Merge branch 'test-sandboxing' of https://github.com/matthewbauer/nix

src/libstore/build.cc

@@ -2357,17 +2357,37 @@ void DerivationGoal::startBuilder()
                 flags |= CLONE_NEWNET;
 
             pid_t child = clone(childEntry, stack + stackSize, flags, this);
-            if (child == -1 && errno == EINVAL)
+            if (child == -1 && errno == EINVAL) {
                 /* Fallback for Linux < 2.13 where CLONE_NEWPID and
                    CLONE_PARENT are not allowed together. */
-                child = clone(childEntry, stack + stackSize, flags & ~CLONE_NEWPID, this);
+                flags &= ~CLONE_NEWPID;
+                child = clone(childEntry, stack + stackSize, flags, this);
+            }
+            if (child == -1 && (errno == EPERM || errno == EINVAL)) {
+                /* Some distros patch Linux to not allow unpriveleged
+                 * user namespaces. If we get EPERM or EINVAL, try
+                 * without CLONE_NEWUSER and see if that works.
+                 */
+                flags &= ~CLONE_NEWUSER;
+                child = clone(childEntry, stack + stackSize, flags, this);
+            }
+            /* Otherwise exit with EPERM so we can handle this in the
+               parent. This is only done when sandbox-fallback is set
+               to true (the default). */
+            if (child == -1 && (errno == EPERM || errno == EINVAL) && settings.sandboxFallback)
+                _exit(1);
             if (child == -1) throw SysError("cloning builder process");
 
             writeFull(builderOut.writeSide.get(), std::to_string(child) + "\n");
             _exit(0);
         }, options);
 
-        if (helper.wait() != 0)
+        int res = helper.wait();
+        if (res != 0 && settings.sandboxFallback) {
+            useChroot = false;
+            tmpDirInSandbox = tmpDir;
+            goto fallback;
+        } else if (res != 0)
             throw Error("unable to start build process");
 
         userNamespaceSync.readSide = -1;
@@ -2398,6 +2418,7 @@ void DerivationGoal::startBuilder()
     } else
 #endif
     {
+    fallback:
         options.allowVfork = !buildUser && !drv->isBuiltin();
         pid = startProcess([&]() {
             runChild();

src/libstore/globals.hh

@@ -209,6 +209,9 @@ public:
         "The paths to make available inside the build sandbox.",
         {"build-chroot-dirs", "build-sandbox-paths"}};
 
+    Setting<bool> sandboxFallback{this, true, "sandbox-fallback",
+        "Whether to disable sandboxing when the kernel doesn't allow it."};
+
     Setting<PathSet> extraSandboxPaths{this, {}, "extra-sandbox-paths",
         "Additional paths to make available inside the build sandbox.",
         {"build-extra-chroot-dirs", "build-extra-sandbox-paths"}};

src/nix/run.cc

@@ -199,7 +199,10 @@ void chrootHelper(int argc, char * * argv)
     uid_t gid = getgid();
 
     if (unshare(CLONE_NEWUSER | CLONE_NEWNS) == -1)
-        throw SysError("setting up a private mount namespace");
+        /* Try with just CLONE_NEWNS in case user namespaces are
+           specifically disabled. */
+        if (unshare(CLONE_NEWNS) == -1)
+            throw SysError("setting up a private mount namespace");
 
     /* Bind-mount realStoreDir on /nix/store. If the latter mount
        point doesn't already exists, we have to create a chroot