Re: [PATCH bpf-next v5 0/3] add support for writable bare tracepoint

Hi Steven,

On 10/4/2021 10:46 PM, Steven Rostedt wrote:

On Mon,  4 Oct 2021 17:48:54 +0800
Hou Tao [off-list ref] wrote:

quoted

The main idea comes from patchset "writable contexts for bpf raw
tracepoints" [1], but it only supports normal tracepoint with
associated trace event under tracefs. Now we have one use case
in which we add bare tracepoint in VFS layer, and update
file::f_mode for specific files. The reason using bare tracepoint
is that it doesn't form a ABI and we can change it freely. So
add support for it in BPF.

Are the VFS maintainers against adding a trace event with just a pointer as
an interface?

Not tried yet, but considering that VFS maintainer refused to have tracepoint in
VFS layer, I'm not sure it is worth trying.

That is, it only gives you a pointer to what is passed in, but does not
give you anything else to form any API against it.
This way, not only does BPF have access to this information, so do the
other tracers, through the new eprobe interface:

Or in a opposite way can eprobe add support for bare tracepoint ?

  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/Documentation/trace?id=7491e2c442781a1860181adb5ab472a52075f393

(I just realized we are missing updates to the Documentation directory).

event probes allows one to attach to an existing trace event, and then
create a new trace event that can read through pointers. It uses the same
interface that kprobes has.

Just adding trace events to VFS that only have pointers would allow all of
BPF, perf and ftrace access as eprobes could then get the data you are
looking for.

-- Steve
.