selftests/bpf: Remove unnecessary direct read of local percpu kptr

For the second argument of bpf_kptr_xchg(), if the reg type contains
MEM_ALLOC and MEM_PERCPU, which means a percpu allocation,
after bpf_kptr_xchg(), the argument is marked as MEM_RCU and MEM_PERCPU
if in rcu critical section. This way, re-reading from the map value
is not needed. Remove it from the percpu_alloc_array.c selftest.

Without previous kernel change, the test will fail like below:

  0: R1=ctx(off=0,imm=0) R10=fp0
  ; int BPF_PROG(test_array_map_10, int a)
  0: (b4) w1 = 0                        ; R1_w=0
  ; int i, index = 0;
  1: (63) *(u32 *)(r10 -4) = r1         ; R1_w=0 R10=fp0 fp-8=0000????
  2: (bf) r2 = r10                      ; R2_w=fp0 R10=fp0
  ;
  3: (07) r2 += -4                      ; R2_w=fp-4
  ; e = bpf_map_lookup_elem(&array, &index);
  4: (18) r1 = 0xffff88810e771800       ; R1_w=map_ptr(off=0,ks=4,vs=16,imm=0)
  6: (85) call bpf_map_lookup_elem#1    ; R0_w=map_value_or_null(id=1,off=0,ks=4,vs=16,imm=0)
  7: (bf) r6 = r0                       ; R0_w=map_value_or_null(id=1,off=0,ks=4,vs=16,imm=0) R6_w=map_value_or_null(id=1,off=0,ks=4,vs=16,imm=0)
  ; if (!e)
  8: (15) if r6 == 0x0 goto pc+81       ; R6_w=map_value(off=0,ks=4,vs=16,imm=0)
  ; bpf_rcu_read_lock();
  9: (85) call bpf_rcu_read_lock#87892          ;
  ; p = e->pc;
  10: (bf) r7 = r6                      ; R6=map_value(off=0,ks=4,vs=16,imm=0) R7_w=map_value(off=0,ks=4,vs=16,imm=0)
  11: (07) r7 += 8                      ; R7_w=map_value(off=8,ks=4,vs=16,imm=0)
  12: (79) r6 = *(u64 *)(r6 +8)         ; R6_w=percpu_rcu_ptr_or_null_val_t(id=2,off=0,imm=0)
  ; if (!p) {
  13: (55) if r6 != 0x0 goto pc+13      ; R6_w=0
  ; p = bpf_percpu_obj_new(struct val_t);
  14: (18) r1 = 0x12                    ; R1_w=18
  16: (b7) r2 = 0                       ; R2_w=0
  17: (85) call bpf_percpu_obj_new_impl#87883   ; R0_w=percpu_ptr_or_null_val_t(id=4,ref_obj_id=4,off=0,imm=0) refs=4
  18: (bf) r6 = r0                      ; R0=percpu_ptr_or_null_val_t(id=4,ref_obj_id=4,off=0,imm=0) R6=percpu_ptr_or_null_val_t(id=4,ref_obj_id=4,off=0,imm=0) refs=4
  ; if (!p)
  19: (15) if r6 == 0x0 goto pc+69      ; R6=percpu_ptr_val_t(ref_obj_id=4,off=0,imm=0) refs=4
  ; p1 = bpf_kptr_xchg(&e->pc, p);
  20: (bf) r1 = r7                      ; R1_w=map_value(off=8,ks=4,vs=16,imm=0) R7=map_value(off=8,ks=4,vs=16,imm=0) refs=4
  21: (bf) r2 = r6                      ; R2_w=percpu_ptr_val_t(ref_obj_id=4,off=0,imm=0) R6=percpu_ptr_val_t(ref_obj_id=4,off=0,imm=0) refs=4
  22: (85) call bpf_kptr_xchg#194       ; R0_w=percpu_ptr_or_null_val_t(id=6,ref_obj_id=6,off=0,imm=0) refs=6
  ; if (p1) {
  23: (15) if r0 == 0x0 goto pc+3       ; R0_w=percpu_ptr_val_t(ref_obj_id=6,off=0,imm=0) refs=6
  ; bpf_percpu_obj_drop(p1);
  24: (bf) r1 = r0                      ; R0_w=percpu_ptr_val_t(ref_obj_id=6,off=0,imm=0) R1_w=percpu_ptr_val_t(ref_obj_id=6,off=0,imm=0) refs=6
  25: (b7) r2 = 0                       ; R2_w=0 refs=6
  26: (85) call bpf_percpu_obj_drop_impl#87882          ;
  ; v = bpf_this_cpu_ptr(p);
  27: (bf) r1 = r6                      ; R1_w=scalar(id=7) R6=scalar(id=7)
  28: (85) call bpf_this_cpu_ptr#154
  R1 type=scalar expected=percpu_ptr_, percpu_rcu_ptr_, percpu_trusted_ptr_

The R1 which gets its value from R6 is a scalar. But before insn 22, R6 is
  R6=percpu_ptr_val_t(ref_obj_id=4,off=0,imm=0)
Its type is changed to a scalar at insn 22 without previous patch.
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230827152821.2001129-1-yonghong.song@linux.devSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/progs/percpu_alloc_array.c

@@ -146,10 +146,6 @@ int BPF_PROG(test_array_map_10)
 			/* race condition */
 			bpf_percpu_obj_drop(p1);
 		}
-
-		p = e->pc;
-		if (!p)
-			goto out;
 	}
 
 	v = bpf_this_cpu_ptr(p);