Re: coffread.c extension for DLLs without debugging symbols

["Raoul Gough" <RaoulGough@yahoo.co.uk>]

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"Christopher Faylor" <cgf@redhat.com> wrote in message
news:20030104205130.GA9784@redhat.com...
> On Sat, Jan 04, 2003 at 04:25:16PM -0000, Raoul Gough wrote:
> >"Christopher Faylor" <cgf@redhat.com> wrote in message
> >news:20030104044408.GA7364@redhat.com...
> >> P.S.  Assuming that this works as advertised, I assume that you
will
> >> have no objections to my releasing a new version of gdb for
windows
> >> with this patch in it before the patch makes its way into the
official
> >> gdb repository, right?  I think there are a few users in the
cygwin
> >> mailing list that would appreciate this change.
> >
> >By all means - please do. You may as well use the new diff for the
> >comment fixes and the strncmp business.
>
> Ok.  Coming soon to a cygwin mirror near you.

Chris, I've done some more testing and discovered a problem with the
virtual memory address determination mechanism. I've been using
bfd_get_section_vma to find out where each section is in memory, but
this is not always correct. BFD returns the DLL's *preferred* load
address, but it may actually have been relocated during loading
because of address space clashes. It seems obvious now, but BFD is not
the right place to get the needed information. I'm looking into the
win32-nat code that handles DLL load events, which *does* know about
the real load address. Unfortunately, this really will make the code
win32 specific. Unless someone else can suggest anything better?

Actually, this seems like a generic bug in GDB's DLL handling - by
compiling with debugging symbols, the problem is even visible without
my patch. Have a look at the attached example, which shows GDB 5.2.1
getting it wrong.

Regards,
Raoul Gough.


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$ cat dlleg.c
__attribute((__dllexport__)) void fn () { }
__attribute((__dllexport__)) char hello[] = "Hello world";
__attribute((__dllexport__)) int init_data = 42;
__attribute((__dllexport__)) int uninit_data;
$ cat dlleg2.c
__attribute((__dllexport__)) void fn2 () { }
__attribute((__dllexport__)) char hello2[] = "Hello again";
__attribute((__dllexport__)) int init_data2 = 44;
__attribute((__dllexport__)) int uninit_data2;
$ cat dllegmain.c
__attribute((__dllimport__)) void fn ();
__attribute((__dllimport__)) char hello[];
__attribute((__dllimport__)) int init_data;
__attribute((__dllimport__)) int uninit_data;

__attribute((__dllimport__)) void fn2 ();
__attribute((__dllimport__)) char hello2[];
__attribute((__dllimport__)) int init_data2;
__attribute((__dllimport__)) int uninit_data2;

int main ()
{
  fn();
  uninit_data = init_data;

  fn2();
  uninit_data2 = init_data2;

  return 0;
}
$ gcc -g -c dlleg.c
$ gcc -g -c dlleg2.c
$ gcc -g -c dllegmain.c
$ #
$ # Note - by omitting the --enable-auto-image-base linker flag, both
$ # DLLs get the same load address by default (0x1000000). At load
$ # time, one of them gets relocated
$ #
$ gcc -shared -o dlleg.dll dlleg.o
$ gcc -shared -o dlleg2.dll dlleg2.o
$ gcc -o dllegmain dllegmain.o dlleg.dll dlleg2.dll
$ gdb dllegmain
GNU gdb 5.2.1
Copyright 2002 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "mingw32"...
(gdb) break main
Breakpoint 1 at 0x401050: file dllegmain.c, line 13.
(gdb) run
Starting program: f:\Users\Raoul\gdb/dllegmain.exe 

Breakpoint 1, main () at dllegmain.c:13
13	  fn();
(gdb) x/s &hello
0x10002000 <hello2>:	 "Hello again"
(gdb) #
(gdb) # Uh oh. The symbol "hello" should point to the string
(gdb) # "Hello world", and hello2 should be "Hello again"
(gdb) # (see dlleg.c and dlleg2.c above)
(gdb) #
(gdb) x/s &hello2
0x10002000 <hello2>:	 "Hello again"
(gdb) #
(gdb) # win32-nat.c knows the real load addresses:
(gdb) #
(gdb) info dll
DLL Name                        Load Address
F:\cygwin\bin\cygwin1.dll       61001000
F:\WINNT\system32\kernel32.dll  77e81000
f:\Users\Raoul\gdb\dlleg2.dll   10001000
f:\Users\Raoul\gdb\dlleg.dll    00331000
F:\WINNT\system32\advapi32.dll  77db1000
F:\WINNT\system32\rpcrt4.dll    77d41000
F:\WINNT\system32\user32.dll    77e11000
F:\WINNT\system32\gdi32.dll     77f41000
(gdb) #
(gdb) # Using this information, we can find the other string
(gdb) # manually. dlleg2.dll got loaded at it's preferred
(gdb) # address, but dlleg.dll was relocated.
(gdb) #
(gdb) x/s 0x332000
0x332000:	 "Hello world"
(gdb) quit
The program is running.  Exit anyway? (y or n) y
$