Re: [PATCH] livepatch: account for patch offset when applying NOP patch

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Mar 30, 2022 at 10:03:11AM +0200, Jan Beulich wrote:
> While not triggered by the trivial xen_nop in-tree patch on
> staging/master, that patch exposes a problem on the stable trees, where
> all functions have ENDBR inserted. When NOP-ing out a range, we need to
> account for this. Handle this right in livepatch_insn_len().
> 
> Fixes: 6974c75180f1 ("xen/x86: Livepatch: support patching CET-enhanced 
> functions")
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> Only build tested, as I don't have a live patching environment available.
> 
> For Arm this assumes that the patch_offset field starts out as zero; I
> think we can make such an assumption, yet otoh on x86 explicit
> initialization was added by the cited commit.
> 
> --- a/xen/include/xen/livepatch.h
> +++ b/xen/include/xen/livepatch.h
> @@ -90,7 +90,7 @@ static inline
>  unsigned int livepatch_insn_len(const struct livepatch_func *func)
>  {
>      if ( !func->new_addr )
> -        return func->new_size;
> +        return func->new_size - func->patch_offset;
>  
>      return ARCH_PATCH_INSN_SIZE;
>  }

Don't you also need to move the call to livepatch_insn_len() in
arch_livepatch_apply() after func->patch_offset has been adjusted to
account for ENDBR presence?

Thanks, Roger.