Re: Seccomp implications for glibc wrapper function changes

On 11/09/2017 12:58 PM, Michael Kerrisk (man-pages) wrote:

[CC += Kees, in case he has some comments]

On 9 November 2017 at 08:17, Michael Kerrisk (man-pages)
[off-list ref] wrote:

quoted

Hi Florian,

On 8 November 2017 at 07:24, Florian Weimer [off-list ref] wrote:

quoted

On 11/07/2017 09:35 PM, Michael Kerrisk (man-pages) wrote:

quoted

This change broke my code that was doing seccomp filtering for the
open() system call number (__NR_open). The breakage in question is not
serious, since this was really just demonstration code. However, I
want to raise awareness that these sorts of changes have the potential
to possibly cause breakages for some code using seccomp, and note that
I think such changes should not be made lightly or gratuitously.

I have the opposite view: We should make such changes as often as possible,
to remind people that seccomp filters (and certain SELinux and AppArmor
policies) are incompatible with the GNU/Linux model, where everything is
developed separately and not maintained within a single source tree (unlike
say OpenBSD).  This means that you really can't deviate from the upstream
Linux userspace ABI (in the broadest possible sense) and still expect things
to work.

I know that people like to slap seccomp filters on everything today, but
without careful examination, that is likely to introduce bugs (particularly
on rarely used code paths).  It can also cause the process to switch to
legacy interfaces with known issues (e.g., reading from /dev/urandom instead
of getrandom, without waiting for the kernel to signal initialization of the
pool).

Thanks. The above is a good summary of the counterpoints to my initial argument.

Florian, taking your and Adhemerval's useful comments into account, I
added the following text to the seccomp(2) manual page:

[[
    Caveats
        There  are  various  subtleties  to consider when applying seccomp
        filters to a program, including the following:

        *  Some traditional system calls have  user-space  implementations
           in the vdso(7) on many architectures.  Notable examples include
           clock_gettime(2), gettimeofday(2), and time(2).  On such archi‐
           tectures, seccomp filtering for these system calls will have no
           effect.

I think the situation is more complicated for many of those because they 
can still perform system calls on their fallback paths.  So it's one 
more case where seccomp can give you unpredictable failures.

Rest looks good to me.  Thanks for the writeup.

Florian