selftests/bpf: Fix verifier_typedef test on 32-bit

This BPF assembly test accesses a struct field using a fixed offset based
on a 64-bit environment, without any debug info usable for relocation or
rewriting of accesses. This fails on 32-bit armhf:

  tester_init:PASS:tester_log_buf 0 nsec
  process_subtest:PASS:obj_open_mem 0 nsec
  process_subtest:PASS:specs_alloc 0 nsec
  run_subtest:PASS:obj_open_mem 0 nsec
  libbpf: prog 'resolve_typedef': BPF program load failed: -EACCES
  libbpf: prog 'resolve_typedef': failed to load: -EACCES
  libbpf: failed to load object 'verifier_typedef'
  run_subtest:FAIL:unexpected_load_failure unexpected error: -13 (errno 13)
  VERIFIER LOG:
  =============
  Global function resolve_typedef() doesn't return scalar. Only those are supported.
  0: R1=ctx() R10=fp0
  ; asm volatile ("                                       \ @ verifier_typedef.c:12
  0: (79) r1 = *(u64 *)(r1 +0)
  func 'bpf_fentry_test_sinfo' arg0 has btf_id 10943 type STRUCT 'skb_shared_info'
  1: R1_w=ptr_skb_shared_info()
  1: (79) r2 = *(u64 *)(r1 +56)
  access beyond the end of member netmem (mend:4) in struct skb_frag with off 0 size 8
  processed 2 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
  =============
  torvalds#561/1   verifier_typedef/typedef: resolve:FAIL
  torvalds#561     verifier_typedef:FAIL

Fix this by rewriting resolve_typedef() in C with explicit field references
which can be run-time patched, resulting in 32-bit appropriate code:

  int resolve_typedef(unsigned long long * ctx):
  ; int BPF_PROG(resolve_typedef, struct skb_shared_info *si)
     0: (b7) r2 = 44
  ; int BPF_PROG(resolve_typedef, struct skb_shared_info *si)
     1: (79) r1 = *(u64 *)(r1 +0)
     2: (0f) r1 += r2
  ; volatile netmem_ref tmp __attribute__((unused)) = si->frags->netmem;
     3: (61) r1 = *(u32 *)(r1 +0)

Fixes: 2597a25 ("selftests/bpf: Add test to exercise typedef walking")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>

Author: guidosarducci

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

@@ -2,22 +2,17 @@
 
 #include <vmlinux.h>
 #include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
 #include "bpf_misc.h"
 
 SEC("fentry/bpf_fentry_test_sinfo")
 __description("typedef: resolve")
 __success __retval(0)
-__naked void resolve_typedef(void)
+int BPF_PROG(resolve_typedef, struct skb_shared_info *si)
 {
-	asm volatile ("					\
-	r1 = *(u64 *)(r1 +0);				\
-	r2 = *(u64 *)(r1 +%[frags_offs]);		\
-	r0 = 0;						\
-	exit;						\
-"	:
-	: __imm_const(frags_offs,
-		      offsetof(struct skb_shared_info, frags))
-	: __clobber_all);
+	volatile netmem_ref tmp __attribute__((unused)) = si->frags->netmem;
+
+	return 0;
 }
 
 char _license[] SEC("license") = "GPL";