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.
bab31d1451