On Thu, Sep 3, 2009 at 9:20 PM, Bean<bean12...@gmail.com> 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 <phco...@gmail.com>
* 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/
>
> _______________________________________________
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> Grub-devel@gnu.org
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>
>
--
Regards
Vladimir 'phcoder' Serbinenko
Personal git repository: http://repo.or.cz/w/grub2/phcoder.git
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