Re: [PATCH v2] livepatch: account for patch offset when applying NOP patch

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

On Thu, Mar 31, 2022 at 08:42:47AM +0200, Jan Beulich wrote:
> On 30.03.2022 19:04, Roger Pau Monné wrote:
> > On Wed, Mar 30, 2022 at 01:05:31PM +0200,>> --- a/xen/arch/x86/livepatch.c
> >> +++ b/xen/arch/x86/livepatch.c
> >> @@ -157,9 +157,15 @@ void noinline arch_livepatch_apply(struc
> >>       * loaded hotpatch (to avoid racing against other fixups 
> >> adding/removing
> >>       * ENDBR64 or similar instructions).
> >>       */
> >> -    if ( is_endbr64(old_ptr) || is_endbr64_poison(func->old_addr) )
> >> +    if ( len >= ENDBR64_LEN &&
> > 
> > Sorry, didn't realize before, but shouldn't this check be using
> > old_size instead of len (which is based on new_size)?
> 
> Yes and no: In principle yes, but with len == func->new_size in the NOP
> case, and with arch_livepatch_verify_func() guaranteeing new_size <=
> old_size, the check is still fine for that case. Plus: If new_size was
> less than 4 _but_ there's an ENDBR64 at the start, what would we do? I
> think there's more that needs fixing in this regard. So I guess I'll
> make a v3 with this extra fix dropped and with the livepatch_insn_len()
> invocation simply moved. After all the primary goal is to get the
> stable trees unstuck.

Right, I agree to try and get the stable trees unblocked ASAP.

I think the check for ENDBR is only relevant when we are patching the
function with a jump, otherwise the new replacement code should
contain the ENDBR instruction already?

Thanks, Roger.