Re: [RFC PATCH 0/2] tracing/user_events: Remote write ABI

From: Mathieu Desnoyers
Date: Sat Oct 29 2022 - 09:58:41 EST


On 2022-10-28 18:17, Beau Belgrave wrote:
On Fri, Oct 28, 2022 at 05:50:04PM -0400, Mathieu Desnoyers wrote:
On 2022-10-27 18:40, Beau Belgrave wrote:

[...]


NOTE:
User programs that wish to have the enable bit shared across forks
either need to use a MAP_SHARED allocated address or register a new
address and file descriptor. If MAP_SHARED cannot be used or new
registrations cannot be done, then it's allowable to use MAP_PRIVATE
as long as the forked children never update the page themselves. Once
the page has been updated, the page from the parent will be copied over
to the child. This new copy-on-write page will not receive updates from
the kernel until another registration has been performed with this new
address.

This seems rather odd. I would expect that if a parent process registers
some instrumentation using private mappings for enabled state through the
user events ioctl, and then forks, the child process would seamlessly be
traced by the user events ABI while being able to also change the enabled
state from the userspace tracer libraries (which would trigger COW).
Requiring the child to re-register to user events is rather odd.


It's the COW that is the problem, see below.

What is preventing us from tracing the child without re-registration in this
scenario ?


Largely knowing when the COW occurs on a specific page. We don't make
the mappings, so I'm unsure if we can ask to be notified easily during
these times or not. If we could, that would solve this. I'm glad you are
thinking about this. The note here was exactly to trigger this
discussion :)

I believe this is the same as a Futex, I'll take another look at that
code to see if they've come up with anything regarding this.

Any ideas?

Based on your description of the symptoms, AFAIU, upon registration of a given user event associated with a mm_struct, the user events ioctl appears to translates the virtual address into a page pointer immediately, and keeps track of that page afterwards. This means it loses track of the page when COW occurs.

Why not keep track of the registered virtual address and struct_mm associated with the event rather than the page ? Whenever a state change is needed, the virtual-address-to-page translation will be performed again. If it follows a COW, it will get the new copied page. If it happens that no COW was done, it should map to the original page. If the mapping is shared, the kernel would update that shared page. If the mapping is private, then the kernel would COW the page before updating it.

Thoughts ?

Thanks,

Mathieu


Thanks,
-Beau

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com