Re: [PATCH 2/2] kernel: Support compiling out the prctl syscall

[Josh Triplett]

On Tue, Nov 08, 2016 at 04:40:02PM -0800, Kees Cook wrote:
> On Tue, Nov 8, 2016 at 4:18 PM, Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote:
> > Some embedded systems can do without the prctl syscall, saving some
> > space.
> >
> > This also avoids regular increases in tinyconfig size as people add more
> > non-optional functionality to prctl (observed via the 0-day kernel
> > infrastructure).
> >
> > bloat-o-meter results:
> >
> > add/remove: 0/3 grow/shrink: 0/1 up/down: 0/-2143 (-2143)
> > function                                     old     new   delta
> > offsets                                       23      12     -11
> > prctl_set_auxv                                97       -     -97
> > sys_prctl                                    794       -    -794
> > prctl_set_mm                                1241       -   -1241
> > Total: Before=1902583, After=1900440, chg -0.11%
> >
> > Signed-off-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> 
> I'm absolutely a fan of doing this, but I wonder how this interacts
> with the LSMs that define prctl hooks, etc. I wouldn't expect a system
> that didn't want prctl to want an LSM, but maybe the LSMs all need to
> depend on CONFIG_PRCTL now?

I did think about that (as well as SECCOMP), but I did confirm that the
kernel builds fine with allyesconfig minus CONFIG_PRCTL.  An LSM that
wants to restrict access to some prctls should be fine with no process
having any access to prctl. :)  Beyond that, anything wanting
configuration via LSM (such as SECCOMP) still exists and functions, even
if you can't access it from outside the kernel.