[PATCHv2] gdb: linux-namespaces: enter user namespace when appropriate

[Andrew Burgess <aburgess@redhat.com>]

From: Benjamin Berg <benjamin@sipsolutions.net>

In v2:

  - Update the test to ignore a warning seen when running the test on
    a machine with libc debug information installed, but without the
    libc source being available, e.g.:

    warning: 46     ../sysdeps/unix/sysv/linux/arm/libc-do-syscall.S: No such file or directory

    This was causing some CI failures to be reported from Linaro.

  - Rebased to current upstream/master.

--

The use of user namespaces is required for normal users to use mount
namespaces.  Consider trying this as an unprivileged user:

  $ unshare --mount /bin/true
  unshare: unshare failed: Operation not permitted

The problem here is that an unprivileged user doesn't have the
required permissions to create a new mount namespace.  If, instead, we
do this:

  $ unshare --mount --map-root-user /bin/true

then this will succeed.  The new option causes unshare to create a
user namespace in which the unprivileged user is mapped to UID/GID 0,
and so gains all privileges (inside the namespace), the user is then
able to create the mount namespace as required.

So, how does this relate to GDB?

When a user attaches to a process running in a separate mount
namespace, GDB makes use of a separate helper process (see
linux_mntns_get_helper in nat/linux-namespaces.c), which will then use
the `setns` function to enter (or try to enter) the mount namespace of
the process GDB is attaching too.  The helper process will then handle
file I/O requests received from GDB, and return the results back to
GDB, this allows GDB to access files within the mount namespace.

The problem here is that, switching to a mount namespace requires that
a process hold CAP_SYS_CHROOT and CAP_SYS_ADMIN capabilities within
its user namespace (actually it's a little more complex, see 'man 2
setns').  Assuming GDB is running as an unprivileged user, then GDB
will not have the required permissions.

However, if GDB enters the user namespace that the `unshare` process
created, then the current user will be mapped to UID/GID 0, and will
have the required permissions.

And so, this patch extends linux_mntns_access_fs (in
nat/linux-namespace.c) to first try and switch to the user namespace
of the inferior before trying to switch to the mount namespace.  If
the inferior does have a user namespace, and does have elevated
privileges within that namespace, then this first switch by GDB will
mean that the second step, into the mount namespace, will succeed.

If there is no user namespace, or the inferior doesn't have elevated
privileges within the user namespace, then the switch into the mount
namespace will fail, just as it currently does, and the user will need
to give elevated privileges to GDB via some other mechanism (e.g. run
as root).

This work was originally posted here:

  https://inbox.sourceware.org/gdb-patches/20230321120126.1418012-1-benjamin@sipsolutions.net

I (Andrew Burgess) have made some cleanups to the code to comply with
GDB's coding standard, and the test is entirely mine.  This commit
message is also entirely mine -- the original message was very terse
and required the reader to understand how the various namespaces
work and interact.  The above is my attempt to document what I now
understand about the problem being fixed.

I've left the original author in place as the core of the GDB change
itself is largely as originally presented, but any inaccuracies in the
commit message, or problems with the test, are all mine.

Co-Authored-by: Andrew Burgess <aburgess@redhat.com>
---
 gdb/nat/linux-namespaces.c                    |  24 +++-
 .../gdb.base/user-namespace-attach.c          |  35 +++++
 .../gdb.base/user-namespace-attach.exp        | 136 ++++++++++++++++++
 3 files changed, 194 insertions(+), 1 deletion(-)
 create mode 100644 gdb/testsuite/gdb.base/user-namespace-attach.c
 create mode 100644 gdb/testsuite/gdb.base/user-namespace-attach.exp

diff --git a/gdb/nat/linux-namespaces.c b/gdb/nat/linux-namespaces.c
index a3a38d82e64..4b5cb373e7a 100644
--- a/gdb/nat/linux-namespaces.c
+++ b/gdb/nat/linux-namespaces.c
@@ -880,7 +880,7 @@ linux_mntns_access_fs (pid_t pid)
   struct stat sb;
   struct linux_mnsh *helper;
   ssize_t size;
-  int fd;
+  int fd, fd_user = -1;
 
   if (pid == getpid ())
     return MNSH_FS_DIRECT;
@@ -897,6 +897,8 @@ linux_mntns_access_fs (pid_t pid)
     {
       int save_errno = errno;
       close (fd);
+      if (fd_user >= 0)
+	close (fd_user);
       errno = save_errno;
     };
 
@@ -906,6 +908,13 @@ linux_mntns_access_fs (pid_t pid)
   if (sb.st_ino == ns->id)
     return MNSH_FS_DIRECT;
 
+  struct linux_ns *ns_user = linux_ns_get_namespace (LINUX_NS_USER);
+  if (ns_user != nullptr)
+    {
+      const char *ns_filename = linux_ns_filename (ns_user, pid);
+      fd_user = gdb_open_cloexec (ns_filename, O_RDONLY, 0).release ();
+    }
+
   helper = linux_mntns_get_helper ();
   if (helper == NULL)
     return MNSH_FS_ERROR;
@@ -914,6 +923,19 @@ linux_mntns_access_fs (pid_t pid)
     {
       int result, error;
 
+      /* Try to enter the user namespace first.  The current user might
+	 have elevated privileges within the user namespace, which would
+	 then allow the attempt to enter the mount namespace to succeed.  */
+      if (fd_user >= 0)
+	{
+	  size = mnsh_send_setns (helper, fd_user, 0);
+	  if (size < 0)
+	    return MNSH_FS_ERROR;
+
+	  if (mnsh_recv_int (helper, &result, &error) != 0)
+	    return MNSH_FS_ERROR;
+	}
+
       size = mnsh_send_setns (helper, fd, 0);
       if (size < 0)
 	return MNSH_FS_ERROR;
diff --git a/gdb/testsuite/gdb.base/user-namespace-attach.c b/gdb/testsuite/gdb.base/user-namespace-attach.c
new file mode 100644
index 00000000000..684ce1cfc66
--- /dev/null
+++ b/gdb/testsuite/gdb.base/user-namespace-attach.c
@@ -0,0 +1,35 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2025 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program 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 General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <sys/types.h>
+#include <unistd.h>
+#include <stdio.h>
+
+volatile int spin_p = 1;
+
+int
+main ()
+{
+  alarm (60);
+
+  printf ("pid = %lld\n", ((long long) getpid ()));
+
+  while (spin_p)
+    sleep (1);
+
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.base/user-namespace-attach.exp b/gdb/testsuite/gdb.base/user-namespace-attach.exp
new file mode 100644
index 00000000000..c01bc5ca583
--- /dev/null
+++ b/gdb/testsuite/gdb.base/user-namespace-attach.exp
@@ -0,0 +1,136 @@
+# Copyright 2025 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program 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 General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Check that GDB can attach to a process started using 'unshare'.  The
+# inferior is started in a separate mnt namespace.
+
+require can_spawn_for_attach
+
+standard_testfile
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile] == -1} {
+    return
+}
+
+# This test relies (at least in some parts) on the sysroot being
+# 'target:'.  Grab the current sysroot now so we can skip those tests
+# if the board file has changed the sysroot.
+set sysroot ""
+set test "show sysroot"
+gdb_test_multiple $test $test {
+    -re -wrap "The current system root is \"(.*)\"\\." {
+	set sysroot $expect_out(1,string)
+    }
+}
+
+# Start a process using 'unshare FLAGS', then attach to the process
+# from GDB.  Check that the attach worked as expected.
+proc run_test { flags } {
+
+    # If FLAGS contains '--mount' then a separate mnt namespace will
+    # be created, in which case the executable will have been read
+    # from the 'target:'.  Otherwise, the executable will have been
+    # read from the local filesystem, and there will be no prefix.
+    #
+    # Of course, this only applies if the sysroot is 'target:', some
+    # boards change this, so skip these tests on those boards.
+    if { [lsearch -exact [split $flags " "] "--mount"] != -1 } {
+	if { $::sysroot ne "target:" } {
+	    return
+	}
+
+	set prefix "target:"
+    } else {
+	set prefix ""
+    }
+
+
+
+    set inferior_spawn_id \
+	[spawn_wait_for_attach [list "unshare $flags $::binfile"]]
+    if { $inferior_spawn_id == -1 } {
+	unsupported "failed to spawn for attach"
+	return
+    }
+
+    set inferior_pid [spawn_id_get_pid $inferior_spawn_id]
+
+    clean_restart
+
+    set saw_bad_warning false
+    gdb_test_multiple "attach $inferior_pid" "attach to inferior" {
+	-re "^attach $::decimal\r\n" {
+	    exp_continue
+	}
+
+	-re "^warning: \[^\r\n\]+: could not open as an executable file: \[^\r\n\]+\r\n" {
+	    set saw_bad_warning true
+	    exp_continue
+	}
+
+	-re "^warning: \[^\r\n\]+: can't open to read symbols: \[^\r\n\]+\r\n" {
+	    set saw_bad_warning true
+	    exp_continue
+	}
+
+	-re "^warning: Could not load vsyscall page because no executable was specified\r\n" {
+	    # This warning is a secondary consequence of the above bad
+	    # warnings, so don't count this as a bad warnings, ignore
+	    # it instead.
+	    exp_continue
+	}
+
+	-re "^warning:\\s+$::decimal\\s*\[^\r\n\]: No such file or directory\r\n" {
+	    # This unrelated warning is seen when GDB stops in libc,
+	    # and the source code for libc is not available.
+	    exp_continue
+	}
+
+	-re "^warning: \[^\r\n\]+\r\n" {
+	    # If we ignore "other" warnings then, should the above
+	    # warnings strings change we'll start ignoring the bad
+	    # warnings, and the test will appear to pass.
+	    #
+	    # If you are seeing a warning here that really has nothing
+	    # to do with the test failing, then the correct solution
+	    # is to add a new regexp to specifically match _that_
+	    # warning, and ignore it.
+	    set saw_bad_warning true
+	    exp_continue
+	}
+
+	-re "^$::gdb_prompt $" {
+	    gdb_assert { !$saw_bad_warning } $gdb_test_name
+	}
+
+	-re "^\[^\r\n\]*\r\n" {
+	    exp_continue
+	}
+    }
+
+    # Ensure GDB could access the executable.
+    set binfile_re [string_to_regexp $::binfile]
+    gdb_test "info inferiors" \
+	"\r\n\\*\\s+$::decimal\\s+\[^\r\n\]+\\s+${prefix}${binfile_re}\\s*"
+}
+
+set test_flags [list \
+		    "--mount --map-root-user" \
+		    "--user" \
+		    "--user --map-root-user"]
+
+foreach_with_prefix flags $test_flags {
+    run_test $flags
+}

base-commit: add73a101f5e8225d4055563c52adaa64c8fb11d
-- 
2.47.1