[gcc r14-6195] btf: avoid wrong DATASEC entries for extern vars [PR112849]

[gcc r14-6195] btf: avoid wrong DATASEC entries for extern vars [PR112849]

[David Faust]

https://gcc.gnu.org/g:b8cf266f4ca4ff89704b190c827abf9ea7e7b5bf

commit r14-6195-gb8cf266f4ca4ff89704b190c827abf9ea7e7b5bf
Author: David Faust <david.faust@oracle.com>
Date:   Mon Dec 4 14:08:03 2023 -0800

    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.

Diff:
---
 gcc/btfout.cc                                  | 10 ++++++++-
 gcc/testsuite/gcc.dg/debug/btf/btf-datasec-3.c | 28 ++++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/gcc/btfout.cc b/gcc/btfout.cc
index a5e0d640e19..db4f1084f85 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 00000000000..297340cabfa
--- /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 } } */