bpf: Fix subreg optimization for BPF_FETCH

All 32-bit variants of BPF_FETCH (add, and, or, xor, xchg, cmpxchg)
define a 32-bit subreg and thus have zext_dst set. Their encoding,
however, uses dst_reg field as a base register, which causes
opt_subreg_zext_lo32_rnd_hi32() to zero-extend said base register
instead of the one the insn really defines (r0 or src_reg).

Fix by properly choosing a register being defined, similar to how
check_atomic() already does that.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210210204502.83429-1-iii@linux.ibm.com

kernel/bpf/verifier.c

@@ -10957,6 +10957,7 @@ static int opt_subreg_zext_lo32_rnd_hi32(struct bpf_verifier_env *env,
 	for (i = 0; i < len; i++) {
 		int adj_idx = i + delta;
 		struct bpf_insn insn;
+		u8 load_reg;
 
 		insn = insns[adj_idx];
 		if (!aux[adj_idx].zext_dst) {
@@ -10999,9 +11000,27 @@ static int opt_subreg_zext_lo32_rnd_hi32(struct bpf_verifier_env *env,
 		if (!bpf_jit_needs_zext())
 			continue;
 
+		/* zext_dst means that we want to zero-extend whatever register
+		 * the insn defines, which is dst_reg most of the time, with
+		 * the notable exception of BPF_STX + BPF_ATOMIC + BPF_FETCH.
+		 */
+		if (BPF_CLASS(insn.code) == BPF_STX &&
+		    BPF_MODE(insn.code) == BPF_ATOMIC) {
+			/* BPF_STX + BPF_ATOMIC insns without BPF_FETCH do not
+			 * define any registers, therefore zext_dst cannot be
+			 * set.
+			 */
+			if (WARN_ON(!(insn.imm & BPF_FETCH)))
+				return -EINVAL;
+			load_reg = insn.imm == BPF_CMPXCHG ? BPF_REG_0
+							   : insn.src_reg;
+		} else {
+			load_reg = insn.dst_reg;
+		}
+
 		zext_patch[0] = insn;
-		zext_patch[1].dst_reg = insn.dst_reg;
-		zext_patch[1].src_reg = insn.dst_reg;
+		zext_patch[1].dst_reg = load_reg;
+		zext_patch[1].src_reg = load_reg;
 		patch = zext_patch;
 		patch_len = 2;
 apply_patch_buffer: