Re: [Xen-devel] [PATCH] libxc: remove most of tools/libxc/xc_dom_compat_linux.c

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Tue, 2015-10-06 at 14:06 +0100, Andrew Cooper wrote:
> On 06/10/15 13:58, Wei Liu wrote:
> > On Tue, Oct 06, 2015 at 01:52:16PM +0100, Andrew Cooper wrote:
> > > On 06/10/15 12:35, Juergen Gross wrote:
> > > > In tools/libxc/xc_dom_compat_linux.c only xc_linux_build() is
> > > > currently
> > > > being used by an in-tree component (qemu-xen). All other functions
> > > > are
> > > > superfluous wrappers of the domain builder which can be removed.
> > > > 
> > > > Suggested-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > > > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > > Sorry, but NACK.  They are *not* superfluous.
> > > 
> > > XenServer uses them; xc_dom_linux_build() is the only way to apply
> > > XSA-25 size restrictions to PV kernels and initrds.
> > > 
> > Could you explain a bit more about this? How could this particular
> > function apply restrictions?
> > 
> > Unfortunately XSA-25 is not on XSA page so I have no idea what that is.
> > 
> > Wei.
> 
> Older XSAs are listed on the wiki
> http://wiki.xen.org/wiki/Security_Announcements
> 
> XSA-25 introduced xc_dom_{kernel,ramdisk}_max_size() to set upper
> decompression limits on the kernel and initrd.
> 
> To use them, a struct xc_dom_image *dom is needed, which means the only
> safe way of constructing PV domains is
> 
> xc_dom_allocate()
> xc_dom_kernel_max_size()
> xc_dom_ramdisk_max_size()
> xc_dom_linux_build()
> 
> To have the decompression limits in place before decompression starts.

No it isn't.

xc_dom_linux_build is implemented in terms of the non-compat xc_dom_*
functions, so it should be possible to do what you want with out using the
compat wrapper.

If there is some obscure reason this isn't the case then we should fix
that, not carry around the compat options for ever as a workaround (fixes
include but are not limited to promoting xc_dom_linux_build into a non
-compat helper).

Ian.


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