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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Oct 06, 2015 at 02:17:41PM +0100, Ian Campbell wrote:
> 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).
> 

I agree with this reasoning so

Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>

> Ian.

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