Re: [PATCH bpf-next 06/29] bpf: Add bpf_arg/bpf_ret_value helpers for tracing programs

On Tue, Nov 30, 2021 at 11:13 PM Andrii Nakryiko
[off-list ref] 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.
Plenty of things to consider. Like should it return an error
at run-time or verification time when a particular arch is not supported.
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.