Re: [PATCH] Bug fix for loadenv.c

[Vladimir 'phcoder' Serbinenko]

On Thu, Sep 3, 2009 at 9:20 PM, Bean<address@hidden> wrote:
> Hi,
>
> This is the bug fix for command/loadenv.c. Here is a small program to
> illustrate what happen:
Update your branch:

2009-09-01  Vladimir Serbinenko  <address@hidden>

        * commands/loadenv.c (check_blocklists): Fix off-by-one error.
        (write_blocklists): Likewise.

>
> #include <stdio.h>
> #include <stdlib.h>
>
> struct data
> {
>  struct data *next;
>  int length;
> };
>
> struct data *
> new_data ()
> {
>  struct data *p;
>
>  p = malloc (sizeof (*p));
>  p->length = 1;
>  p->next = 0;
>
>  return p;
> }
>
> int
> main ()
> {
>  int index;
>  struct data *head, *p;
>
>  head = new_data ();
>  for (p = head, index = 0; p; p = p->next, index += p->length)
>    printf ("%d\n", index);
>
>  return 0;
> }
>
> Compile it with gcc-4.0 from OSX:
> i686-apple-dawin8-gcc-4.0.1 -Os test.c
>
> The assembly dump shows:
>
> L4:
>        movl    %edi, 4(%esp)
>        movl    -28(%ebp), %eax
>        movl    %eax, (%esp)
>        call    L_printf$stub
>        movl    (%esi), %esi
>        addl    4(%esi), %edi
>        jmp     L4
>
> Why would it uses unconditional jumps ? I think it's some over
> optimization in gcc. We have index += p->length after p = p->next, if
> p is null, p->length would be invalid, so gcc figure that p would
> never be null and the end condition is always false. In fact, the bug
> is in the code itself, even if it tests p, in OS environment, access
> null pointer is not allowed, so index += p->length can't be executed
> anyway.
>
> The correct way is:
>
>  for (p = head, index = 0; p;)
>  {
>    printf ("%d\n", index);
>   p = p->next;
>  if (p)
>   index += p->length;
>  }
>
>
> --
> Bean
>
> gitgrub home: http://github.com/grub/grub/
> my fork page: http://github.com/bean123/grub/
>
> _______________________________________________
> Grub-devel mailing list
> address@hidden
> http://lists.gnu.org/mailman/listinfo/grub-devel
>
>



-- 
Regards
Vladimir 'phcoder' Serbinenko

Personal git repository: http://repo.or.cz/w/grub2/phcoder.git