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Re: chroot's userspec option
From: |
Ken Werner |
Subject: |
Re: chroot's userspec option |
Date: |
Fri, 28 Feb 2014 11:34:38 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.3.0 |
On 02/27/2014 05:45 PM, Pádraig Brady wrote:
On 02/27/2014 03:48 PM, Ken Werner wrote:
Hi,
I noticed when using chroot's --userspec option the Gnulib's parse_user_spec function
gets called that leads the glibc to dlopen libnss_compat.so.2 (probably getpwnam() that
triggers the libc's NSS mechanism). Since parse_user_spec is called after the chroot
system call the new root directory will be searched. I guess this means that the chroot
utility attempts to parse the user spec in the "guest" environment. Is this
behavior intended? In my case the chroot environment contains a libnss_compat.so.2 that's
not compatible and the chroot utility fails with:
/usr/bin/chroot: relocation error: /lib/libnss_compat.so.2: symbol
_nss_files_parse_pwent, version GLIBC_2.0 not defined in file libc.so.6 with
link time reference
As soon as I LD_PRELOAD libnss_compat.so.2 the "host" environment is used to
parse the user spec. If this is the intended behavior it would be better if chroot calls
the parse_user_spec prior issuing the chroot syscall. Any thoughts? :)
This issue was noted previously with an explicit workaround:
http://lists.gnu.org/archive/html/coreutils/2011-07/msg00057.html
Then again with an implicit workaround:
http://lists.gnu.org/archive/html/coreutils/2012-05/msg00018.html
I had mentioned an amendment to that but there was no response.
I'll look at a fix now to do:
t_ids = parse_user_spec(); //outside chroot
ids = parse_user_spec(); //inside chroot
if (!ids)
ids = t_ids;
Thank you for providing those pointers! I have to admit that it's still
not clear to me whether the userspec option is supposed to lookup the
user/group using the A) the old or B) the new root. In case of A) the
fix would be call parse_user_spec prior switching to the new root. While
B) is not trivial to support imho. The way it's implemented by now
assumes the libc's NSS plugins of the new root are compatible to the
libc of the old root. As you noticed that's not the case when chrooting
into a 32bit userland on a 64bit system (and there are many more cases).
Since I do not really depend on the uid/gid lookup I wondered why
getpwnam() and getgrnam() are still called even if numeric IDs are
provided rather than the names. It turns out the code [1] only skips the
lookup if the IDs are prefixed with '+'. For example:
chroot --userspec=+1234:+1234 /path/to/new/root
Unfortunately the --groups option doesn't have a way to skip the lookup
currently [2]. It calls getgrgid/getgrnam that probably trigger libc's
NSS plugins as well.
I guess the first thing would be to discuss and decide which approach is
the desired one - then to post patches that changes the docs+code
accordingly. /me ducks ;)
[1] Gnulib's userspec.c:parse_user_spec calls parse_with_separator that
skips the lookup in case the first char is a '+':
http://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=blob;f=lib/userspec.c;h=1be9266eb54638a2624d0a9205d8e68fd516205e;hb=HEAD#l160
[2] Coretutil's chroot.c:main calls set_additional_groups() that calls
getgrgid/getgrnam:
http://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=blob;f=src/chroot.c;h=50bb2537ea7df4b963e151bbcb54c217533f32d0;hb=HEAD#l66
Regards,
Ken