Subject: Re: bug#15926: RFE: unlink command already uses 'unlink' call; make 'rm' use 'remove' call

From: Bernhard Voelker <[email-masked]>
Date: Wed, 27 Nov 2013 00:08:08 +0100

On 11/25/2013 08:31 AM, Bernhard Voelker wrote:
> On 11/25/2013 02:10 AM, Pádraig Brady wrote:
>> So this might race with rm being preempted between the touch() and the exit(),
>> causing a false failure? It's probably best to not kill if the 'x' file exists,
>> as it's very unlikely that rm will run as normal in that case.
> 
> outch, the race is even more likely for the newly added "rm -r file1 / file2"
> case where unlinkat() is called twice without the immediate _exit() due to
> CU_TEST_SKIP_EXIT=1.
> 
> I have to reconsider, but won't have time for that today.

Hi Padraig,

I simplified it by using timeout(1), as you already suggested:

  if [ "$CU_TEST_SKIP_EXIT" = 1 ]; then
    # Pass on this variable into 'rm's environment.
    skip_exit='CU_TEST_SKIP_EXIT=1'
  fi

  timeout --signal=KILL 2 \
    env LD_PRELOAD=./k.so $skip_exit \
      rm -rv --one-file-system "$@" > out 2> err

I also changed the other things you mentioned.

Finally, I had to pass --interactive=never to the last test to
avoid the "descend into write-protected directory?" prompt.
This is since rm has been executed in the background.

Next try attached.

Have a nice day,
Berny
>From a15731bdf727e3574a11fbff8634dfd1beabf82e Mon Sep 17 00:00:00 2001
From: Bernhard Voelker <[email-masked]>
Date: Tue, 26 Nov 2013 23:50:08 +0100
Subject: [PATCH] tests: add a test for rm -r "/"

* tests/rm/r-root.sh: Add a non-root test.
* tests/local.mk (all_tests): Mention the test.
---
 tests/local.mk     |   1 +
 tests/rm/r-root.sh | 239 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 240 insertions(+)
 create mode 100755 tests/rm/r-root.sh

diff --git a/tests/local.mk b/tests/local.mk
index 3c92425..1888b21 100644
--- a/tests/local.mk
+++ b/tests/local.mk
@@ -201,6 +201,7 @@ all_tests =					\
   tests/rm/r-2.sh				\
   tests/rm/r-3.sh				\
   tests/rm/r-4.sh				\
+  tests/rm/r-root.sh				\
   tests/rm/readdir-bug.sh			\
   tests/rm/rm1.sh				\
   tests/touch/empty-file.sh			\
diff --git a/tests/rm/r-root.sh b/tests/rm/r-root.sh
new file mode 100755
index 0000000..26db030
--- /dev/null
+++ b/tests/rm/r-root.sh
@@ -0,0 +1,239 @@
+#!/bin/sh
+# Try to remove '/' recursively.
+
+# Copyright (C) 2013 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/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ rm
+
+# POSIX mandates rm(1) to skip '/' arguments.  This test verifies this mandated
+# behavior as well as the --preserve-root and --no-preserve-root options.
+# Especially the latter case is a live fire exercise as rm(1) is supposed to
+# enter the unlinkat() system call.  Therefore, limit the risk as much
+# as possible -- if there's a bug this test would wipe the system out!
+
+# Faint-hearted: skip this test for the 'root' user.
+skip_if_root_
+
+# Pull the teeth from rm(1) by intercepting the unlinkat() system call via the
+# LD_PRELOAD environment variable.  This requires shared libraries to work.
+require_gcc_shared_
+
+cat > k.c <<'EOF' || framework_failure_
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+int unlinkat (int dirfd, const char *pathname, int flags)
+{
+  /* Prove that LD_PRELOAD works: create the evidence file "x".  */
+  fclose (fopen ("x", "w"));
+
+  /* Immediately terminate, unless indicated otherwise.  */
+  if (! getenv("CU_TEST_SKIP_EXIT"))
+    _exit (0);
+
+  /* Pretend success.  */
+  return 0;
+}
+EOF
+
+# Then compile/link it:
+gcc -Wall --std=gnu99 -shared -fPIC -ldl -O2 k.c -o k.so \
+  || framework_failure_ 'failed to build shared library'
+
+# Verify that "rm -r dir" basically works.
+mkdir   dir || framework_failure_
+rm -r   dir || framework_failure_
+test -d dir && framework_failure_
+
+# Now verify that intercepting unlinkat() works:
+# rm(1) must succeed as before, but this time both the evidence file "x"
+# and the test directory "dir" must exist afterwards.
+mkdir   dir || framework_failure_
+LD_PRELOAD=./k.so \
+rm -r   dir || framework_failure_
+test -d dir || skip_ "internal test failure: maybe LD_PRELOAD doesn't work?"
+test -f x   || skip_ "internal test failure: maybe LD_PRELOAD doesn't work?"
+
+#-------------------------------------------------------------------------------
+# exercise_rm_r_root: shell function to test "rm -r '/'"
+# The caller must provide the FILE to remove as well as any options
+# which should be passed to 'rm'.
+# Paranoia mode on:
+# For the worst case where both rm(1) would fail to refuse to process the "/"
+# argument (in the cases without the --no-preserve-root option), and
+# intercepting the unlinkat(1) system call would fail (which actually already
+# has been proven to work above), and the current non root user has
+# write access to "/", limit the damage to the current file system via
+# the --one-file-system option.
+# Furthermore, run rm(1) via timeout(1) that kills that process after
+# a maximum of 2 seconds.
+exercise_rm_r_root ()
+{
+  # Remove the evidence file "x"; verify that.
+  rm -f x   || framework_failure_
+  test -f x && framework_failure_
+
+  local skip_exit=
+  if [ "$CU_TEST_SKIP_EXIT" = 1 ]; then
+    # Pass on this variable into 'rm's environment.
+    skip_exit='CU_TEST_SKIP_EXIT=1'
+  fi
+
+  timeout --signal=KILL 2 \
+    env LD_PRELOAD=./k.so $skip_exit \
+      rm -rv --one-file-system "$@" > out 2> err
+
+  return $?
+}
+
+# "rm -r /" without --no-preserve-root should output the following
+# diagnostic error message.
+cat <<EOD > exp || framework_failure_
+rm: it is dangerous to operate recursively on '/'
+rm: use --no-preserve-root to override this failsafe
+EOD
+
+#-------------------------------------------------------------------------------
+# Exercise "rm -r /" without and with the --preserve-root option.
+# Also exercise the synonyms '///' and '////' which would normally go into
+# the 'synonyms' test category below; but due to the way gnulib's fts_open()
+# performs trimming of trailing slashes ("///" -> "/"), the error diagnostic
+# is the same as for a bare "/", see the ROOT_DEV_INO_WARN macro.
+# Expect a non-Zero exit status.
+for opts in '/' '--preserve-root /' '///' '////'; do
+  exercise_rm_r_root $opts \
+    && fail=1
+
+  # Expect nothing in 'out' and the above error diagnostic in 'err'.
+  # As rm(1) should have skipped the "/" argument, it does not call unlinkat().
+  # Therefore, the evidence file "x" should not exist.
+  compare /dev/null out || fail=1
+  compare exp       err || fail=1
+  test -f x             && fail=1
+
+  # Do nothing more if this test failed.
+  test $fail = 1 && { cat out; cat err; Exit $fail; }
+done
+
+#-------------------------------------------------------------------------------
+# Exercise "rm -r file1 / file2".
+# Expect a non-Zero exit status representing failure to remove "/",
+# yet 'file1' and 'file2' should be removed.
+: > file1 || framework_failure_
+: > file2 || framework_failure_
+
+# Now that we know that 'rm' won't call the unlinkat() system function for "/",
+# we could probably execute it without the LD_PRELOAD'ed safety net.
+# Nevertheless, it's still better to use it for this test.
+# Tell the unlinkat() replacement function to not _exit(0) immediately
+# by setting the following variable.
+CU_TEST_SKIP_EXIT=1
+
+exercise_rm_r_root --preserve-root file1 '/' file2 \
+  && fail=1
+
+unset CU_TEST_SKIP_EXIT
+
+cat <<EOD > out_removed
+removed 'file1'
+removed 'file2'
+EOD
+
+# The above error diagnostic should appear in 'err'.
+# Both 'file1' and 'file2' should be removed.  Simply verify that in the
+# "out" file, as the replacement unlinkat() dummy did not remove them.
+# Expect the evidence file "x" to exist.
+compare out_removed out || fail=1
+compare exp         err || fail=1
+test -f x               || fail=1
+
+# Do nothing more if this test failed.
+test $fail = 1 && { cat out; cat err; Exit $fail; }
+
+#-------------------------------------------------------------------------------
+# Exercise various synonyms of "/" including symlinks to it.
+# The error diagnostic slightly differs from that of the basic "/" case above.
+cat <<EOD > exp_same || framework_failure_
+rm: it is dangerous to operate recursively on 'FILE' (same as '/')
+rm: use --no-preserve-root to override this failsafe
+EOD
+
+# Some combinations have a trailing "." or "..".  This triggers another check
+# in the code first and therefore leads to a different diagnostic.  However,
+# we want to test anyway to protect against future reordering of the checks
+# in the code.
+cat <<EOD > exp_dot || framework_failure_
+rm: refusing to remove '.' or '..' directory: skipping 'FILE'
+EOD
+
+# Prepare a few symlinks to "/".
+ln -s /        rootlink  || framework_failure_
+ln -s rootlink rootlink2 || framework_failure_
+ln -sr /       rootlink3 || framework_failure_
+
+for file in    \
+  'rootlink/'  \
+  'rootlink2/' \
+  'rootlink3/' \
+  '//'         \
+  '//.'        \
+  '/./'        \
+  '/../'       \
+  '/.././'     \
+  '/bin/..' ; do
+
+  exercise_rm_r_root --preserve-root "$file" \
+    && fail=1
+
+  sed "s,FILE,$file," exp_same > exp2     || framework_failure_
+  sed "s,FILE,$file," exp_dot  > exp_dot2 || framework_failure_
+
+  # Check against the "refusing to remove '.' or '..'" diagnostic.
+  compare exp_dot2 err \
+    && continue
+
+  compare /dev/null out || fail=1
+  compare exp2      err || fail=1
+  test -f x             && fail=1
+
+  # Do nothing more if this test failed.
+  test $fail = 1 && { cat out; cat err; Exit $fail; }
+done
+
+#-------------------------------------------------------------------------------
+# Until now, it was all just fun.
+# Now exercise the --no-preserve-root option with which rm(1) should enter
+# the intercepted unlinkat() system call.
+# As the interception code terminates the process immediately via _exit(0),
+# the exit status should be 0.
+# Use the option --interactive=never to bypass the following prompt:
+#   "rm: descend into write-protected directory '/'?"
+exercise_rm_r_root  --interactive=never --no-preserve-root '/' \
+  || fail=1
+
+# The 'err' file should not contain the above error diagnostic.
+grep "^rm: it is dangerous to operate recursively on '/'" err \
+  && fail=1
+
+# Instead, rm(1) should have called the intercepted unlinkat() function,
+# i.e. the evidence file "x" should exist.
+test -f x || fail=1
+
+test $fail = 1 && { cat out; cat err; }
+
+Exit $fail
-- 
1.8.3.1