Re: [PATCH 3/6] seccomp: Implement constant action bitmaps

[Kees Cook]

On Wed, Sep 23, 2020 at 07:36:47PM -0500, YiFei Zhu wrote:
> On Wed, Sep 23, 2020 at 6:29 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> > In order to optimize these cases from O(n) to O(1), seccomp can
> > use bitmaps to immediately determine the desired action. A critical
> > observation in the prior paragraph bears repeating: the common case for
> > syscall tests do not check arguments. For any given filter, there is a
> > constant mapping from the combination of architecture and syscall to the
> > seccomp action result. (For kernels/architectures without CONFIG_COMPAT,
> > there is a single architecture.). As such, it is possible to construct
> > a mapping of arch/syscall to action, which can be updated as new filters
> > are attached to a process.
> 
> Would you mind educating me how this patch plan one handling MIPS? For
> one kernel they seem to have up to three arch numbers per build,
> AUDIT_ARCH_MIPS{,64,64N32}. Though ARCH_TRACE_IGNORE_COMPAT_SYSCALLS
> does not seem to be defined for MIPS so I'm assuming the syscall
> numbers are the same, but I think it is possible some client uses that
> arch number to pose different constraints for different processes, so
> it would better not accelerate them rather than break them.

I'll take a look, but I'm hoping it won't be too hard to fit into what
I've got designed so for to deal with x86_x32. (Will MIPS want this
optimization at all?)

-- 
Kees Cook