[Toybox] unshare/nsenter on centos 6.6.

enh enh at google.com
Thu Sep 29 08:34:05 PDT 2016

i'm led to believe (based on "i can't run glibc 2.15 binaries on
centos" bugs such as
https://code.google.com/p/android/issues/detail?id=197074) that even
centos 6.7 is only glibc 2.12!

"enterprise" == "unusably stale".

On Wed, Sep 28, 2016 at 1:18 PM, Rob Landley <rob at landley.net> wrote:
> I have a report that toybox won't build under centos 6.6 because
> unshare() and nsenter() exist in linux/sched.h but not in libc.so. While
> I can fix this with basically:
> @@ -63,9 +64,11 @@ config NSENTER
>  #define FOR_nsenter
>  #include "toys.h"
> +#include <sys/syscall.h>
>  #include <linux/sched.h>
> -int unshare(int flags);
> -int setns(int fd, int nstype);
> +
> +#define unshare(flags) syscall(SYS_unshare, flags)
> +#define setns(fd, nstype) syscall(SYS_setns, fd, nstype)
> I'm unsure what the right approach is here?
> There's a compile-time probe checking to see if unshare() exists in the
> headers, but I'm using linux/sched.h instead of sched.h because the
> unshare(2) man page says:
>    Feature Test Macro Requirements for glibc (see
>    feature_test_macros(7)):
>        unshare():
>            Since glibc 2.14:
>                _GNU_SOURCE
>            Before glibc 2.14:
>                _BSD_SOURCE || _SVID_SOURCE
>                    /* _GNU_SOURCE also suffices */
> I.E. they used to expose unshare() by default when you #included
> <sched.h>, then they decided to break existing code, and that unshare
> hadn't been created by Linux but had instead been created by The Hurd.
> Or possibly they were declaring this new kernel syscall to be a
> glibc-specific extension and refusing to wrap it by default? I don't
> know, either way this is INSANE and I'm not going along with it. So I
> included the linux header to get the function definition, but trusted
> libc's system call wrappers to still be there 9because that's not
> related to which headers you #included. Yes, they care hugely about
> namespace pollution on your #includes, but the functions are still there
> in libc. Jazzhands!)
> So centos has a kernel with the system call, but their libc isn't
> wrapping the system call. Except... the probe I wrote is not just
> compiling but linking (in scripts/genconfig.sh the probesymbol
> TOYBOX_CONTAINER call does not have -c after it, meaning gcc will
> attempt to compile _and_ link the result, and report failure if it
> couldn't link the binary either.)
> I THINK what the reporter was hitting (don't currently have a centos
> test system) is setns() not being there. Because the probe was testing
> for unshare() but not setns(), and the command uses both. So this whole
> "do I trust the libc wrapper functions to be there when I know they
> broke their headers for crazy political reasons, or do I just do the
> syscall() myself" is a side issue. It sounds like there was a kernel
> version that had unshare() but not setns(), and centos fell in that hole
> and provides a strange broken build environment in the name of "stability".
> Yeah, the unshare system call was added in 2006, nsenter in 2010. So the
> fix isn't to change toys/other/nsenter.c, but to add setns() to the
> probe and say "nope, centos can't do this command" when the probe fails
> due to the 6+ year old kernel. (If I did the syscall myself, it wouldn't
> have the SYS_setns in sys/syscall.h anyway...)
> That said... is it worse to prototype the wrapper functions myself, or
> to make the syscall myself? (It's one of those "six of one" things where
> I'm not really happy with either answer...)
> Rob
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Elliott Hughes - http://who/enh - http://jessies.org/~enh/
Android native code/tools questions? Mail me/drop by/add me as a reviewer.

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