Re: [PATCH] [5/12] x86_64: Make patching more robust, fix paravirt issue

[Jeremy Fitzhardinge <jeremy@goop.org>]

Andi Kleen wrote:
> Commit 19d36ccdc34f5ed444f8a6af0cbfdb6790eb1177 "x86: Fix alternatives
> and kprobes to remap write-protected kernel text" uses code which is
> being patched for patching.
>
> In particular, paravirt_ops does patching in two stages: first it
> calls paravirt_ops.patch, then it fills any remaining instructions
> with nop_out().  nop_out calls text_poke() which calls
> lookup_address() which calls pgd_val() (aka paravirt_ops.pgd_val):
> that call site is one of the places we patch.
>
> If we always do patching as one single call to text_poke(), we only
> need make sure we're not patching the memcpy in text_poke itself.
> This means the prototype to paravirt_ops.patch needs to change, to
> marshal the new code into a buffer rather than patching in place as it
> does now.  It also means all patching goes through text_poke(), which
> is known to be safe (apply_alternatives is also changed to make a
> single patch).
>   

Hi Andi,

This patch breaks Xen booting.  I get infinite recursive faults during
patching when this patch is present.  If I boot with
"noreplace-paravirt" it works OK, and it works as expected if I back
this patch out.  I haven't tracked down the exact failure mode; its a
little hard to debug because it overwrites all kernel memory with
recursive fault stackframes and then finally traps out to Xen when it
hits the bottom of memory.

I think we should back this one out before .23.

    J