Re: [patch] Fix a glitch in debugging 32-bit process with 64-bit GDB.

[Paul Pluzhnikov <ppluzhnikov at google dot com>]

On Mon, Dec 8, 2008 at 6:07 PM, Daniel Jacobowitz <drow@false.org> wrote:

>> We end up with an impossible target address of 0x1ffffbf88.
>
> Conclusion doesn't follow from example

Well, that's the value I observed in the debugger...

> Do you mean that SYMBOL_VALUE is -16U or -16UL, and
> that's where the wrapping comes from?  But SYMBOL_VALUE is a long,
> signed.  Is SYMBOL_VALUE (long) 0xfffffff0?

Yes:

(top) p *var
$1 = {ginfo = {name = 0x16feaa0 "ar_ptr", value = {ivalue =
4294967280, block = 0xfffffff0,
      bytes = 0xfffffff0 <Address 0xfffffff0 out of bounds>, address =
4294967280, chain = 0xfffffff0}, language_specific = {cplus_specific =
{
        demangled_name = 0x0}}, language = language_c, section = 0,
obj_section = 0x0}, type = 0x16fe418, symtab = 0x1702900,
  domain = VAR_DOMAIN, aclass = LOC_ARG, is_argument = 1, line = 2742,
ops = 0x0, aux_value = 0x0, hash_next = 0x0}

That value was set here:

#0  define_symbol (valu=4294967280, string=0x2aaaabd3285c
"ar_ptr:p(0,32)", desc=2742, type=160, objfile=0xed4070)
    at ../../src/gdb/stabsread.c:936
#1  0x0000000000545f98 in process_one_symbol (type=160, desc=2742,
valu=4294967280, name=0x2aaaabd3285c "ar_ptr:p(0,32)",
    section_offsets=0xe9bcf0, objfile=0xed4070) at ../../src/gdb/dbxread.c:3178
#2  0x0000000000545001 in read_ofile_symtab (pst=0xf9cde8) at
../../src/gdb/dbxread.c:2600
...

And that (AFAICT) is coming from BFD.

So it appears that BFD gives us 0x00000000fffffff0, but GDB expects
0xfffffffffffffff0

> If that's the case then the debug reader might be to blame.

This is coming from a rather old glibc-2.2.2, compiled with
non-exstent :) old compiler:

  GNU CC version 2.96 20000731 (Red Hat Linux 7.1 2.96-79).

Here is what 'objdump -g /lib/i686/libc.so.6' has to say:

static mchunkptr chunk_alloc (arena *ar_ptr /* 0x00000000fffffff0 */,
size_t nb /* 0x00000000ffffffec */)
{ /* 0x000000000007f2f0 */
  { /* 0x000000000007f2f0 */
    register mbinptr q /* 0x0000000000000002 */;
    register mchunkptr bck /* 0x0000000000000007 */;
...

And here is 'objdump --stabs':

105431 FUN    0      2746   0007f2f0 317480 chunk_alloc:f(0,25)
105432 PSYM   0      2742   fffffff0 317500 ar_ptr:p(0,32)
105433 PSYM   0      2742   ffffffec 317515 nb:p(3,2)


-- 
Paul Pluzhnikov