Re: [Xen-devel] [PATCH] libelf: allow having HYPERCALL_PAGE entry before VIRT_BASE in __xen_guest section.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.10.17 at 14:03, <gregory.herrero@xxxxxxxxxx> wrote:
> When filling __xen_guest section of a guest, user may define
> HYPERCALL_PAGE earlier than VIRT_BASE in the section leading to an
> incorrect hypercall page address since an undefined virt_base could be
> used to compute hypercall page address.
> If there is no VIRT_BASE entry in __xen_guest section, default value of
> 0 is used for virt_base. Thus, setting hypercall page address to
> HYPERCALL_PAGE value is correct in this case too.

Do you have an example of a guest kernel that is doing this? I
ask because the __xen_guest section has been deprecated for
many years - everyone should be using the notes section
instead nowadays.

> --- a/xen/common/libelf/libelf-dominfo.c
> +++ b/xen/common/libelf/libelf-dominfo.c
> @@ -330,14 +330,21 @@ elf_errorstatus elf_xen_parse_guest_info(struct 
> elf_binary *elf,
>  
>          /* longs */
>          if ( !strcmp(name, "VIRT_BASE") )
> +        {
>              parms->virt_base = strtoull(value, NULL, 0);
> +            if ( parms->virt_hypercall != UNSET_ADDR )
> +                params->virt_hypercall += params->virt_base;
> +        }
>          if ( !strcmp(name, "VIRT_ENTRY") )
>              parms->virt_entry = strtoull(value, NULL, 0);
>          if ( !strcmp(name, "ELF_PADDR_OFFSET") )
>              parms->elf_paddr_offset = strtoull(value, NULL, 0);
>          if ( !strcmp(name, "HYPERCALL_PAGE") )
> -            parms->virt_hypercall = (strtoull(value, NULL, 0) << 12) +
> -                parms->virt_base;
> +        {
> +            parms->virt_hypercall = (strtoull(value, NULL, 0) << 12);
> +            if ( parms->virt_base != UNSET_ADDR )
> +                params->virt_hypercall += params->virt_base;
> +        }

I think you rather want to do this once after the loop. That'll
then also take care of there being multiple VIRT_BASE entries.

Jan


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