On Mon, Aug 15, 2022 at 07:31:23AM -0700, Alexei Starovoitov wrote: > On Mon, Aug 15, 2022 at 7:13 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > > > > On Wed, 11 Dec 2019 13:30:13 +0100 > > Björn Töpel <bjorn.topel@xxxxxxxxx> wrote: > > > > > From: Björn Töpel <bjorn.topel@xxxxxxxxx> > > > > > > The BPF dispatcher is a multi-way branch code generator, mainly > > > targeted for XDP programs. When an XDP program is executed via the > > > bpf_prog_run_xdp(), it is invoked via an indirect call. The indirect > > > call has a substantial performance impact, when retpolines are > > > enabled. The dispatcher transform indirect calls to direct calls, and > > > therefore avoids the retpoline. The dispatcher is generated using the > > > BPF JIT, and relies on text poking provided by bpf_arch_text_poke(). > > > > > > The dispatcher hijacks a trampoline function it via the __fentry__ nop > > > > Why was the ftrace maintainers not Cc'd on this patch? I would have NACKED > > it. Hell, it wasn't even sent to LKML! This was BPF being sneaky in > > updating major infrastructure of the Linux kernel without letting the > > stakeholders of this change know about it. > > > > For some reason, the BPF folks think they own the entire kernel! > > > > When I heard that ftrace was broken by BPF I thought it was something > > unique they were doing, but unfortunately, I didn't investigate what they > > were doing at the time. > > ftrace is still broken and refusing to accept the fact doesn't make it > non-broken. Alexei, stop this. The 'call __fentry__' sites are owned by ftrace. Always have been. If BPF somehow thinks it can use them without telling ftrace then it's BPF that's broken. > > Then they started sending me patches to hide fentry locations from ftrace. > > And even telling me that fentry != ftrace > > It sounds that you've invented nop5 and kernel's ability > to replace nop5 with a jump or call. Ftrace has introduced the mcount/fentry patching into the kernel and has always owned it for those sites. There is a lot of other text writing not owned by ftrace. But the fentry sites are ftrace's. Ftrace was also the one that got us the text_poke_bp() infrastructure and got it reviewed by the CPU vendors. Since then we've grown static_branch and static_call, they have their own patch sites and do no interfere with ftrace. > ftrace should really stop trying to own all of the kernel text rewrites. > It's in the way. Like this case. It doesn't. It hasn't. But it *does* own the fentry sites. > It was implemented long before static_calls made it to the kernel > and it's different. It wasn't long before. Yes it landed a few months prior to the static_call work, but the whole static_call thing was in progress for a long long time. Anyway, yes it is different. But it's still very much broken. You simply cannot step on __fentry__ sites like that.
