[PATCH v2] bpf: make sure CO-RE relocs are typed with struct BTF_KIND_STRUCT

[Cupertino Miranda]

Hi everyone,

Here is the v2 for the patch in this thread:
  https://gcc.gnu.org/pipermail/gcc-patches/2024-October/665378.html
Please noticed that commit message was adapted to new content.

Regards,
Cupertino

Based on observation within bpf-next selftests and comparisson of GCC
and clang compiled code, the BPF loader expects all CO-RE relocations to
point to BTF non const and non volatile type nodes.
---
 gcc/btfout.cc                                 |  2 +-
 gcc/config/bpf/btfext-out.cc                  |  7 ++++
 gcc/ctfc.h                                    |  2 +
 .../gcc.target/bpf/core-attr-const.c          | 40 +++++++++++++++++++
 4 files changed, 50 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.target/bpf/core-attr-const.c

diff --git a/gcc/btfout.cc b/gcc/btfout.cc
index 8b91bde8798..24f62ec1a52 100644
--- a/gcc/btfout.cc
+++ b/gcc/btfout.cc
@@ -167,7 +167,7 @@ get_btf_kind (uint32_t ctf_kind)
 
 /* Convenience wrapper around get_btf_kind for the common case.  */
 
-static uint32_t
+uint32_t
 btf_dtd_kind (ctf_dtdef_ref dtd)
 {
   if (!dtd)
diff --git a/gcc/config/bpf/btfext-out.cc b/gcc/config/bpf/btfext-out.cc
index 095c35b894b..30266a2d384 100644
--- a/gcc/config/bpf/btfext-out.cc
+++ b/gcc/config/bpf/btfext-out.cc
@@ -320,6 +320,13 @@ bpf_core_reloc_add (const tree type, const char * 
section_name,
   ctf_container_ref ctfc = ctf_get_tu_ctfc ();
   ctf_dtdef_ref dtd = ctf_lookup_tree_type (ctfc, type);
 
+  /* Make sure CO-RE type is never the const or volatile version.  */
+  if ((btf_dtd_kind (dtd) == BTF_KIND_CONST
+       || btf_dtd_kind (dtd) == BTF_KIND_VOLATILE)
+      && kind >= BPF_RELO_FIELD_BYTE_OFFSET
+      && kind <= BPF_RELO_FIELD_RSHIFT_U64)
+    dtd = dtd->ref_type;
+
   /* Buffer the access string in the auxiliary strtab.  */
   bpfcr->bpfcr_astr_off = 0;
   gcc_assert (accessor != NULL);
diff --git a/gcc/ctfc.h b/gcc/ctfc.h
index 41e1169f271..e5967f590f9 100644
--- a/gcc/ctfc.h
+++ b/gcc/ctfc.h
@@ -465,4 +465,6 @@ extern void btf_mark_type_used (tree);
 extern int ctfc_get_dtd_srcloc (ctf_dtdef_ref, ctf_srcloc_ref);
 extern int ctfc_get_dvd_srcloc (ctf_dvdef_ref, ctf_srcloc_ref);
 
+extern uint32_t btf_dtd_kind (ctf_dtdef_ref dtd);
+
 #endif /* GCC_CTFC_H */
diff --git a/gcc/testsuite/gcc.target/bpf/core-attr-const.c 
b/gcc/testsuite/gcc.target/bpf/core-attr-const.c
new file mode 100644
index 00000000000..da6113a3faf
--- /dev/null
+++ b/gcc/testsuite/gcc.target/bpf/core-attr-const.c
@@ -0,0 +1,40 @@
+/* Test to make sure CO-RE access relocs point to non const versions of the
+   type.  */
+
+/* { dg-do compile } */
+/* { dg-options "-O0 -dA -gbtf -mco-re -masm=normal" } */
+
+struct S {
+  int a;
+  int b;
+  int c;
+} __attribute__((preserve_access_index));
+
+void
+func (struct S * s)
+{
+  int *x;
+  int *y;
+  int *z;
+  struct S tmp;
+  const struct S *cs = s;
+  volatile struct S *vs = s;
+
+  /* 0:2 */
+  x = &(s->c);
+
+  /* 0:1 */
+  y = (int *) &(cs->b);
+
+  /* 0:1 */
+  z = (int *) &(vs->b);
+
+  *x = 4;
+  *y = 4;
+  *z = 4;
+}
+
+/* Both const and non const struct type should have the same bpfcr_type. */
+/* { dg-final { scan-assembler-times "0x1\t# bpfcr_type \\(struct S\\)" 1 } } 
*/
+/* { dg-final { scan-assembler-times "0x1\t# bpfcr_type \\(const struct S\\)" 
1 } } */
+/* { dg-final { scan-assembler-times "0x1\t# bpfcr_type \\(volatile struct 
S\\)" 1 } } */
-- 
2.39.5