Re: [uml-devel] [RFC] PATCH 3/4 - Time virtualization : PTRACE_SYSCALL_MASK

[Jeff Dike <jdike@addtoit.com>]

On Mon, May 01, 2006 at 09:51:27AM -0400, Daniel Jacobowitz wrote:
> On Fri, Apr 28, 2006 at 09:49:56PM -0400, Jeff Dike wrote:
> > On Fri, Apr 28, 2006 at 10:28:46PM +0200, Blaisorblade wrote:
> > > bitmask = 0;
> > > set_bit(__NR_tee, bitmask);
> > > ptrace(PTRACE_SET_TRACEONLY, bitmask);
> > 
> > Yup, I like this.
> 
> I really recommend you not do this.  

> Suppose the kernel knows about 32 more syscalls than userspace.  It's
> going to read extra bits out of the bitmask that userspace didn't
> initialize!

The example above is a sketch, not a fully formed, compilable user.  Every
proposed interface has had the mask length passed in - in the case
above in the data argument.

> Also, if you store the mask with the child process, it risks surprising
> existing tracers: attach, set mask, detach, then the next time someone
> attaches an old version of strace some syscalls will be "hidden".

Not if the mask only survives for the duration of a PTRACE_ATTACH, and
the mask is released on PTRACE_DETACH.

				Jeff