Re: [PATCH v3 linux-trace 1/8] tracing: attach eBPF programs to tracepoints and syscalls

On Tue, Feb 10, 2015 at 04:22:50PM -0800, Alexei Starovoitov wrote:

quoted

quoted

not all tools use libtraceevent.
gdb calls perf_event_open directly:
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=blob;f=gdb/nat/linux-btrace.c
and parses PERF_RECORD_SAMPLE as a binary.
In this case it's branch records, but I think we never said anywhere
that PERF_SAMPLE_IP | PERF_SAMPLE_ADDR should come
in this particular order.

What particular order? Note, that's a hardware event, not a software
one.

yes, but gdb assumes that 'u64 ip' precedes, 'u64 addr'
when attr.sample_type = IP | ADDR whereas this is an
internal order of 'if' statements inside perf_output_sample()...

This is indeed promised in the data layout description in
include/uapi/linux/perf_event.h.

There is no other way to find where fields are; perf relies on
predetermined order of fields coupled with the requested field bitmask.

So we promise the order: id, ip, pid, tid, time, addr,.. etc.

So if you request IP and ADDR but none of the other fields, then you
know your sample will start with IP and then contain ADDR.

The traceevent thing has a debug/trace-fs format description of fields
that is supposed to be used.