Add a helper binary to create a basic Secure Computing filter.scratch/seccomp

The binary uses the 'seccomp' helper library.  The library isn't
needed to load the generated Secure Computing filter.

* configure.ac: Check for 'seccomp' header and library.

* lib-src/seccomp-filter.c: New helper binary to generate a generic
Secure Computing filter for GNU/Linux.

* lib-src/Makefile.in (DONT_INSTALL): Add 'seccomp-filter' helper
binary if possible.
(all): Add Secure Computing filter file if possible.
(seccomp-filter$(EXEEXT)): Compile helper binary.
(seccomp-filter.bpf seccomp-filter.pfc): Generate filter files.

* test/src/emacs-tests.el (emacs-tests/seccomp/allows-stdout)
(emacs-tests/seccomp/forbids-subprocess): New unit tests.

* test/Makefile.in (src/emacs-tests.log): Add dependency on the helper
binary.

3 files changed, 48 insertions, 0 deletions

diff --git a/test/Makefile.in b/test/Makefile.in
index b6cf6493e32..670bdd6c3da 100644
--- a/test/Makefile.in
+++ b/test/Makefile.in
@@ -272,6 +272,8 @@ $(test_module): $(test_module:${SO}=.c) ../src/emacs-module.h
           $(srcdir)/../lib/timespec.c $(srcdir)/../lib/gettime.c
 endif
 
+src/emacs-tests.log: ../lib-src/seccomp-filter.c
+
 ## Check that there is no 'automated' subdirectory, which would
 ## indicate an incomplete merge from an older version of Emacs where
 ## the tests were arranged differently.
diff --git a/test/src/emacs-resources/seccomp-filter.bpf b/test/src/emacs-resources/seccomp-filter.bpf
new file mode 120000
index 00000000000..b3d603d0aeb
--- /dev/null
+++ b/test/src/emacs-resources/seccomp-filter.bpf
@@ -0,0 +1 @@
+../../../lib-src/seccomp-filter.bpf
\ No newline at end of file
diff --git a/test/src/emacs-tests.el b/test/src/emacs-tests.el
index 7618a9c6752..e9333fe9d32 100644
--- a/test/src/emacs-tests.el
+++ b/test/src/emacs-tests.el
@@ -25,7 +25,9 @@
 
 (require 'cl-lib)
 (require 'ert)
+(require 'ert-x)
 (require 'rx)
+(require 'subr-x)
 
 (ert-deftest emacs-tests/seccomp/absent-file ()
   (skip-unless (string-match-p (rx bow "SECCOMP" eow)
@@ -128,4 +130,47 @@ to `make-temp-file', which see."
                           (concat "--seccomp=" filter))
             0)))))
 
+(ert-deftest emacs-tests/seccomp/allows-stdout ()
+  (let ((emacs
+         (expand-file-name invocation-name invocation-directory))
+        (filter (ert-resource-file "seccomp-filter.bpf"))
+        (process-environment nil))
+    (skip-unless (file-executable-p emacs))
+    (skip-unless (file-readable-p filter))
+    ;; The --seccomp option is processed early, without filename
+    ;; handlers.  Therefore remote or quoted filenames wouldn't work.
+    (should-not (file-remote-p filter))
+    (cl-callf file-name-unquote filter)
+    (with-temp-buffer
+      (let ((status (call-process
+                     emacs nil t nil
+                     "--quick" "--batch"
+                     (concat "--seccomp=" filter)
+                     (format "--eval=%S" '(message "Hi")))))
+        (ert-info ((format "Process output: %s" (buffer-string)))
+          (should (eql status 0)))
+        (should (equal (string-trim (buffer-string)) "Hi"))))))
+
+(ert-deftest emacs-tests/seccomp/forbids-subprocess ()
+  (let ((emacs
+         (expand-file-name invocation-name invocation-directory))
+        (filter (ert-resource-file "seccomp-filter.bpf"))
+        (process-environment nil))
+    (skip-unless (file-executable-p emacs))
+    (skip-unless (file-readable-p filter))
+    ;; The --seccomp option is processed early, without filename
+    ;; handlers.  Therefore remote or quoted filenames wouldn't work.
+    (should-not (file-remote-p filter))
+    (cl-callf file-name-unquote filter)
+    (with-temp-buffer
+      (let ((status
+             (call-process
+              emacs nil t nil
+              "--quick" "--batch"
+              (concat "--seccomp=" filter)
+              (format "--eval=%S" `(call-process ,emacs nil nil nil
+                                                 "--version")))))
+        (ert-info ((format "Process output: %s" (buffer-string)))
+          (should-not (eql status 0)))))))
+
 ;;; emacs-tests.el ends here