Re: AUDIT_ARCH_ and __NR_syscall constants for seccomp filters
Hi Paul,
thanks for your response!
On Mo, 2021-06-28T12:59-0400, Paul Moore wrote:
On Mon, Jun 28, 2021 at 9:25 AM Thomas Weißschuh
<linux(a)weissschuh.net> wrote:
>
> Hi everyone,
>
> there does not seem to be a way to access the AUDIT_ARCH_ constant that matches
> the currently visible syscall numbers (__NR_...) from the kernel uapi headers.
Looking at Linus' current tree I see the AUDIT_ARCH_* defines in
include/uapi/linux/audit.h; looking on my system right now I see the
defines in /usr/include/linux/audit.h. What kernel repository and
distribution are you using?
I am using ArchLinux and also have all these defines.
> Questions:
>
> Is it really necessary to validate the arch value when syscall numbers are
> already target-specific?
> (If not, should this be added to the docs?)
Checking the arch/ABI value is important so that you can ensure that
you are using the syscall number in the proper context. For example,
look at the access(2) syscall: it is undefined on some ABIs and can
take either a value of 20, 21, or 33 depending on the arch/ABI.
Unfortunately this is rather common.
But when if I am not hardcoding the syscall numbers but use the
__NR_access kernel define then I should always get the correct number for the
ABI I am compiling for (or an error if the syscall does not exist), no?
Checking the arch/ABI value is also handy if you want to quickly
disallow certain ABIs on a system that supports multiple ABI, e.g.
disabling 32-bit x86 on a 64-bit x86_64 system.
> Would it make sense to expose the audit arch matching the syscall numbers in
> the uapi headers?
Yes, which is why the existing headers do so ;) If you don't see the
header files I mentioned above, it may be worth checking your kernel
source repository and your distribution's installed kernel header
files.
I do see constants for all the possible ABIs but not one constant that always
represents the one I am currently compiling for.
The same way the syscall number defines always give me the syscall number for
the currently targeted ABI.
Thomas