Re: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor

On Sun, Aug 2, 2020 at 11:54 AM Madhavan T. Venkataraman
[off-list ref] wrote:

More responses inline..

On 7/28/20 12:31 PM, Andy Lutomirski wrote:

quoted

quoted

On Jul 28, 2020, at 6:11 AM, madvenka@linux.microsoft.com wrote:

From: "Madhavan T. Venkataraman" [off-list ref]

2. Use existing kernel functionality.  Raise a signal, modify the
state, and return from the signal.  This is very flexible and may not
be all that much slower than trampfd.

Let me understand this. You are saying that the trampoline code
would raise a signal and, in the signal handler, set up the context
so that when the signal handler returns, we end up in the target
function with the context correctly set up. And, this trampoline code
can be generated statically at build time so that there are no
security issues using it.

Have I understood your suggestion correctly?

yes.

So, my argument would be that this would always incur the overhead
of a trip to the kernel. I think twice the overhead if I am not mistaken.
With trampfd, we can have the kernel generate the code so that there
is no performance penalty at all.

I feel like trampfd is too poorly defined at this point to evaluate.
There are three general things it could do.  It could generate actual
code that varies by instance.  It could have static code that does not
vary.  And it could actually involve a kernel entry.

If it involves a kernel entry, then it's slow.  Maybe this is okay for
some use cases.

If it involves only static code, I see no good reason that it should
be in the kernel.

If it involves dynamic code, then I think it needs a clearly defined
use case that actually requires dynamic code.

Also, signals are asynchronous. So, they are vulnerable to race conditions.
To prevent other signals from coming in while handling the raised signal,
we would need to block and unblock signals. This will cause more
overhead.

If you're worried about raise() racing against signals from out of
thread, you have bigger problems to deal with.