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

[Paul Pluzhnikov <ppluzhnikov@google.com>]

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On Sat, Dec 13, 2008 at 8:01 AM, Joel Brobecker <brobecker@adacore.com> wrote:

> > - leave GDB broken and wait for newer glibc :(
>
> I'm just realizing that this might be a very very visible problem,
> if most offsets are negative. Is that the case?

Now that you've mentioned it, I see that there is a trivial test case:

int bar(int x) { return x; }
int foo(int x) { int y = x; return bar(y); }
int main() { return foo(0); }

gcc -gstabs -m32 t.c && gdb64 -q ./a.out

(gdb) b bar
Breakpoint 1 at 0x8048303: file t.c, line 1.
(gdb) r

Breakpoint 1, bar (x=0) at t.c:1
1	int bar(int x) { return x; }
(gdb) up
#1  0x0804831f in foo (x=0) at t.c:2
2	int foo(int x) { int y = x; return bar(y); }
(gdb) info locals
y = Cannot access memory at address 0xffffd954


> > - do sign extension, but only for N_LSYM and N_PSYM symbols as
> >  a heuristic.
>
> It seems to me that this should work without negative effect
> (we have to be a little careful because there is a 64bit stabs
> extension on Tru64).

AFAICT, Tru64 goes through ECOFF reader in mdebugread.c, and so
wouldn't be affected by a change to dbxread.c

> After all, values for these types of symbols
> are always offsets to some location inside a frame, and I can't see
> a frame being that big.

Especially in a 32-bit executable ...

> > - pre-scan the objfile to determine highest link address, and
> >  do sign extension if that address is (well) below 0x80000000
>
> Seems too much work, IMO.

Right.

So here is a 3rd attempt.

Thanks,
--
Paul Pluzhnikov


2008-12-12  Paul Pluzhnikov  <ppluzhnikov@google.com>

        * dbxread.c (read_ofile_symtab): Sign-extend 32-bit N_LSYM and
        N_PSYM STABS values for 64-bit GDB.

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Index: dbxread.c
===================================================================
RCS file: /cvs/src/src/gdb/dbxread.c,v
retrieving revision 1.98
diff -u -p -u -r1.98 dbxread.c
--- dbxread.c	1 Oct 2008 16:41:27 -0000	1.98
+++ dbxread.c	13 Dec 2008 18:07:31 -0000
@@ -2597,6 +2597,11 @@ read_ofile_symtab (struct partial_symtab
 
       if (type & N_STAB)
 	{
+	  if (sizeof (nlist.n_value) > 4
+	      && (type == N_LSYM || type == N_PSYM))
+	    /* These are very likely to be 32-bit negative.
+	       Sign-extend them for 64-bit GDB.  */
+	    nlist.n_value = (nlist.n_value ^ 0x80000000) - 0x80000000;
 	  process_one_symbol (type, nlist.n_desc, nlist.n_value,
 			      namestring, section_offsets, objfile);
 	}