bpf_probe_read() split: bpftrace RFC

Brendan Gregg


This is the biggest change afoot to the bpftrace API, and I think we
can sort it out quickly without fuss, but it is worth sharing here.
This is from https://github.com/iovisor/bpftrace/issues/614 .

bpftrace currently allows pointer dereferencing via *addr, and
str(addr) for strings. But the future split of bpf_probe_read() into
bpf_probe_read_user() and bpf_probe_read_kernel() (to support SPARC,
etc) may break a lot of bpftrace tools and documentation. Or it may
not, if we are clever about it.

The proposal is this: add the following bpftrace builtins:

- uptr(addr): dereference user address
- ustr(addr): fetch NULL-terminated user string
- kptr(addr): dereference kernel address
- kstr(addr): fetch NULL-terminated kernel string

AND, to introduce a "context" for probe actions -- user or kernel --
where *addr and str(addr) work relative to that context. The context
would be:

- kprobes/kretprobes: kernel
- uprobes/uretprobes: user
- tracepoints: kernel (with the exception of syscall tracepoints: user)
- other probe types: kernel

It's possible that this context approach leaves us with zero broken
tools and documentation (ie, there are zero cases so far where we even
need to use uptr/ustr/kptr/kstr). I'm still checking and looking for
exceptions. Where you can help: can you think of a syscall tracepoint
that has a kernel address as an argument? Or another non-syscall
tracepoint that has a user-address as an argument? Or can you think of
any other problem with this plan?



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