Re: [Xen-devel] [PATCH] xen/setup: add paranoid index check and warning

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 14/10/14 02:19, Martin Kelly wrote:
> In a call to set_phys_range_identity, i-1 is used without checking that
> i is non-zero. Although unlikely, a bug in the code before it could
> cause the value to be 0, leading to erroneous behavior. This patch adds
> a check against 0 value and a corresponding warning.

This can only happen if the toolstack supplies a memory map with zero
entries.  I could see justification for adding a panic at the top of
this function in this case, but I can't see the usefulness of this warning.

> --- a/arch/x86/xen/setup.c
> +++ b/arch/x86/xen/setup.c
> @@ -671,7 +671,10 @@ char * __init xen_memory_setup(void)
>        * PFNs above MAX_P2M_PFN are considered identity mapped as
>        * well.
>        */
> -     set_phys_range_identity(map[i-1].addr / PAGE_SIZE, ~0ul);
> +     if (i > 0)
> +             set_phys_range_identity(map[i-1].addr / PAGE_SIZE, ~0ul);
> +     else
> +             WARN(1, "Something went wrong clamping memory to a factor of 
> EXTRA_MEM_RATIO!");
>  
>       /*
>        * In domU, the ISA region is normal, usable memory, but we
> 


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