strace of io_uring events?

[Matthew Wilcox]

On Wed, Jul 15, 2020 at 07:35:50AM -0700, Andy Lutomirski wrote:
> > On Jul 15, 2020, at 4:12 AM, Miklos Szeredi <miklos at szeredi.hu> wrote:
> > 
> > <feff>Hi,

feff?  Are we doing WTF-16 in email now?  ;-)

> > 
> > This thread is to discuss the possibility of stracing requests
> > submitted through io_uring.   I'm not directly involved in io_uring
> > development, so I'm posting this out of  interest in using strace on
> > processes utilizing io_uring.
> > 
> > io_uring gives the developer a way to bypass the syscall interface,
> > which results in loss of information when tracing.  This is a strace
> > fragment on  "io_uring-cp" from liburing:
> > 
> > io_uring_enter(5, 40, 0, 0, NULL, 8)    = 40
> > io_uring_enter(5, 1, 0, 0, NULL, 8)     = 1
> > io_uring_enter(5, 1, 0, 0, NULL, 8)     = 1
> > ...
> > 
> > What really happens are read + write requests.  Without that
> > information the strace output is mostly useless.
> > 
> > This loss of information is not new, e.g. calls through the vdso or
> > futext fast paths are also invisible to strace.  But losing filesystem
> > I/O calls are a major blow, imo.
> > 
> > What do people think?
> > 
> > From what I can tell, listing the submitted requests on
> > io_uring_enter() would not be hard.  Request completion is
> > asynchronous, however, and may not require  io_uring_enter() syscall.
> > Am I correct?
> > 
> > Is there some existing tracing infrastructure that strace could use to
> > get async completion events?  Should we be introducing one?
> > 
> > 
> 
> Let’s add some seccomp folks. We probably also want to be able to run seccomp-like filters on io_uring requests. So maybe io_uring should call into seccomp-and-tracing code for each action.

Adding Stefano since he had a complementary proposal for iouring
restrictions that weren't exactly seccomp.