[Ksummit-discuss] [CORE TOPIC] Kernel tinification: shrinking the kernel and avoiding size regressions

[josh at joshtriplett.org]

On Fri, May 09, 2014 at 06:59:16PM +0200, Bird, Tim wrote:
> On Friday, May 09, 2014 9:22 AM Josh Triplett wrote:
> > 
> > On Fri, May 02, 2014 at 01:11:03PM -0400, Dave Jones wrote:
> > > - how much configurability here is too much ?
> > >   r_f_p was an obvious candidate because it's.. well, nasty.  Some of the
> > >   more straightforward syscalls may not be such a big deal, but then we
> > >   have CONFIG's for kcmp and other 'simple' syscalls already..
> > 
> > We need a more systematic mechanism, I think.  CONFIG_SYSCALL_FOO for
> > every possible FOO seems too much, even for classes of syscalls.
> > Ideally, we could feed in a table of syscalls collected by some
> > analysis of the target userspace, and the kernel will then have exactly
> > those syscalls.
> 
> In my system, I set it up so that every syscall had it's own
> SYSCALL_DEFINE macro. and then used a single header file
> consisting of lines like:
> #define syscall_setreuid16_unused 1
> 
> The SYSCALL_DEFINE macros would then control whether the
> syscall was extern'ed or not.  A separate mechanism converted
> the CALL macro in calls.S (on ARM) to use sys_ni_syscall, and
> LTO made the (now unreferenced) function evaporate.
> 
> Overall, this allowed control of every syscall with a single easily
> generated (or easily hand-edited) header file.  And, with a stub
> header file, everything worked as without the changes.
> 
> The header file was auto-generated by tools that scanned the
> user-space programs for all possible syscall sequences.
> 
> In hindsight this system could probably be improved with some
> extra tweaking to the base SYSCALL_DEFINE macros, to make
> it so no source changes were required at the function definition sites.

Another possibility: make all the syscall functions garbage-collectable,
and only keep those referenced from the actual syscall table.  Then
generate the syscall table accordingly.

- Josh Triplett