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Use unshare() instead of clone()

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It turns out that using clone() to start a child process is unsafe in
a multithreaded program. It can cause the initialisation of a build
child process to hang in setgroups(), as seen several times in the
build farm:

The reason is that Glibc thinks that the other threads of the parent
exist in the child, so in setxid_mark_thread() it tries to get a futex
that has been acquired by another thread just before the clone(). With
fork(), Glibc runs pthread_atfork() handlers that take care of this
(in particular, __reclaim_stacks()). But clone() doesn't do that.

Fortunately, we can use fork()+unshare() instead of clone() to set up
private namespaces.

See also https://www.mail-archive.com/lxc-devel@lists.linuxcontainers.org/msg03434.html.
This commit is contained in:
Eelco Dolstra 2014-08-21 14:08:09 +02:00
parent fefd3650d4
commit 524f89f139
1 changed files with 35 additions and 55 deletions
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90
src/libstore/build.cc
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@ -1575,13 +1575,6 @@ void chmod_(const Path & path, mode_t mode)
}
int childEntry(void * arg)
{
((DerivationGoal *) arg)->initChild();
return 1;
}
void DerivationGoal::startBuilder()
{
startNest(nest, lvlInfo, format(
@ -1897,48 +1890,10 @@ void DerivationGoal::startBuilder()
/* Create a pipe to get the output of the builder. */
builderOut.create();
/* Fork a child to build the package. Note that while we
currently use forks to run and wait for the children, it
shouldn't be hard to use threads for this on systems where
fork() is unavailable or inefficient.
If we're building in a chroot, then also set up private
namespaces for the build:
- The PID namespace causes the build to start as PID 1.
Processes outside of the chroot are not visible to those on
the inside, but processes inside the chroot are visible from
the outside (though with different PIDs).
- The private mount namespace ensures that all the bind mounts
we do will only show up in this process and its children, and
will disappear automatically when we're done.
- The private network namespace ensures that the builder cannot
talk to the outside world (or vice versa). It only has a
private loopback interface.
- The IPC namespace prevents the builder from communicating
with outside processes using SysV IPC mechanisms (shared
memory, message queues, semaphores). It also ensures that
all IPC objects are destroyed when the builder exits.
- The UTS namespace ensures that builders see a hostname of
localhost rather than the actual hostname.
*/
#if CHROOT_ENABLED
if (useChroot) {
char stack[32 * 1024];
pid = clone(childEntry, stack + sizeof(stack) - 8,
CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWNET | CLONE_NEWIPC | CLONE_NEWUTS | SIGCHLD, this);
} else
#endif
{
pid = fork();
if (pid == 0) initChild();
}
if (pid == -1) throw SysError("unable to fork");
/* Fork a child to build the package. */
pid = startProcess([&]() {
initChild();
});
/* parent */
pid.setSeparatePG(true);
@ -1965,14 +1920,41 @@ void DerivationGoal::initChild()
try { /* child */
_writeToStderr = 0;
restoreAffinity();
commonChildInit(builderOut);
#if CHROOT_ENABLED
if (useChroot) {
/* Set up private namespaces for the build:
- The PID namespace causes the build to start as PID 1.
Processes outside of the chroot are not visible to
those on the inside, but processes inside the chroot
are visible from the outside (though with different
PIDs).
- The private mount namespace ensures that all the bind
mounts we do will only show up in this process and
its children, and will disappear automatically when
we're done.
- The private network namespace ensures that the
builder cannot talk to the outside world (or vice
versa). It only has a private loopback interface.
- The IPC namespace prevents the builder from
communicating with outside processes using SysV IPC
mechanisms (shared memory, message queues,
semaphores). It also ensures that all IPC objects
are destroyed when the builder exits.
- The UTS namespace ensures that builders see a
hostname of localhost rather than the actual
hostname.
*/
if (unshare(CLONE_NEWNS | CLONE_NEWNET | CLONE_NEWIPC | CLONE_NEWUTS) == -1)
throw SysError("setting up private namespaces");
/* Initialise the loopback interface. */
AutoCloseFD fd(socket(PF_INET, SOCK_DGRAM, IPPROTO_IP));
if (fd == -1) throw SysError("cannot open IP socket");
@ -2169,8 +2151,6 @@ void DerivationGoal::initChild()
writeToStderr("while setting up the build environment: " + string(e.what()) + "\n");
_exit(1);
}
abort(); /* never reached */
}