bpf: preserve constant zero when doing partial register restore

Similar to special handling of STACK_ZERO, when reading 1/2/4 bytes from
stack from slot that has register spilled into it and that register has
a constant value zero, preserve that zero and mark spilled register as
precise for that. This makes spilled const zero register and STACK_ZERO
cases equivalent in their behavior.
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231205184248.1502704-7-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -4767,22 +4767,39 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env,
 				copy_register_state(&state->regs[dst_regno], reg);
 				state->regs[dst_regno].subreg_def = subreg_def;
 			} else {
+				int spill_cnt = 0, zero_cnt = 0;
+
 				for (i = 0; i < size; i++) {
 					type = stype[(slot - i) % BPF_REG_SIZE];
-					if (type == STACK_SPILL)
+					if (type == STACK_SPILL) {
+						spill_cnt++;
 						continue;
+					}
 					if (type == STACK_MISC)
 						continue;
-					if (type == STACK_ZERO)
+					if (type == STACK_ZERO) {
+						zero_cnt++;
 						continue;
+					}
 					if (type == STACK_INVALID && env->allow_uninit_stack)
 						continue;
 					verbose(env, "invalid read from stack off %d+%d size %d\n",
 						off, i, size);
 					return -EACCES;
 				}
-				mark_reg_unknown(env, state->regs, dst_regno);
-				insn_flags = 0; /* not restoring original register state */
+
+				if (spill_cnt == size &&
+				    tnum_is_const(reg->var_off) && reg->var_off.value == 0) {
+					__mark_reg_const_zero(&state->regs[dst_regno]);
+					/* this IS register fill, so keep insn_flags */
+				} else if (zero_cnt == size) {
+					/* similarly to mark_reg_stack_read(), preserve zeroes */
+					__mark_reg_const_zero(&state->regs[dst_regno]);
+					insn_flags = 0; /* not restoring original register state */
+				} else {
+					mark_reg_unknown(env, state->regs, dst_regno);
+					insn_flags = 0; /* not restoring original register state */
+				}
 			}
 			state->regs[dst_regno].live |= REG_LIVE_WRITTEN;
 		} else if (dst_regno >= 0) {