From b8cf266f4ca4ff89704b190c827abf9ea7e7b5bf Mon Sep 17 00:00:00 2001
From: David Faust <david.faust@oracle.com>
Date: Mon, 4 Dec 2023 14:08:03 -0800
Subject: [PATCH] btf: avoid wrong DATASEC entries for extern vars [PR112849]

The process of creating BTF_KIND_DATASEC records involves iterating
through variable declarations, determining which section they will be
placed in, and creating an entry in the appropriate DATASEC record
accordingly.

For variables without e.g. an explicit __attribute__((section)), we use
categorize_decl_for_section () to identify the appropriate named section
and corresponding BTF_KIND_DATASEC record.

This was incorrectly being done for 'extern' variable declarations as
well as non-extern ones, which meant that extern variable declarations
could result in BTF_KIND_DATASEC entries claiming the variable is
allocated in some section such as '.bss' without any knowledge whether
that is actually true. That resulted in errors building the Linux kernel
BPF selftests.

This patch corrects btf_collect_datasec () to avoid assuming a section
for extern variables, and only emit BTF_KIND_DATASEC entries for them if
they have a known section.

gcc/
	PR debug/112849
	* btfout.cc (btf_collect_datasec): Avoid incorrectly creating an
	entry in a BTF_KIND_DATASEC record for extern variable decls without
	a known section.

gcc/testsuite/
	PR debug/112849
	* gcc.dg/debug/btf/btf-datasec-3.c: New test.
---
 gcc/btfout.cc                                 | 10 ++++++-
 .../gcc.dg/debug/btf/btf-datasec-3.c          | 28 +++++++++++++++++++
 2 files changed, 37 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.dg/debug/btf/btf-datasec-3.c

diff --git a/gcc/btfout.cc b/gcc/btfout.cc
index a5e0d640e198..db4f1084f85c 100644
--- a/gcc/btfout.cc
+++ b/gcc/btfout.cc
@@ -486,7 +486,15 @@ btf_collect_datasec (ctf_container_ref ctfc)
 
       /* Mark extern variables.  */
       if (DECL_EXTERNAL (node->decl))
-	dvd->dvd_visibility = BTF_VAR_GLOBAL_EXTERN;
+	{
+	  dvd->dvd_visibility = BTF_VAR_GLOBAL_EXTERN;
+
+	  /* PR112849: avoid assuming a section for extern decls without
+	     an explicit section, which would result in incorrectly
+	     emitting a BTF_KIND_DATASEC entry for them.  */
+	  if (node->get_section () == NULL)
+	    continue;
+	}
 
       const char *section_name = get_section_name (node);
       if (section_name == NULL)
diff --git a/gcc/testsuite/gcc.dg/debug/btf/btf-datasec-3.c b/gcc/testsuite/gcc.dg/debug/btf/btf-datasec-3.c
new file mode 100644
index 000000000000..297340cabfa4
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/debug/btf/btf-datasec-3.c
@@ -0,0 +1,28 @@
+/* PR debug/112849
+   Test that we do not incorrectly create BTF_KIND_DATASEC entries for
+   extern decls with no known section.  */
+
+/* { dg-do compile } */
+/* { dg-options "-O0 -gbtf -dA" } */
+
+extern int VERSION __attribute__((section (".version")));
+
+extern int test_bss1;
+extern int test_data1;
+
+int test_bss2;
+int test_data2 = 2;
+
+int
+foo (void)
+{
+  test_bss2 = VERSION;
+  return test_bss1 + test_data1 + test_data2;
+}
+
+/* There should be 3 DATASEC entries total.  Of the extern decls, only VERSION
+   has a known section; entries are not created for the other two.  */
+/* { dg-final { scan-assembler-times "bts_type" 3 } } */
+/* { dg-final { scan-assembler-times "bts_type: \\(BTF_KIND_VAR 'test_data2'\\)" 1 } } */
+/* { dg-final { scan-assembler-times "bts_type: \\(BTF_KIND_VAR 'test_bss2'\\)" 1 } } */
+/* { dg-final { scan-assembler-times "bts_type: \\(BTF_KIND_VAR 'VERSION'\\)" 1 } } */
-- 
2.39.2