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From: "Thomas Weißschuh" <linux@weissschuh.net>
To: Paul Moore <paul@paul-moore.com>
Cc: linux-audit@redhat.com, bpf@vger.kernel.org
Subject: Re: AUDIT_ARCH_ and __NR_syscall constants for seccomp filters
Date: Mon, 28 Jun 2021 19:13:25 +0200	[thread overview]
Message-ID: <696bf938-c9d2-4b18-9f53-b6ff27035a97@t-8ch.de> (raw)
In-Reply-To: <CAHC9VhSTb75NEPZRm+Tkngv=SW8ntmSpVCrXMHHHWc2qYNZqCA@mail.gmail.com>

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@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

  reply	other threads:[~2021-06-28 17:13 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-28  7:31 Thomas Weißschuh
2021-06-28 16:59 ` Paul Moore
2021-06-28 17:13   ` Thomas Weißschuh [this message]
2021-06-28 17:34     ` Paul Moore
2021-06-28 17:58       ` Thomas Weißschuh
2021-06-28 22:43         ` Paul Moore
2021-06-29 10:40           ` Thomas Weißschuh
2021-06-29 23:41             ` Paul Moore

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