[gcc r13-4708] btf: fix 'extern const void' variables [PR106773]

[gcc r13-4708] btf: fix 'extern const void' variables [PR106773]

[David Faust]

https://gcc.gnu.org/g:4f7aa145b796c044526c93f390e68f3b56a1b30a

commit r13-4708-g4f7aa145b796c044526c93f390e68f3b56a1b30a
Author: David Faust <david.faust@oracle.com>
Date:   Wed Dec 7 11:47:26 2022 -0800

    btf: fix 'extern const void' variables [PR106773]
    
    The eBPF loader expects to find BTF_KIND_VAR records for references to
    extern const void symbols. We were mistakenly identifing these as
    unsupported types, and as a result skipping emitting VAR records for
    them.
    
    In addition, the internal DWARF representation from which BTF is
    produced does not generate 'const' modifier DIEs for the void type,
    which meant in BTF the 'const' qualifier was dropped for 'extern const
    void' variables. This patch also adds support for generating a const
    void type in BTF to correct emission for these variables.
    
            PR target/106773
    
    gcc/
    
            * btfout.cc (btf_collect_datasec): Correct size of void entries.
            (btf_dvd_emit_preprocess_cb): Do not skip emitting variables which
            refer to void types.
            (btf_init_postprocess): Create 'const void' type record if needed and
            adjust variables to refer to it as appropriate.
    
    gcc/testsuite/
    
            * gcc.dg/debug/btf/btf-pr106773.c: New test.

Diff:
---
 gcc/btfout.cc                                 | 44 ++++++++++++++++++++++++---
 gcc/testsuite/gcc.dg/debug/btf/btf-pr106773.c | 25 +++++++++++++++
 2 files changed, 65 insertions(+), 4 deletions(-)

diff --git a/gcc/btfout.cc b/gcc/btfout.cc
index 677e8324424..204b11d4e9f 100644
--- a/gcc/btfout.cc
+++ b/gcc/btfout.cc
@@ -350,6 +350,8 @@ btf_collect_datasec (ctf_container_ref ctfc)
       tree size = DECL_SIZE_UNIT (node->decl);
       if (tree_fits_uhwi_p (size))
 	info.size = tree_to_uhwi (size);
+      else if (VOID_TYPE_P (TREE_TYPE (node->decl)))
+	info.size = 1;
 
       /* Offset is left as 0 at compile time, to be filled in by loaders such
 	 as libbpf.  */
@@ -441,7 +443,7 @@ btf_dvd_emit_preprocess_cb (ctf_dvdef_ref *slot, ctf_container_ref arg_ctfc)
     return 1;
 
   /* Do not add variables which refer to unsupported types.  */
-  if (btf_removed_type_p (var->dvd_type))
+  if (!voids.contains (var->dvd_type) && btf_removed_type_p (var->dvd_type))
     return 1;
 
   arg_ctfc->ctfc_vars_list[num_vars_added] = var;
@@ -1075,15 +1077,49 @@ btf_init_postprocess (void)
 {
   ctf_container_ref tu_ctfc = ctf_get_tu_ctfc ();
 
-  size_t i;
-  size_t num_ctf_types = tu_ctfc->ctfc_types->elements ();
-
   holes.create (0);
   voids.create (0);
 
   num_types_added = 0;
   num_types_created = 0;
 
+  /* Workaround for 'const void' variables.  These variables are sometimes used
+     in eBPF programs to address kernel symbols.  DWARF does not generate const
+     qualifier on void type, so we would incorrectly emit these variables
+     without the const qualifier.
+     Unfortunately we need the TREE node to know it was const, and we need
+     to create the const modifier type (if needed) now, before making the types
+     list.  So we can't avoid iterating with FOR_EACH_VARIABLE here, and then
+     again when creating the DATASEC entries.  */
+  ctf_id_t constvoid_id = CTF_NULL_TYPEID;
+  varpool_node *var;
+  FOR_EACH_VARIABLE (var)
+    {
+      if (!var->decl)
+	continue;
+
+      tree type = TREE_TYPE (var->decl);
+      if (type && VOID_TYPE_P (type) && TYPE_READONLY (type))
+	{
+	  dw_die_ref die = lookup_decl_die (var->decl);
+	  if (die == NULL)
+	    continue;
+
+	  ctf_dvdef_ref dvd = ctf_dvd_lookup (tu_ctfc, die);
+	  if (dvd == NULL)
+	    continue;
+
+	  /* Create the 'const' modifier type for void.  */
+	  if (constvoid_id == CTF_NULL_TYPEID)
+	    constvoid_id = ctf_add_reftype (tu_ctfc, CTF_ADD_ROOT,
+					    dvd->dvd_type, CTF_K_CONST, NULL);
+	  dvd->dvd_type = constvoid_id;
+	}
+    }
+
+  size_t i;
+  size_t num_ctf_types = tu_ctfc->ctfc_types->elements ();
+
   if (num_ctf_types)
     {
       init_btf_id_map (num_ctf_types + 1);
diff --git a/gcc/testsuite/gcc.dg/debug/btf/btf-pr106773.c b/gcc/testsuite/gcc.dg/debug/btf/btf-pr106773.c
new file mode 100644
index 00000000000..f90fa773a4b
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/debug/btf/btf-pr106773.c
@@ -0,0 +1,25 @@
+/* Test BTF generation for extern const void symbols.
+   BTF_KIND_VAR records should be emitted for such symbols if they are used,
+   as well as a corresponding entry in the appropriate DATASEC record.  */
+
+/* { dg-do compile } */
+/* { dg-options "-O0 -gbtf -dA" } */
+
+/* Expect 1 variable record only for foo, with 'extern' (2) linkage.  */
+/* { dg-final { scan-assembler-times "\[\t \]0xe000000\[\t \]+\[^\n\]*btv_info" 1 } } */
+/* { dg-final { scan-assembler-times "\[\t \]0x2\[\t \]+\[^\n\]*btv_linkage" 1 } } */
+
+/* { dg-final { scan-assembler-times "ascii \"foo.0\"\[\t \]+\[^\n\]*btf_string" 1 } } */
+
+/* { dg-final { scan-assembler-times "0\[\t \]+\[^\n\]*bts_offset" 1 } } */
+/* { dg-final { scan-assembler-times "1\[\t \]+\[^\n\]*bts_size" 1 } } */
+
+extern const void foo __attribute__((weak)) __attribute__((section (".ksyms")));
+extern const void bar __attribute__((weak)) __attribute__((section (".ksyms")));
+
+unsigned long func () {
+  unsigned long x = (unsigned long) &foo;
+
+  return x;
+}
+