Re: [Xen-devel] [RFC] x86/boot: Don't use BDA value if it's suspiciously small

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 26/08/16 10:09, Sylvain Munaut wrote:
> If we have an multiboot value and the value we got from the BDA
> seems too small, use the safe one
>
> Signed-off-by: Sylvain Munaut <s.munaut@xxxxxxxxxxxxxxxxxxxx>
> ---
> I need this when using linux-as-a-bootloader (i.e. kexec into Xen) because
> the BDA is just zero at that point (not entirely sure why tbh).
Does Linux reclam and reuse the BDA by this point in its boot?

> This is the simplest patch I could come up with and that shouldn't change
> anything for system currently booting. But if the multiboot infos are
> present, I'm not sure why not use that preferentially since the values
> from the BDA / EBDA could just be random garbage while the multiboot header
> has at least some minimal validation (checksum + magic).
If we have mutliboot memory information available (should be all of the 
time), we should use that in preference to peeking at the BDA/EBDA
> An error message if no sane value ( i.e. > 64k at min ) can be found at
> all could be printed too. Took me some time to trace this down and some
> serial output would have been welcome :)
All of this code is rather hairy.  Unfortunately, there is no good 
logging method available at this point.
~Andrew

> Comments welcome wrt to what people think is best and I can re-spin this.
>
>   xen/arch/x86/boot/head.S | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
> index 85770e8..d79fcc5 100644
> --- a/xen/arch/x86/boot/head.S
> +++ b/xen/arch/x86/boot/head.S
> @@ -108,6 +108,8 @@ __start:
>           shl     $10-4,%edx
>           cmp     %eax,%edx           /* compare with BDA value */
>           cmovb   %edx,%eax           /* and use the smaller */
> +        cmp     $0x1000,%eax        /* or if the BDA value is too small */
> +        cmovb   %edx,%eax           /* (and probably not valid) */
>   
>   2:      /* Reserve 64kb for the trampoline */
>           sub     $0x1000,%eax

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