Re: [GIT PULL] x86/urgent for v5.11-rc7

[Peter Zijlstra]

On Mon, Feb 08, 2021 at 09:33:00AM -0600, Josh Poimboeuf wrote:
> On Mon, Feb 08, 2021 at 10:02:06AM -0500, Steven Rostedt wrote:
> > On Sun, 7 Feb 2021 16:45:40 -0600
> > Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > 
> > > > I do suspect involved people should start thinking about how they want
> > > > to deal with functions starting with
> > > > 
> > > >         endbr64
> > > >         call __fentry__
> > > > 
> > > > instead of the call being at the very top of the function.  
> > > 
> > > FWIW, objtool's already fine with it (otherwise we would have discovered
> > > the need to disable fcf-protection much sooner).
> > 
> > And this doesn't really affect tracing (note, another user that might be
> > affected is live kernel patching).
> 
> Good point, livepatch is indeed affected.  Is there a better way to get
> the "call __fentry__" address for a given function?
> 
> 
> /*
>  * Convert a function address into the appropriate ftrace location.
>  *
>  * Usually this is just the address of the function, but on some architectures
>  * it's more complicated so allow them to provide a custom behaviour.
>  */
> #ifndef klp_get_ftrace_location
> static unsigned long klp_get_ftrace_location(unsigned long faddr)
> {
> 	return faddr;
> }
> #endif

I suppose the trivial fix is to see if it points to endbr64 and if so,
increment the addr by the length of that.