Use sandbox fallback when cloning fails in builder

When sandbox-fallback = true (the default), the Nix builder will fall
back to disabled sandbox mode when the kernel doesn’t allow users to
set it up. This prevents hard errors from occuring in tricky places,
especially the initial installer. To restore the previous behavior,
users can set:

  sandbox-fallback = false

in their /etc/nix/nix.conf configuration.

2 files changed, 14 insertions, 1 deletions

diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index c10005839..dd08ce7d7 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -2316,13 +2316,22 @@ void DerivationGoal::startBuilder()
                 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(EPERM);
             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 == EPERM && settings.sandboxFallback) {
+            useChroot = false;
+            goto fallback;
+        } else if (res != 0)
             throw Error("unable to start build process");
 
         userNamespaceSync.readSide = -1;
@@ -2353,6 +2362,7 @@ void DerivationGoal::startBuilder()
     } else
 #endif
     {
+    fallback:
         options.allowVfork = !buildUser && !drv->isBuiltin();
         pid = startProcess([&]() {
             runChild();
diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index 0af8215d1..cc9534b27 100644
--- a/src/libstore/globals.hh
+++ b/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"}};