On Wed, Dec 01, 2021 at 09:59:57AM -0800, Andrii Nakryiko wrote: > On Wed, Dec 1, 2021 at 9:37 AM Alexei Starovoitov > <alexei.starovoitov@xxxxxxxxx> wrote: > > > > On Tue, Nov 30, 2021 at 11:13 PM Andrii Nakryiko > > <andrii.nakryiko@xxxxxxxxx> wrote: > > > > > > Hm... I'd actually try to keep kprobe BTF-free. We have fentry for > > > cases where BTF is present and the function is simple enough (like <=6 > > > args, etc). Kprobe is an escape hatch mechanism when all the BTF > > > fanciness just gets in the way (retsnoop being a primary example from > > > my side). What I meant here was that bpf_get_arg(int n) would read > > > correct fields from pt_regs that map to first N arguments passed in > > > the registers. What we currently have with PT_REGS_PARM macros in > > > bpf_tracing.h, but with a proper unified BPF helper. > > > > and these macros are arch specific. > > which means that it won't be a trivial patch to add bpf_get_arg() > > support for kprobes. > > no one suggested it would be trivial :) things worth doing are usually > non-trivial, as can be evidenced by Jiri's patch set > > > Plenty of things to consider. Like should it return an error > > at run-time or verification time when a particular arch is not supported. > > See my other replies to Jiri, I'm more and more convinced that dynamic > is the way to go for things like this, where the safety of the kernel > or BPF program are not compromised. > > But you emphasized an important point, that it's probably good to > allow users to distinguish errors from reading actual value 0. There > are and will be situations where argument isn't available or some > combination of conditions are not supported. So I think, while it's a > bit more verbose, these forms are generally better: > > int bpf_get_func_arg(int n, u64 *value); > int bpf_get_func_ret(u64 *value); > > WDYT? ok, good preparation for kprobe code quirks described by Alexei > > > Or argument 6 might be available on one arch, but not on the other. > > 32-bit CPU regs vs 64-bit regs of BPF, etc. > > I wouldn't attempt to mix this work with current patches. > > Oh, I didn't suggest doing it as part of this already huge and > complicated set. But I think it's good to think a bit ahead and design > the helper API appropriately, at the very least. > > And again, I think bpf_get_func_arg/bpf_get_func_ret deserve their own > patch set where we can discuss all this independently from > multi-attach. > good ;-) thanks, jirka
