Re: [Xen-devel] PING ARM [PATCH v2] xen/cmdline: Fix buggy strncmp(s, LITERAL, ss - s) construct

[Julien Grall <julien.grall@xxxxxxx>]

Hi Jan,

On 1/15/19 8:47 AM, Jan Beulich wrote:
> > > > On 14.01.19 at 18:16, <julien.grall@xxxxxxx> wrote:
> > Hi Andrew,
> >
> > On 14/01/2019 16:59, Andrew Cooper wrote:
> > > On 14/01/2019 16:07, Julien Grall wrote:
> > > > On 14/01/2019 15:17, Andrew Cooper wrote:
> > > > > > diff --git a/xen/arch/arm/efi/efi-boot.h b/xen/arch/arm/efi/efi-boot.h
> > > > > > index ca655ff..22a86ec 100644
> > > > > > --- a/xen/arch/arm/efi/efi-boot.h
> > > > > > +++ b/xen/arch/arm/efi/efi-boot.h
> > > > > > @@ -212,7 +212,7 @@ EFI_STATUS __init
> > > > > > fdt_add_uefi_nodes(EFI_SYSTEM_TABLE *sys_table,
> > > > > >                 break;
> > > > > >               type = fdt_getprop(fdt, node, "device_type", &len);
> > > > > > -        if ( type && strncmp(type, "memory", len) == 0 )
> > > > > > +        if ( type && len == 6 && strncmp(type, "memory", 6) == 0 )
> > > > string property terminates with NUL and is included in the len. So I
> > > > don't think this change is correct.
> > > Are you saying that len is 7 here then?
> > Yes. But I don't think this change is necessary as we already include NUL in the
> > comparison.
> If len is 7, then indeed you do. Looking at fdt_get_property_by_offset()
> I can't see though where the guarantee comes from that the returned
> string is nul-terminated, as it's prop->len which gets handed back. IOW
> if you e.g. get back "mem" (with or without a nul terminator) and len 3,
> then strncmp() would still return zero.
From section 2.2.4, when the propery value is a string then it should 
be null-terminated. So your example is invalid from a DT spec point of view.
Validating a Device-Tree (other than the bindings) is a pain so you 
always have to provide a blob complaint with the spec [1].
Cheers,

[1] 
https://github.com/devicetree-org/devicetree-specification/releases/tag/v0.2

> Jan
--
Julien Grall

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