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

Kees Cook
2016-11-09T00:40:08Z

On Tue, Nov 8, 2016 at 4:18 PM, Josh Triplett <josh@joshtriplett.org> 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@joshtriplett.org>

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?

-Kees

> ---
> init/Kconfig | 12 ++++++++++++
> kernel/Makefile | 3 ++-
> kernel/sys_ni.c | 1 +
> 3 files changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/init/Kconfig b/init/Kconfig
> index 34407f1..1dd671c 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1408,6 +1408,18 @@ config MULTIUSER
>
> If unsure, say Y here.
>
> +config PRCTL
> + bool "prctl syscall" if EXPERT
> + default y
> + help
> + This option enables the prctl syscall, used for a variety of
> + operations on the current process.
> +
> + If building an embedded system where no applications or libraries use
> + prctl, you can disable this option to save space.
> +
> + If unsure, say Y here.
> +
> config SGETMASK_SYSCALL
> bool "sgetmask/ssetmask syscalls support" if EXPERT
> def_bool PARISC || MN10300 || BLACKFIN || M68K || PPC || MIPS || X86 || SPARC || CRIS || MICROBLAZE || SUPERH
> diff --git a/kernel/Makefile b/kernel/Makefile
> index 37c6d4c..43fb4ca 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -9,9 +9,10 @@ obj-y = fork.o exec_domain.o panic.o \
> extable.o params.o \
> kthread.o sys_ni.o nsproxy.o \
> notifier.o ksysfs.o cred.o reboot.o \
> - async.o range.o smpboot.o ucount.o prctl.o
> + async.o range.o smpboot.o ucount.o
>
> obj-$(CONFIG_MULTIUSER) += groups.o
> +obj-$(CONFIG_PRCTL) += prctl.o
>
> ifdef CONFIG_FUNCTION_TRACER
> # Do not trace internal ftrace files
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index 635482e..84fd646 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -175,6 +175,7 @@ cond_syscall(sys_setfsgid);
> cond_syscall(sys_capget);
> cond_syscall(sys_capset);
> cond_syscall(sys_copy_file_range);
> +cond_syscall(sys_prctl);
>
> /* arch-specific weak syscall entries */
> cond_syscall(sys_pciconfig_read);
> --
> git-series 0.8.11

--
Kees Cook
Nexus Security