On Wed, Feb 06, 2019 at 08:59:44AM +0000, Julien Thierry wrote:
> Hi Torsten,
>
> On 18/01/2019 16:39, Torsten Duwe wrote:
>
> > --- a/arch/arm64/kernel/ftrace.c
> > +++ b/arch/arm64/kernel/ftrace.c
> > @@ -133,17 +163,45 @@ int ftrace_make_call(struct dyn_ftrace *
> > return ftrace_modify_code(pc, old, new, true);
> > }
> >
> > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> > +int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
> > + unsigned long addr)
> > +{
> > + unsigned long pc = rec->ip + REC_IP_BRANCH_OFFSET;
> > + u32 old, new;
> > +
> > + old = aarch64_insn_gen_branch_imm(pc, old_addr, true);
> > + new = aarch64_insn_gen_branch_imm(pc, addr, true);
> > +
> > + return ftrace_modify_code(pc, old, new, true);
> > +}
> > +#endif
> > +
> > /*
> > * Turn off the call to ftrace_caller() in instrumented function
> > */
> > int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
> > unsigned long addr)
> > {
> > - unsigned long pc = rec->ip;
> > + unsigned long pc = rec->ip + REC_IP_BRANCH_OFFSET;
>
> Sorry to come back on this patch again, but I was looking at the ftrace
> code a bit, and I see that when processing the ftrace call locations,
> ftrace calls ftrace_call_adjust() on every ip registered as mcount
> caller (or in our case patchable entries). This ftrace_call_adjust() is
> arch specific, so I was thinking we could place the offset in here once
> and for all so we don't have to worry about it in the future.
Now that you mention it - yes indeed that's the correct facility to fix
the deviating address, as Steve has also confirmed. I had totally forgotten
about this hook.
> Also, I'm unsure whether it would be safe, but we could patch the "mov
> x9, lr" there as well. In theory, this would be called at init time
> (before secondary CPUs are brought up) and when loading a module (so I'd
> expect no-one is executing that code *yet*.
>
> If this is possible, I think it would make things a bit cleaner.
This is in fact very tempting, but it will introduce a nasty side effect
to ftrace_call_adjust. Is there any obvious documentation that specifies
guarantees about ftrace_call_adjust being called exactly once for each site?
Torsten
