/dev/shm inconsistency in chroot

* /dev/shm inconsistency in chroot
@ 2014-01-23 19:21 Sree Harsha Totakura
  2014-01-23 19:56 ` Mark H Weaver
  0 siblings, 1 reply; 10+ messages in thread
From: Sree Harsha Totakura @ 2014-01-23 19:21 UTC (permalink / raw)
  To: guix-devel, nix-dev


[-- Attachment #1.1.1: Type: text/plain, Size: 2222 bytes --]

Hi,

It has been known for a while that any access to /dev/shm fails inside
chroot if the host system has /dev/shm symlinked to /run/shm.

The suggested method to deal with this until now is to remove the
symlink from the host system, create /dev/shm directory and mount a
tmpfs on it.

I spent sometime investigating why this is needed and here are some
points I noted: from libstore/build.cc the daemon tries to bind mount
some paths from the host system into the chroot directory.  Among these
are /dev and /dev/pts.  The daemon also mounts a tmpfs file system under
chrootdir+/dev/shm should the path /dev/shm exist.

Due to bind mounting /dev, on systems where /dev/shm is a symlink, the
symlink is also present in the chrootdir+/dev.  Since the symlink points
to /run/shm, and since /run is not bind mounted by default into the
chrootdir+/run, the symlink is broken inside chroot.

The above problem can be addressed by passing --chroot-directory=/run to
the guix-daemon which then includes /run into the list of mounts that
are to be bind mounted inside chroot.  This will resolve the
chroot+/dev/shm symlink properly.  Yet, the accesses to /dev/shm fail
inside the chroot.  This is because the mount statement in build.cc for
mounting tmpfs at chroot+/dev/shm mounts the tmpfs at /run/shm the
target of the symlink and since /run tree is made private the mount does
not propagate into the bind mounted chroot+/run tree.  In the chroot,
this leaves the /dev/shm symlink to point to the directory /run/shm and
obviously any shared memory accesses fail.

This problem can be fixed finally by passing
--chroot-directory=/run/shm.  Although the tmpfs mount for
chroot+/dev/shm still ends up mounting tmpfs at /run/shm, since /run/shm
is now bind mounted to chroot+/run/shm, inside the chroot the symlink
/dev/shm points to /run/shm which is now a tmpfs.  The shared memory
accesses work fine here.  However there is caveat: since /run/shm is
bindmounted any files from the host system from that directory are also
present in the chroot.

To address this further and to get rid of having the user to pass
--chroot-directory flag, I propose the attached patch.

Regards,
Sree

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1.1.2: 0001-Create-tmpfs-on-dev-shm-after-chrooting.patch --]
[-- Type: text/x-patch; name="0001-Create-tmpfs-on-dev-shm-after-chrooting.patch", Size: 2198 bytes --]

From 359fda36b6dcabea79a76b56e10d4d67702f545f Mon Sep 17 00:00:00 2001
From: Sree Harsha Totakura <sreeharsha@totakura.in>
Date: Thu, 23 Jan 2014 20:11:57 +0100
Subject: [PATCH] Create tmpfs on /dev/shm after chrooting.

src/libstore/build.cc: Create tmpfs on /dev/shm after chrooting.  If /dev/shm is a link, create required directory for mounting.
---
 src/libstore/build.cc |   17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 4329d9a..b01bf92 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -2054,12 +2054,6 @@ void DerivationGoal::initChild()
             if (mount("none", (chrootRootDir + "/proc").c_str(), "proc", 0, 0) == -1)
                 throw SysError("mounting /proc");
 
-            /* Mount a new tmpfs on /dev/shm to ensure that whatever
-               the builder puts in /dev/shm is cleaned up automatically. */
-            if (pathExists("/dev/shm"))
-                if (mount("none", (chrootRootDir + "/dev/shm").c_str(), "tmpfs", 0, 0) == -1)
-                    throw SysError("mounting /dev/shm");
-
             /* Do the chroot().  Below we do a chdir() to the
                temporary build directory to make sure the current
                directory is in the chroot.  (Actually the order
@@ -2067,6 +2061,17 @@ void DerivationGoal::initChild()
                tmpRootDit/tmpDir are the same directories.) */
             if (chroot(chrootRootDir.c_str()) == -1)
                 throw SysError(format("cannot change root directory to `%1%'") % chrootRootDir);
+
+            /* Mount a new tmpfs on /dev/shm to ensure that whatever
+               the builder puts in /dev/shm is cleaned up automatically. */
+            if (pathExists ("/dev/shm"))
+            {
+                Path target = "/dev/shm";
+                if (isLink(target) && !pathExists(target = readLink(target)))
+                    createDirs(target);
+                if (mount("none", "/dev/shm", "tmpfs", 0, 0) == -1)
+                    throw SysError("mounting /dev/shm");
+            }
         }
 #endif
 
-- 
1.7.10.4


[-- Attachment #1.2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 259 bytes --]

[-- Attachment #2: Type: text/plain, Size: 149 bytes --]

_______________________________________________
nix-dev mailing list
nix-dev@lists.science.uu.nl
http://lists.science.uu.nl/mailman/listinfo/nix-dev

^ permalink raw reply related	[flat|nested] 10+ messages in thread