[PATCH] libbacktrace: Fix up DWARF5 .debug_line handling [PR98716]

[Jakub Jelinek]

Hi!

When GCC (since the switch to -gdwarf-5 by default) is configured and built
against recent binutils (2.35.1 if slightly patched, or 2.36 or trunk),
the assembler emits DWARF5 .debug_line rather than DWARF4 or older
.debug_line.

Seems some DWARF5 support has been added to libbacktrace, but there is one
problem.  The DWARF5 spec (like DWARF4 spec) says that the initial value of
file is 1, but unlike DWARF4 and earlier which had in the filename table
entries starting with 1, DWARF5 has an 0 entry before that (which is
supposed to match DW_AT_name and DW_AT_comps_dir pair in the .debug_info
CU).

The following patch fixes that.

On i686-linux when built against those new binutils this fixes (the
c-c++-common tests for both C and C++):
-FAIL: c-c++-common/asan/alloca_big_alignment.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  output pattern test
-FAIL: c-c++-common/asan/alloca_big_alignment.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  output pattern test
-FAIL: c-c++-common/asan/alloca_detect_custom_size.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  output pattern test
-FAIL: c-c++-common/asan/alloca_detect_custom_size.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  output pattern test
-FAIL: c-c++-common/asan/alloca_overflow_partial.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  output pattern test
-FAIL: c-c++-common/asan/alloca_overflow_partial.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  output pattern test
-FAIL: c-c++-common/asan/alloca_overflow_right.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  output pattern test
-FAIL: c-c++-common/asan/alloca_overflow_right.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  output pattern test
-FAIL: c-c++-common/asan/alloca_underflow_left.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  output pattern test
-FAIL: c-c++-common/asan/alloca_underflow_left.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  output pattern test
-FAIL: c-c++-common/asan/global-overflow-1.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  output pattern test
-FAIL: c-c++-common/asan/global-overflow-1.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  output pattern test
-FAIL: c-c++-common/asan/heap-overflow-1.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  output pattern test
-FAIL: c-c++-common/asan/heap-overflow-1.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  output pattern test
-FAIL: c-c++-common/asan/null-deref-1.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  output pattern test
-FAIL: c-c++-common/asan/null-deref-1.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  output pattern test
-FAIL: c-c++-common/asan/sanity-check-pure-c-1.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  output pattern test
-FAIL: c-c++-common/asan/sanity-check-pure-c-1.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  output pattern test
-FAIL: c-c++-common/asan/stack-overflow-1.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  output pattern test
-FAIL: c-c++-common/asan/stack-overflow-1.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  output pattern test
-FAIL: c-c++-common/asan/strncpy-overflow-1.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  output pattern test
-FAIL: c-c++-common/asan/strncpy-overflow-1.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  output pattern test
-FAIL: c-c++-common/asan/use-after-free-1.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  output pattern test
-FAIL: c-c++-common/asan/use-after-free-1.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  output pattern test
-FAIL: g++.dg/asan/large-func-test-1.C   -O2 -flto -fno-use-linker-plugin -flto-partition=none  output pattern test
-FAIL: g++.dg/asan/large-func-test-1.C   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  output pattern test
-FAIL: TestCaller

Bootstrapped/regtested on x86_64-linux and i686-linux (2.35.1 binutils but
not patched) and i686-linux (latest binutils), x86_64-linux regtest still
pending, ok for trunk?

2021-01-18  Jakub Jelinek  <jakub@redhat.com>

	PR debug/98716
	* dwarf.c (read_line_program): For DWARF5 .debug_line headers,
	start with hdr->filenames[1] rather than hdr->filenames[0].

--- libbacktrace/dwarf.c.jj	2021-01-04 10:25:53.495067802 +0100
+++ libbacktrace/dwarf.c	2021-01-18 14:27:05.034589998 +0100
@@ -2715,8 +2715,11 @@ read_line_program (struct backtrace_stat
 
   address = 0;
   op_index = 0;
-  if (hdr->filenames_count > 0)
-    reset_filename = hdr->filenames[0];
+  /* The initial file is file with index 1.  In DWARF4 and earlier
+     filename table starts with entry 1, while in DWARF5 it starts
+     with entry 0 which should match the CU's DW_AT_name attribute.  */
+  if (hdr->filenames_count > (hdr->version >= 5))
+    reset_filename = hdr->filenames[hdr->version >= 5];
   else
     reset_filename = "";
   filename = reset_filename;

	Jakub