In the chroot, make all mounted filesystems private

This is required on systemd, which mounts filesystems as "shared"
subtrees.  Changes to shared trees in a private mount namespace are
propagated to the outside world, which is bad.

src/libstore/build.cc

@@ -1853,6 +1853,24 @@ void DerivationGoal::initChild()
             char domainname[] = "(none)"; // kernel default
             setdomainname(domainname, sizeof(domainname));
 
+            /* Make all filesystems private.  This is necessary
+               because subtrees may have been mounted as "shared"
+               (MS_SHARED).  (Systemd does this, for instance.)  Even
+               though we have a private mount namespace, mounting
+               filesystems on top of a shared subtree still propagates
+               outside of the namespace.  Making a subtree private is
+               local to the namespace, though, so setting MS_PRIVATE
+               does not affect the outside world. */
+            Strings mounts = tokenizeString(readFile("/proc/self/mountinfo", true), "\n");
+            foreach (Strings::iterator, i, mounts) {
+                Strings fields = tokenizeString(*i, " ");
+                assert(fields.size() >= 5);
+                Strings::iterator j = fields.begin();
+                std::advance(j, 4);
+                if (mount(0, j->c_str(), 0, MS_PRIVATE, 0) == -1)
+                    throw SysError(format("unable to make filesystem `%1%' private") % *j);
+            }
+
             /* Bind-mount all the directories from the "host"
                filesystem that we want in the chroot
                environment. */

src/libutil/util.cc

@@ -224,12 +224,12 @@ string readFile(int fd)
 }
 
 
-string readFile(const Path & path)
+string readFile(const Path & path, bool drain)
 {
     AutoCloseFD fd = open(path.c_str(), O_RDONLY);
     if (fd == -1)
         throw SysError(format("opening file `%1%'") % path);
-    return readFile(fd);
+    return drain ? drainFD(fd) : readFile(fd);
 }
 
 

src/libutil/util.hh

@@ -63,7 +63,7 @@ Strings readDirectory(const Path & path);
 
 /* Read the contents of a file into a string. */
 string readFile(int fd);
-string readFile(const Path & path);
+string readFile(const Path & path, bool drain = false);
 
 /* Write a string to a file. */
 void writeFile(const Path & path, const string & s);