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From: "dtaylor at emc dot com" <sourceware-bugzilla@sourceware.org>
To: gdb-prs@sourceware.org
Subject: [Bug gdb/17520] New: structure offset wrong when 1/4 GB or greater
Date: Wed, 29 Oct 2014 14:36:00 -0000
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X-Bugzilla-Version: 7.8
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https://sourceware.org/bugzilla/show_bug.cgi?id=17520

            Bug ID: 17520
           Summary: structure offset wrong when 1/4 GB or greater
           Product: gdb
           Version: 7.8
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: dtaylor at emc dot com

When a structure member's offset is 1/4 GB or greater from the start
of the structure, GDB computes the wrong value.

When computing the offset of a structure member, GDB computes first
the BITS from the start of the structure, returns that as a 32 bit
signed quantity and then divides the result by 8.

While this can show up on 32 bit systems, it is especially bad on
64 bit systems.  We have systems with over 500 GB of memory and have
run into the problem.

Here's a short program that illustrates the bug.  The actual struct
where the problem was observed has over 250 members and is over 1/2 GB
in size.

#include <stdio.h>

struct big_struct
{
  char first[0x10000000 + 16];
  long second;
} big_struct;

int
main (int argc, char *argv[])
{
  printf ("sizeof (big_struct) 0x%lx,\n&big_struct %p,\n&big_struct.second
%p\noffset %lx\n",
      sizeof (big_struct),
      (void *)&big_struct,
      (void *)&big_struct.second,
      __builtin_offsetof(struct big_struct, second));
  return (0);
}

If I run it, the output is:

    sizeof (big_struct) 0x10000018,
    &big_struct 0x601040,
    &big_struct.second 0x10601050
    offset 10000010

But, in GDB 7.8:

    (gdb) p &big_struct
    $1 = (struct big_struct *) 0x601040 <big_struct>
    (gdb) p &big_struct.second
    $2 = (long *) 0xfffffffff0601050

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