On Mon, 8 Feb 2021, Steven Rostedt wrote:
> On Mon, 8 Feb 2021 16:47:05 +0100
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> > > /*
> > > * 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
powerpc has this
static inline unsigned long klp_get_ftrace_location(unsigned long faddr)
{
/*
* Live patch works only with -mprofile-kernel on PPC. In this case,
* the ftrace location is always within the first 16 bytes.
*/
return ftrace_location_range(faddr, faddr + 16);
}
> > I suppose the trivial fix is to see if it points to endbr64 and if so,
> > increment the addr by the length of that.
>
> I thought of that too. But one thing that may be possible, is to use
> kallsym. I believe you can get the range of a function (start and end of
> the function) from kallsyms. Then ask ftrace for the addr in that range
> (there should only be one).
And we can do this if a hard-coded value live above is not welcome. If I
remember correctly, we used to have exactly this in the old versions of
kGraft. We walked through all ftrace records, called
kallsyms_lookup_size_offset() on every record's ip and if the offset+ip
matched faddr (in this case), we returned the ip.
Miroslav
