r-root.sh

#!/bin/sh
# Try to remove '/' recursively.

# Copyright (C) 2013-2024 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 <https://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!

# Fainthearted: 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_

# Ensure this variable is unset as it's
# used later in the unlinkat() wrapper.
unset CU_TEST_SKIP_EXIT

# Set this to 0 if you don't have a working gdb but would
# still like to run the test
USE_GDB=1

if test $USE_GDB = 1; then
  case $host_triplet in
    *darwin*) skip_ 'avoiding due to potentially non functioning gdb' ;;
    *) ;;
  esac

  # Use gdb to provide further protection by limiting calls to unlinkat().
  ( timeout 10s gdb --version ) > gdb.out 2>&1
  case $(cat gdb.out) in
    *'GNU gdb'*) ;;
    *) skip_ "can't run gdb";;
  esac
fi

# Break on a line rather than a symbol, to cater for inline functions
break_src="$abs_top_srcdir/src/remove.c"
break_line=$(grep -n ^excise "$break_src") || framework_failure_
break_line=$(echo "$break_line" | cut -d: -f1) || framework_failure_
break_line="$break_src:$break_line"


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_shared_ k.c k.so \
  || framework_failure_ 'failed to build shared library'

# Note breakpoint commands don't work in batch mode
# https://sourceware.org/bugzilla/show_bug.cgi?id=10079
# So we use python to script behavior upon hitting the breakpoint
cat > bp.py <<'EOF.py' || framework_failure_
def breakpoint_handler (event):
  if not isinstance(event, gdb.BreakpointEvent):
    return
  hit_count = event.breakpoints[0].hit_count
  if hit_count == 1:
    gdb.execute('shell touch excise.break')
    gdb.execute('continue')
  elif hit_count > 2:
    gdb.write('breakpoint hit twice already')
    gdb.execute('quit 1')
  else:
    gdb.execute('continue')

gdb.events.stop.connect(breakpoint_handler)
EOF.py

# In order of the sed expressions below, this cleans:
#
# 1. gdb uses the full path when running rm, so remove the leading dirs.
# 2. For some of the "/" synonyms, the error diagnostic slightly differs from
# that of the basic "/" case (see gnulib's fts_open' and ROOT_DEV_INO_WARN):
#   rm: it is dangerous to operate recursively on 'FILE' (same as '/')
# Strip that part off for the following comparison.
clean_rm_err_()
{
  sed "s/.*rm: /rm: /; \
       s/\(rm: it is dangerous to operate recursively on\).*$/\1 '\/'/"
}

#-------------------------------------------------------------------------------
# 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 gdb that limits the number of unlinkat() calls.
exercise_rm_r_root ()
{
  # Remove the evidence files; verify that.
  rm -f x excise.break || framework_failure_
  test -f x && framework_failure_
  test -f excise.break && 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

  if test $USE_GDB = 1; then
    gdb -nx --batch-silent -return-child-result				\
      --eval-command="set exec-wrapper					\
                       env 'LD_PRELOAD=$LD_PRELOAD:./k.so' $skip_exit"	\
      --eval-command="break '$break_line'"				\
      --eval-command='source bp.py'					\
      --eval-command="run -rv --one-file-system $*"			\
      --eval-command='quit'						\
      rm < /dev/null > out 2> err.t
  else
    touch excise.break
    env LD_PRELOAD=$LD_PRELOAD:./k.so $skip_exit \
      rm -rv --one-file-system $* < /dev/null > out 2> err.t
  fi

  ret=$?

  clean_rm_err_ < err.t > err || ret=$?

  return $ret
}

# 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 file / directory must still exist afterward.
mkdir dir || framework_failure_
> file    || framework_failure_

skip=
for file in dir file ; do
  exercise_rm_r_root "$file" || skip=1
  test -e "$file"            || skip=1
  test -f x                  || skip=1
  test -f excise.break       || skip=1  # gdb works and breakpoint hit
  compare /dev/null err      || skip=1

  test "$skip" = 1 \
    && { cat out; cat err; \
         skip_ "internal test failure: maybe LD_PRELOAD or gdb doesn't work?"; }
done

# "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.
# Exercise various synonyms of "/" including symlinks to it.
# Expect a non-Zero exit status.
# Prepare a few symlinks to "/".
ln -s /        rootlink  || framework_failure_
ln -s rootlink rootlink2 || framework_failure_
ln -sr /       rootlink3 || framework_failure_

for opts in           \
  '/'                 \
  '--preserve-root /' \
  '//'                \
  '///'               \
  '////'              \
  'rootlink/'         \
  'rootlink2/'        \
  'rootlink3/'        ; do

  returns_ 1 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 with --no-preserve to ensure shortened equivalent is not allowed.
cat <<EOD > exp_opt || framework_failure_
rm: you may not abbreviate the --no-preserve-root option
EOD
returns_ 1 exercise_rm_r_root --no-preserve / || fail=1
compare exp_opt err || fail=1
test -f x && fail=1

#-------------------------------------------------------------------------------
# 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

returns_ 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 "/" having a trailing "." or ".." in the name.
# 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.
# Expect that other error diagnostic in 'err' and nothing in 'out'.
# Expect a non-Zero exit status.  The evidence file "x" should not exist.
for file in      \
  '//.'          \
  '/./'          \
  '/.//'         \
  '/../'         \
  '/.././'       \
  '/etc/..'      \
  'rootlink/..'  \
  'rootlink2/.'  \
  'rootlink3/./' ; do

  test -d "$file" || continue   # if e.g. /etc does not exist.

  returns_ 1 exercise_rm_r_root --preserve-root "$file" || fail=1

  grep "rm: refusing to remove '\.' or '\.\.' directory: skipping" err \
    || fail=1

  compare /dev/null out  || 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