bpf: Fix access string default for CO-RE type based relocations

Although part of all CO-RE relocation data, type based relocations do
not require an access string.
Initial implementation defined it as an empty string.
On the other hand, libbpf when parsing the CO-RE relocations verifies
that those strings would contain "0", otherwise reports an error.
This patch makes GCC compliant with libbpf expectations.

gcc/Changelog:
	* config/bpf/btfext-out.cc (cpf_core_reloc_add): Correct for new code.
	Add assert to validate the string is set.
	* config/bpf/core-builtins.cc (cr_final): Make string struct
	field as const.
	(process_enum_value): Correct for field type change.
	(process_type): Set access string to "0".

gcc/testsuite/ChangeLog:
	* gcc.target/bpf/core-builtin-type-based.c: Correct.
	* gcc.target/bpf/core-builtin-type-id.c: Correct.

4 files changed, 11 insertions, 6 deletions

diff --git a/gcc/config/bpf/btfext-out.cc b/gcc/config/bpf/btfext-out.cc
index 00d2501..7ec438f 100644
--- a/gcc/config/bpf/btfext-out.cc
+++ b/gcc/config/bpf/btfext-out.cc
@@ -299,8 +299,9 @@ bpf_core_reloc_add (const tree type, const char * section_name,
 
   /* Buffer the access string in the auxiliary strtab.  */
   bpfcr->bpfcr_astr_off = 0;
-  if (accessor != NULL)
-    bpfcr->bpfcr_astr_off = btf_ext_add_string (accessor);
+  gcc_assert (accessor != NULL);
+  bpfcr->bpfcr_astr_off = btf_ext_add_string (accessor);
+
   bpfcr->bpfcr_type = get_btf_id (ctf_lookup_tree_type (ctfc, type));
   bpfcr->bpfcr_insn_label = label;
   bpfcr->bpfcr_kind = kind;
diff --git a/gcc/config/bpf/core-builtins.cc b/gcc/config/bpf/core-builtins.cc
index 4256fea..70b14e4 100644
--- a/gcc/config/bpf/core-builtins.cc
+++ b/gcc/config/bpf/core-builtins.cc
@@ -205,7 +205,7 @@ struct cr_local
 /* Core Relocation Final data */
 struct cr_final
 {
-  char *str;
+  const char *str;
   tree type;
   enum btf_core_reloc_kind kind;
 };
@@ -868,8 +868,10 @@ process_enum_value (struct cr_builtins *data)
 	{
 	  if (TREE_VALUE (l) == expr)
 	    {
-	      ret.str = (char *) ggc_alloc_atomic ((index / 10) + 1);
-	      sprintf (ret.str, "%d", index);
+	      char *tmp = (char *) ggc_alloc_atomic ((index / 10) + 1);
+	      sprintf (tmp, "%d", index);
+	      ret.str = (const char *) tmp;
+
 	      break;
 	    }
 	  index++;
@@ -987,7 +989,7 @@ process_type (struct cr_builtins *data)
 	      || data->kind == BPF_RELO_TYPE_MATCHES);
 
   struct cr_final ret;
-  ret.str = NULL;
+  ret.str = ggc_strdup ("0");
   ret.type = data->type;
   ret.kind = data->kind;
 
diff --git a/gcc/testsuite/gcc.target/bpf/core-builtin-type-based.c b/gcc/testsuite/gcc.target/bpf/core-builtin-type-based.c
index 74a8d5a..9d81813 100644
--- a/gcc/testsuite/gcc.target/bpf/core-builtin-type-based.c
+++ b/gcc/testsuite/gcc.target/bpf/core-builtin-type-based.c
@@ -56,3 +56,4 @@ int foo(void *data)
 /* { dg-final { scan-assembler-times "0x8\[\t \]+\[^\n\]*bpfcr_kind" 13 } } BPF_TYPE_EXISTS */
 /* { dg-final { scan-assembler-times "0x9\[\t \]+\[^\n\]*bpfcr_kind" 11 } } BPF_TYPE_SIZE */
 /* { dg-final { scan-assembler-times "0xc\[\t \]+\[^\n\]*bpfcr_kind" 13 } } BPF_TYPE_MATCHES */
+/* { dg-final { scan-assembler-times "bpfcr_astr_off \[(\"\]+0\[(\"\]+" 37 } } */
diff --git a/gcc/testsuite/gcc.target/bpf/core-builtin-type-id.c b/gcc/testsuite/gcc.target/bpf/core-builtin-type-id.c
index 4b23288..9576b91 100644
--- a/gcc/testsuite/gcc.target/bpf/core-builtin-type-id.c
+++ b/gcc/testsuite/gcc.target/bpf/core-builtin-type-id.c
@@ -38,3 +38,4 @@ int foo(void *data)
 /* { dg-final { scan-assembler-times "0\[\t \]+\[^\n\]*bpfcr_type" 0  { xfail *-*-* } } } */
 /* { dg-final { scan-assembler-times "0x6\[\t \]+\[^\n\]*bpfcr_kind" 13 } } BPF_TYPE_ID_LOCAL */
 /* { dg-final { scan-assembler-times "0x7\[\t \]+\[^\n\]*bpfcr_kind" 7 } } BPF_TYPE_ID_TARGET */
+/* { dg-final { scan-assembler-times "bpfcr_astr_off \[(\"\]+0\[(\"\]+" 20 } } */