dwarf_block_to_fb_offset() and 64-bit host

Hi folks,

I've been looking at a problem on a remote Cortex-M target, which notably
happens to have a somewhat atypical property of using an address space of
0x9xxxxxxx - in other words, addresses will have the top bit set.

When debugging, there is a problem with a GDB built for a 64-bit host,
versus one built for 32-bit, all from exactly the same source base. It
manifests with this sort of issue with a 64-bit hosted GDB:

Breakpoint 2, fileio1_main (id=0, id@entry=<error reading variable: Cannot
access memory at address 0x9001917c>)
    at /.../fatfs1.c:431

The cited address is perfectly valid and readable. Whereas on a 32-bit GDB:
Breakpoint 2, fileio1_main (id=0) at /.../fatfs1.c:431

I have tracked down where the 32-bit and 64-bit builds of GDB diverge in
behaviour and it's in dwarf2expr.c's dwarf_block_to_fb_offset():

int
dwarf_block_to_fb_offset (const gdb_byte *buf, const gdb_byte *buf_end,
                          CORE_ADDR *fb_offset_return)
{
  int64_t fb_offset;
[snip]
  buf++;

  buf = gdb_read_sleb128 (buf, buf_end, &fb_offset);
  if (buf == NULL)
    return 0;
  *fb_offset_return = fb_offset;
  if (buf != buf_end || fb_offset != (LONGEST) *fb_offset_return)
    return 0;

  return 1;
}

In the 32-bit (working) build, in one failing example, fb_offset ends up
as -28, whereas *fb_offset_return is 0xffffffe4 - this is because
CORE_ADDR is an *unsigned* 32-bit type (due to bfd_vma). But LONGEST is a
signed long long - 64-bits - so the comparison between fb_offset and
*fb_offset_return ends up comparing -28 and 4294967268. So 0 is returned.

In the 64-bit (buggy) build, fb_offset is still -28 but *fb_offset_return
is 0xffffffffffffffe4 (CORE_ADDR now being an unsigned 64-bit type), which
when cast to LONGEST (which is signed) is still -28. Therefore 1 is
returned. This later results in read_frame_arg()'s call to
ops->read_variable_at_entry incorrectly believing there is a different
value of the variable on entry.

I'm unsure about where the bug really lies. I'm not familiar enough with
Dwarf, but I have to wonder why there's the test for "fb_offset !=
(LONGEST) *fb_offset_return" in the first place.

Any help/advice from anyone with Dwarf experience would be welcome, thanks.

Jifl
-- 
------["Si fractum non sit, noli id reficere"]------       Opinions==mine