALU | BPF_MOV | BPF_X:
 			if (insn->off == 0)
 				emit_mov_reg(&prog,
@@ -3432,6 +3467,11 @@ void bpf_arch_poke_desc_update(struct bpf_jit_poke_descriptor *poke,
 	}
 }
 
+bool bpf_jit_supports_arena(void)
+{
+	return true;
+}
+
 bool bpf_jit_supports_ptr_xchg(void)
 {
 	return true;

include/linux/filter.h

@@ -962,6 +962,7 @@ bool bpf_jit_supports_kfunc_call(void);
 bool bpf_jit_supports_far_kfunc_call(void);
 bool bpf_jit_supports_exceptions(void);
 bool bpf_jit_supports_ptr_xchg(void);
+bool bpf_jit_supports_arena(void);
 void arch_bpf_stack_walk(bool (*consume_fn)(void *cookie, u64 ip, u64 sp, u64 bp), void *cookie);
 bool bpf_helper_changes_pkt_data(void *func);
 

kernel/bpf/core.c

@@ -2932,6 +2932,11 @@ bool __weak bpf_jit_supports_far_kfunc_call(void)
 	return false;
 }
 
+bool __weak bpf_jit_supports_arena(void)
+{
+	return false;
+}
+
 /* Return TRUE if the JIT backend satisfies the following two conditions:
  * 1) JIT backend supports atomic_xchg() on pointer-sized words.
  * 2) Under the specific arch, the implementation of xchg() is the same