Re: [PATCH] avoid undefined behavior due to oversized shifts

["Jan Beulich" <JBeulich at suse dot com>]

>>> On 03.01.13 at 10:17, nick clifton <nickc@redhat.com> wrote:
> Hi Nickolai,
> 
>> In C, shifting a value by more than its bitwidth is undefined behavior.
>> The get_value() function in bfd/elflink.c (invoked as part of
>> bfd_elf_perform_complex_relocation) sometimes shifts a bfd_vma value by
>> more than its bitwidth.
> 
> Actually I think that there is a bigger problems here: If (8 * chunksz) 
> is more than the bitwidth of the bfd_vma type then the get_value() 
> function is never going to return the value that the user expects.  Even 
> if you fix the shifting problem.
> 
> A better patch I think therefore would be something like this:
> 
> diff -u -3 -p -r1.458 elflink.c
> --- bfd/elflink.c       19 Dec 2012 19:45:43 -0000      1.458
> +++ bfd/elflink.c       3 Jan 2013 09:19:34 -0000
> @@ -7919,13 +7919,19 @@ get_value (bfd_vma size,
>   {
>     bfd_vma x = 0;
> 
> +  /* Sanity checks.  */
> +  if (chunksz > sizeof (x)
> +      || (chunksz == sizeof (x) && size > chunksz)
> +      || size < chunksz
> +      || size % chunksz != 0
> +      || input_bfd == NULL
> +      || location == NULL)
> +    abort ();
> +
> 
> Do you agree ?

But that wouldn't eliminate the need for fixing the shifts, as the
checks above don't entirely exclude the chunksz == sizeof(x) case.

Jan