[gdb/symtab] Support .debug_line with DW_FORM_line_strp

I noticed a new gcc option -gdwarf64 and tried it out (using gcc 11.2.1).

With a test-case hello.c:
...
int
main (void)
{
  printf ("hello\n");
  return 0;
}
...
compiled like this:
...
$ gcc -g -gdwarf64 ~/hello.c
...
I ran into:
...
$ gdb -q -batch a.out
DW_FORM_line_strp pointing outside of .debug_line_str section \
  [in module a.out]
...

Debugging gdb revealed that the string offset is:
...
(gdb) up
    objfile=0x182ab70, str_offset=1378684502312,
    form_name=0xeae9b5 "DW_FORM_line_strp")
    at src/gdb/dwarf2/section.c:208
208         error (_("%s pointing outside of %s section [in module %s]"),
(gdb) p /x str_offset
$1 = 0x14100000128
(gdb)
...
which is read when parsing a .debug_line entry at 0x1e0.

Looking with readelf at the 0x1e0 entry, we have:
...
 The Directory Table (offset 0x202, lines 2, columns 1):
  Entry Name
  0     (indirect line string, offset: 0x128): /data/gdb_versions/devel
  1     (indirect line string, offset: 0x141): /home/vries
...
which in a hexdump looks like:
...
  0x00000200 1f022801 00004101 00000201 1f020f02
...

What happens is the following:
- readelf interprets the DW_FORM_line_strp reference to .debug_line_str as
  a 4 byte value, and sees entries 0x00000128 and 0x00000141.
- gdb instead interprets it as an 8 byte value, and sees as first entry
  0x0000014100000128, which is too big so it bails out.

AFAIU, gdb is wrong.  It assumes DW_FORM_line_strp is 8 bytes on the basis
that the corresponding CU is 64-bit DWARF.  However, the .debug_line
contribution has it's own initial_length field, and encodes there that it's
32-bit DWARF.

Fix this by using the correct offset size for DW_FORM_line_strp references
in .debug_line.

Note: the described test-case does trigger this complaint (both with and
without this patch):
...
$ gdb -q -batch -iex "set complaints 10" a.out
During symbol reading: intermixed 32-bit and 64-bit DWARF sections
...

The reason that the CU has 64-bit dwarf is because -gdwarf64 was passed to
gcc.  The reason that the .debug_line entry has 32-bit dwarf is because that's
what gas generates.  Perhaps this is complaint-worthy, but I don't think it
is wrong.

Tested on x86_64-linux, using native and target board dwarf64.exp.

3 files changed, 26 insertions, 6 deletions

diff --git a/gdb/dwarf2/line-header.c b/gdb/dwarf2/line-header.c
index 1519576..852e285 100644
--- a/gdb/dwarf2/line-header.c
+++ b/gdb/dwarf2/line-header.c
@@ -137,7 +137,7 @@ read_checked_initial_length_and_offset (bfd *abfd, const gdb_byte *buf,
 static void
 read_formatted_entries (dwarf2_per_objfile *per_objfile, bfd *abfd,
 			const gdb_byte **bufp, struct line_header *lh,
-			const struct comp_unit_head *cu_header,
+			unsigned int offset_size,
 			void (*callback) (struct line_header *lh,
 					  const char *name,
 					  dir_index d_index,
@@ -187,9 +187,12 @@ read_formatted_entries (dwarf2_per_objfile *per_objfile, bfd *abfd,
 	      break;
 
 	    case DW_FORM_line_strp:
-	      string.emplace
-		(per_objfile->read_line_string (buf, cu_header, &bytes_read));
-	      buf += bytes_read;
+	      {
+		const char *str
+		  = per_objfile->read_line_string (buf, offset_size);
+		string.emplace (str);
+		buf += offset_size;
+	      }
 	      break;
 
 	    case DW_FORM_data1:
@@ -372,7 +375,7 @@ dwarf_decode_line_header  (sect_offset sect_off, bool is_dwz,
     {
       /* Read directory table.  */
       read_formatted_entries (per_objfile, abfd, &line_ptr, lh.get (),
-			      cu_header,
+			      offset_size,
 			      [] (struct line_header *header, const char *name,
 				  dir_index d_index, unsigned int mod_time,
 				  unsigned int length)
@@ -382,7 +385,7 @@ dwarf_decode_line_header  (sect_offset sect_off, bool is_dwz,
 
       /* Read file name table.  */
       read_formatted_entries (per_objfile, abfd, &line_ptr, lh.get (),
-			      cu_header,
+			      offset_size,
 			      [] (struct line_header *header, const char *name,
 				  dir_index d_index, unsigned int mod_time,
 				  unsigned int length)
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 4a963e4..a513cc4 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -20306,6 +20306,18 @@ read_indirect_string (dwarf2_per_objfile *per_objfile, bfd *abfd,
 
 const char *
 dwarf2_per_objfile::read_line_string (const gdb_byte *buf,
+				      unsigned int offset_size)
+{
+  bfd *abfd = objfile->obfd;
+  ULONGEST str_offset = read_offset (abfd, buf, offset_size);
+
+  return per_bfd->line_str.read_string (objfile, str_offset, "DW_FORM_line_strp");
+}
+
+/* See read.h.  */
+
+const char *
+dwarf2_per_objfile::read_line_string (const gdb_byte *buf,
 				      const struct comp_unit_head *cu_header,
 				      unsigned int *bytes_read_ptr)
 {
diff --git a/gdb/dwarf2/read.h b/gdb/dwarf2/read.h
index 1638d85..fe34e3f 100644
--- a/gdb/dwarf2/read.h
+++ b/gdb/dwarf2/read.h
@@ -517,6 +517,11 @@ struct dwarf2_per_objfile
 				const struct comp_unit_head *cu_header,
 				unsigned int *bytes_read_ptr);
 
+  /* Return pointer to string at .debug_line_str offset as read from BUF.
+     The offset_size is OFFSET_SIZE.  */
+  const char *read_line_string (const gdb_byte *buf,
+				unsigned int offset_size);
+
   /* Return true if the symtab corresponding to PER_CU has been set,
      false otherwise.  */
   bool symtab_set_p (const dwarf2_per_cu_data *per_cu) const;