sanity: check for working user namespaces

If user namespaces are not available (typically because AppArmor is
blocking them), alert the user.

We consider network isolation sufficiently important that this is a fatal
error, and the user will need to configure AppArmor to allow bitbake to
create a user namespace.

[ YOCTO #15592 ]

(From OE-Core rev: a069b9f9ee6708022e12970d53262d966ee806ba)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit b6af956fe6e876957a49d4abf425e8c789bf0459)
Signed-off-by: Steve Sakoman <steve@sakoman.com>

meta/classes/sanity.bbclass

@@ -469,6 +469,29 @@ def check_wsl(d):
             bb.warn("You are running bitbake under WSLv2, this works properly but you should optimize your VHDX file eventually to avoid running out of storage space")
     return None
 
+def check_userns():
+    """
+    Check that user namespaces are functional, as they're used for network isolation.
+    """
+
+    # There is a known failure case with AppAmrmor where the unshare() call
+    # succeeds (at which point the uid is nobody) but writing to the uid_map
+    # fails (so the uid isn't reset back to the user's uid). We can detect this.
+    parentuid = os.getuid()
+    pid = os.fork()
+    if not pid:
+        try:
+            bb.utils.disable_network()
+        except:
+            pass
+        os._exit(parentuid != os.getuid())
+
+    ret = os.waitpid(pid, 0)[1]
+    if ret:
+        bb.fatal("User namespaces are not usable by BitBake, possibly due to AppArmor.\n"
+                 "See https://discourse.ubuntu.com/t/ubuntu-24-04-lts-noble-numbat-release-notes/39890#unprivileged-user-namespace-restrictions for more information.")
+
+
 # Require at least gcc version 7.5.
 #
 # This can be fixed on CentOS-7 with devtoolset-6+
@@ -634,6 +657,7 @@ def check_sanity_version_change(status, d):
     status.addresult(check_git_version(d))
     status.addresult(check_perl_modules(d))
     status.addresult(check_wsl(d))
+    status.addresult(check_userns())
 
     missing = ""