Allow for unpriviledged nested containers

If the build itself is run in a container, we may not be able to
fully set up a nested container for test-container testing.
Notably is the mounting of /proc, since it's critical that it
be mounted from within the same PID namespace as its users, and
thus cannot be bind mounted from outside the container like other
mounts.

This patch defaults to using the parent's PID namespace instead of
creating a new one, as this is more likely to be allowed.

If the test needs an isolated PID namespace, it should add the "pidns"
command to its init script.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

4 files changed, 147 insertions, 35 deletions

diff --git a/support/Makefile b/support/Makefile
index 5ddcb8d158..f036a81304 100644
--- a/support/Makefile
+++ b/support/Makefile
@@ -64,6 +64,7 @@ libsupport-routines = \
   support_format_netent \
   support_isolate_in_subprocess \
   support_mutex_pi_monotonic \
+  support_need_proc \
   support_path_support_time64 \
   support_process_state \
   support_ptrace \
diff --git a/support/support.h b/support/support.h
index 73b9fc48f0..d20051da4d 100644
--- a/support/support.h
+++ b/support/support.h
@@ -91,6 +91,11 @@ char *support_quote_string (const char *);
    regular file open for writing, and initially empty.  */
 int support_descriptor_supports_holes (int fd);
 
+/* Predicates that a test requires a working /proc filesystem.  This
+   call will exit with UNSUPPORTED if /proc is not available, printing
+   WHY_MSG as part of the diagnostic.  */
+void support_need_proc (const char *why_msg);
+
 /* Error-checking wrapper functions which terminate the process on
    error.  */
 
diff --git a/support/support_need_proc.c b/support/support_need_proc.c
new file mode 100644
index 0000000000..9b4eab7539
--- /dev/null
+++ b/support/support_need_proc.c
@@ -0,0 +1,35 @@
+/* Indicate that a test requires a working /proc.
+   Copyright (C) 2022 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <unistd.h>
+#include <support/check.h>
+#include <support/support.h>
+
+/* We test for /proc/self/maps since that's one of the files that one
+   of our tests actually uses, but the general idea is if Linux's
+   /proc/ (procfs) filesystem is mounted.  If not, the process exits
+   with an UNSUPPORTED result code.  */
+
+void
+support_need_proc (const char *why_msg)
+{
+#ifdef __linux__
+  if (access ("/proc/self/maps", R_OK))
+    FAIL_UNSUPPORTED ("/proc is not available, %s", why_msg);
+#endif
+}
diff --git a/support/test-container.c b/support/test-container.c
index 25e7f14219..c837c4d758 100644
--- a/support/test-container.c
+++ b/support/test-container.c
@@ -97,6 +97,7 @@ int verbose = 0;
    * mytest.root/mytest.script has a list of "commands" to run:
        syntax:
          # comment
+	 pidns <comment>
          su
          mv FILE FILE
 	 cp FILE FILE
@@ -122,6 +123,8 @@ int verbose = 0;
 
        details:
          - '#': A comment.
+	 - 'pidns': Require a separate PID namespace, prints comment if it can't
+	    (default is a shared pid namespace)
          - 'su': Enables running test as root in the container.
          - 'mv': A minimal move files command.
          - 'cp': A minimal copy files command.
@@ -148,7 +151,7 @@ int verbose = 0;
    * Simple, easy to review code (i.e. prefer simple naive code over
      complex efficient code)
 
-   * The current implementation ist parallel-make-safe, but only in
+   * The current implementation is parallel-make-safe, but only in
      that it uses a lock to prevent parallel access to the testroot.  */
 
 
@@ -227,11 +230,37 @@ concat (const char *str, ...)
   return bufs[n];
 }
 
+/* Like the above, but put spaces between words.  Caller frees.  */
+static char *
+concat_words (char **words, int num_words)
+{
+  int len = 0;
+  int i;
+  char *rv, *p;
+
+  for (i = 0; i < num_words; i ++)
+    {
+      len += strlen (words[i]);
+      len ++;
+    }
+
+  p = rv = (char *) xmalloc (len);
+
+  for (i = 0; i < num_words; i ++)
+    {
+      if (i > 0)
+	p = stpcpy (p, " ");
+      p = stpcpy (p, words[i]);
+    }
+
+  return rv;
+}
+
 /* Try to mount SRC onto DEST.  */
 static void
 trymount (const char *src, const char *dest)
 {
-  if (mount (src, dest, "", MS_BIND, NULL) < 0)
+  if (mount (src, dest, "", MS_BIND | MS_REC, NULL) < 0)
     FAIL_EXIT1 ("can't mount %s onto %s\n", src, dest);
 }
 
@@ -726,6 +755,9 @@ main (int argc, char **argv)
   gid_t original_gid;
   /* If set, the test runs as root instead of the user running the testsuite.  */
   int be_su = 0;
+  int require_pidns = 0;
+  const char *pidns_comment = NULL;
+  int do_proc_mounts = 0;
   int UMAP;
   int GMAP;
   /* Used for "%lld %lld 1" so need not be large.  */
@@ -1011,6 +1043,12 @@ main (int argc, char **argv)
 	      {
 		be_su = 1;
 	      }
+	    else if (nt >= 1 && strcmp (the_words[0], "pidns") == 0)
+	      {
+		require_pidns = 1;
+		if (nt > 1)
+		  pidns_comment = concat_words (the_words + 1, nt - 1);
+	      }
 	    else if (nt == 3 && strcmp (the_words[0], "mkdirp") == 0)
 	      {
 		long int m;
@@ -1068,7 +1106,8 @@ main (int argc, char **argv)
 
 #ifdef CLONE_NEWNS
   /* The unshare here gives us our own spaces and capabilities.  */
-  if (unshare (CLONE_NEWUSER | CLONE_NEWPID | CLONE_NEWNS) < 0)
+  if (unshare (CLONE_NEWUSER | CLONE_NEWNS
+	       | (require_pidns ? CLONE_NEWPID : 0)) < 0)
     {
       /* Older kernels may not support all the options, or security
 	 policy may block this call.  */
@@ -1079,6 +1118,11 @@ main (int argc, char **argv)
 	    check_for_unshare_hints ();
 	  FAIL_UNSUPPORTED ("unable to unshare user/fs: %s", strerror (saved_errno));
 	}
+      /* We're about to exit anyway, it's "safe" to call unshare again
+	 just to see if the CLONE_NEWPID caused the error.  */
+      else if (require_pidns && unshare (CLONE_NEWUSER | CLONE_NEWNS) >= 0)
+	FAIL_EXIT1 ("unable to unshare pid ns: %s : %s", strerror (errno),
+		    pidns_comment ? pidns_comment : "required by test");
       else
 	FAIL_EXIT1 ("unable to unshare user/fs: %s", strerror (errno));
     }
@@ -1094,6 +1138,15 @@ main (int argc, char **argv)
   trymount (support_srcdir_root, new_srcdir_path);
   trymount (support_objdir_root, new_objdir_path);
 
+  /* It may not be possible to mount /proc directly.  */
+  if (! require_pidns)
+  {
+    char *new_proc = concat (new_root_path, "/proc", NULL);
+    xmkdirp (new_proc, 0755);
+    trymount ("/proc", new_proc);
+    do_proc_mounts = 1;
+  }
+
   xmkdirp (concat (new_root_path, "/dev", NULL), 0755);
   devmount (new_root_path, "null");
   devmount (new_root_path, "zero");
@@ -1163,42 +1216,60 @@ main (int argc, char **argv)
 
   maybe_xmkdir ("/tmp", 0755);
 
-  /* Now that we're pid 1 (effectively "root") we can mount /proc  */
-  maybe_xmkdir ("/proc", 0777);
-  if (mount ("proc", "/proc", "proc", 0, NULL) < 0)
-    FAIL_EXIT1 ("Unable to mount /proc: ");
-
-  /* We map our original UID to the same UID in the container so we
-     can own our own files normally.  */
-  UMAP = open ("/proc/self/uid_map", O_WRONLY);
-  if (UMAP < 0)
-    FAIL_EXIT1 ("can't write to /proc/self/uid_map\n");
-
-  sprintf (tmp, "%lld %lld 1\n",
-	   (long long) (be_su ? 0 : original_uid), (long long) original_uid);
-  write (UMAP, tmp, strlen (tmp));
-  xclose (UMAP);
-
-  /* We must disable setgroups () before we can map our groups, else we
-     get EPERM.  */
-  GMAP = open ("/proc/self/setgroups", O_WRONLY);
-  if (GMAP >= 0)
+  if (require_pidns)
     {
-      /* We support kernels old enough to not have this.  */
-      write (GMAP, "deny\n", 5);
-      xclose (GMAP);
+      /* Now that we're pid 1 (effectively "root") we can mount /proc  */
+      maybe_xmkdir ("/proc", 0777);
+      if (mount ("proc", "/proc", "proc", 0, NULL) != 0)
+	{
+	  /* This happens if we're trying to create a nested container,
+	     like if the build is running under podman, and we lack
+	     priviledges.
+
+	     Ideally we would WARN here, but that would just add noise to
+	     *every* test-container test, and the ones that care should
+	     have their own relevent diagnostics.
+
+	     FAIL_EXIT1 ("Unable to mount /proc: ");  */
+	}
+      else
+	do_proc_mounts = 1;
     }
 
-  /* We map our original GID to the same GID in the container so we
-     can own our own files normally.  */
-  GMAP = open ("/proc/self/gid_map", O_WRONLY);
-  if (GMAP < 0)
-    FAIL_EXIT1 ("can't write to /proc/self/gid_map\n");
+  if (do_proc_mounts)
+    {
+      /* We map our original UID to the same UID in the container so we
+	 can own our own files normally.  */
+      UMAP = open ("/proc/self/uid_map", O_WRONLY);
+      if (UMAP < 0)
+	FAIL_EXIT1 ("can't write to /proc/self/uid_map\n");
+
+      sprintf (tmp, "%lld %lld 1\n",
+	       (long long) (be_su ? 0 : original_uid), (long long) original_uid);
+      write (UMAP, tmp, strlen (tmp));
+      xclose (UMAP);
+
+      /* We must disable setgroups () before we can map our groups, else we
+	 get EPERM.  */
+      GMAP = open ("/proc/self/setgroups", O_WRONLY);
+      if (GMAP >= 0)
+	{
+	  /* We support kernels old enough to not have this.  */
+	  write (GMAP, "deny\n", 5);
+	  xclose (GMAP);
+	}
 
-  sprintf (tmp, "%lld %lld 1\n",
-	   (long long) (be_su ? 0 : original_gid), (long long) original_gid);
-  write (GMAP, tmp, strlen (tmp));
-  xclose (GMAP);
+      /* We map our original GID to the same GID in the container so we
+	 can own our own files normally.  */
+      GMAP = open ("/proc/self/gid_map", O_WRONLY);
+      if (GMAP < 0)
+	FAIL_EXIT1 ("can't write to /proc/self/gid_map\n");
+
+      sprintf (tmp, "%lld %lld 1\n",
+	       (long long) (be_su ? 0 : original_gid), (long long) original_gid);
+      write (GMAP, tmp, strlen (tmp));
+      xclose (GMAP);
+    }
 
   if (change_cwd)
     {