Re: [Valgrind-users] Strange behaviour with both 1.9.x and 1.0.x

[Sebastian <sc...@nb...>]

Hi,


On Mon, Apr 14, 2003 at 06:16:18PM -0700, Jeremy Fitzhardinge wrote:

> Well, the behaviour you're seeing is consistent with be_random() returning
> an undefined value in %eax.  FP instructions like fildll immediately check
> the defined-ness of their values (unlike integer instructions, which only
> check for defined-ness when the value is used as a pointer or in a
> conditional instruction).  It may be that be_random() has a bug, or it is
> returning an undefined value as a result of an undefined input. 

unsigned int
be_random (unsigned int max)
{
        unsigned int    tmp;


        if (rand_fd == -1)
                be_randinit ();

        read (rand_fd, &tmp, sizeof (tmp));

        /* 0 denotes special 0 to 2^32 - 1 range
	 */
        if (max == 0)
                return (tmp);

        return (tmp % max);
}

This code looks correct to me. 'rand_fd' leads to /dev/urandom, so to
valgrind it should just look like if 'tmp' is defined by the read(), and the
return value defined by the modulo operation.

0x804aa89 <be_random>:  push   %ebp
0x804aa8a <be_random+1>:        mov    %esp,%ebp
0x804aa8c <be_random+3>:        sub    $0x8,%esp
0x804aa8f <be_random+6>:        cmpl   $0xffffffff,0x8065668
0x804aa96 <be_random+13>:       jne    0x804aa9d <be_random+20>
0x804aa98 <be_random+15>:       call   0x804aa4e <be_randinit>
0x804aa9d <be_random+20>:       sub    $0x4,%esp
0x804aaa0 <be_random+23>:       push   $0x4
0x804aaa2 <be_random+25>:       lea    0xfffffffc(%ebp),%eax
0x804aaa5 <be_random+28>:       push   %eax
0x804aaa6 <be_random+29>:       pushl  0x8065668
0x804aaac <be_random+35>:       call   0x8048a28 <read>
0x804aab1 <be_random+40>:       add    $0x10,%esp
0x804aab4 <be_random+43>:       cmpl   $0x0,0x8(%ebp)
0x804aab8 <be_random+47>:       jne    0x804aac2 <be_random+57>
0x804aaba <be_random+49>:       mov    0xfffffffc(%ebp),%eax
0x804aabd <be_random+52>:       mov    %eax,0xfffffff8(%ebp)
0x804aac0 <be_random+55>:       jmp    0x804aad0 <be_random+71>
0x804aac2 <be_random+57>:       mov    0xfffffffc(%ebp),%eax
0x804aac5 <be_random+60>:       mov    $0x0,%edx
0x804aaca <be_random+65>:       divl   0x8(%ebp)
0x804aacd <be_random+68>:       mov    %edx,0xfffffff8(%ebp)
0x804aad0 <be_random+71>:       mov    0xfffffff8(%ebp),%eax
0x804aad3 <be_random+74>:       leave  
0x804aad4 <be_random+75>:       ret    

(Unoptimized GCC 3.2.3 code).
In any case, there is no way for be_random to return without defining %eax.

 
>      J

ciao,
Sebastian

-- 
-. sc...@nb... -. + http://segfault.net/~scut/ `--------------------.
-' segfault.net/~scut/pgp `' 5453 AC95 1E02 FDA7 50D2 A42D 427E 6DEF 745A 8E07
`- 4 BLU-82/MOAB articles offered, payment due. hi echelon! -----------------'