path: root/src/safe.c

/* safe path traversal functions for 'patch' */

/* Copyright (C) 2015 Free Software Foundation, Inc.

   Written by Tim Waugh <twaugh@redhat.com> and
   Andreas Gruenbacher <agruenba@redhat.com>.

   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 <config.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/resource.h>
#include <sys/time.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <alloca.h>
#include <safe.h>
#include "dirname.h"

#include <hash.h>
#include <xalloc.h>
#include <minmax.h>

#define XTERN extern
#include "common.h"

#include "util.h"
#include "list.h"

#ifndef EFTYPE
# define EFTYPE 0
#endif

static const unsigned int MAX_PATH_COMPONENTS = 1024;

/* Flag to turn the safe_* functions into their unsafe variants; files may then
   lie outside the current working directory. */
bool unsafe;

/* Path lookup results are cached in a hash table + LRU list. When the
   cache is full, the oldest entries are removed.  */

unsigned int dirfd_cache_misses;

struct cached_dirfd {
  struct list_head lru_link;
  struct list_head children_link, children;
  struct cached_dirfd *parent;

  char *name;
  int fd;
};

static Hash_table *cached_dirfds = NULL;
static rlim_t min_cached_fds = 8;
static rlim_t max_cached_fds;
LIST_HEAD (lru_list);

static size_t hash_cached_dirfd (const void *entry, size_t table_size)
{
  const struct cached_dirfd *d = entry;
  size_t strhash = hash_string (d->name, table_size);
  return (strhash * 31 + d->parent->fd) % table_size;
}

static bool compare_cached_dirfds (const void *_a,
				   const void *_b)
{
  const struct cached_dirfd *a = _a;
  const struct cached_dirfd *b = _b;

  return (a->parent->fd == b->parent->fd &&
	  !strcmp (a->name, b->name));
}

static void free_cached_dirfd (struct cached_dirfd *entry)
{
  list_del (&entry->children_link);
  free (entry->name);
  free (entry);
}

static void init_dirfd_cache (void)
{
  struct rlimit nofile;

  if (getrlimit (RLIMIT_NOFILE, &nofile) == 0)
    {
      if (nofile.rlim_cur == RLIM_INFINITY)
        max_cached_fds = RLIM_INFINITY;
      else
	max_cached_fds = MAX (nofile.rlim_cur / 4, min_cached_fds);
    }
  else
    max_cached_fds = min_cached_fds;

  cached_dirfds = hash_initialize (min_cached_fds,
				   NULL,
				   hash_cached_dirfd,
				   compare_cached_dirfds,
				   NULL);

  if (!cached_dirfds)
    xalloc_die ();
}

static struct cached_dirfd *lookup_cached_dirfd (struct cached_dirfd *dir, const char *name)
{
  struct cached_dirfd *entry = NULL;

  if (cached_dirfds)
    {
      struct cached_dirfd key;
      key.parent = dir;
      key.name = (char *) name;
      entry = hash_lookup (cached_dirfds, &key);
    }

  return entry;
}

static void remove_cached_dirfd (struct cached_dirfd *entry)
{
  while (! list_empty (&entry->children))
    {
      struct cached_dirfd *child =
	list_entry (entry->children.next, struct cached_dirfd, children_link);
      list_del_init (&child->children_link);
      /* assert (list_empty (&child->children_link)); */
      hash_delete (cached_dirfds, child);  /* noop when not hashed */
    }
  list_del (&entry->lru_link);
  hash_delete (cached_dirfds, entry);  /* noop when not hashed */
  close (entry->fd);
  free_cached_dirfd (entry);
}

static void insert_cached_dirfd (struct cached_dirfd *entry, int keepfd)
{
  if (cached_dirfds == NULL)
    init_dirfd_cache ();

  if (max_cached_fds != RLIM_INFINITY)
    {
      /* Trim off the least recently used entries */
      while (hash_get_n_entries (cached_dirfds) >= max_cached_fds)
	{
	  struct cached_dirfd *last =
	    list_entry (lru_list.prev, struct cached_dirfd, lru_link);
	  if (&last->lru_link == &lru_list)
	    break;
	  if (last->fd == keepfd)
	    {
	      last = list_entry (last->lru_link.prev, struct cached_dirfd, lru_link);
	      if (&last->lru_link == &lru_list)
		break;
	    }
	  remove_cached_dirfd (last);
	}
    }

  /* Only insert if the parent still exists. */
  if (! list_empty (&entry->children_link))
    assert (hash_insert (cached_dirfds, entry) == entry);
}

static void invalidate_cached_dirfd (int dirfd, const char *name)
{
  struct cached_dirfd dir, key, *entry;
  if (!cached_dirfds)
    return;

  dir.fd = dirfd;
  key.parent = &dir;
  key.name = (char *) name;
  entry = hash_lookup (cached_dirfds, &key);
  if (entry)
    remove_cached_dirfd (entry);
}

/* Put the looked up path back onto the lru list.  Return the file descriptor
   of the top entry.  */
static int put_path (struct cached_dirfd *entry)
{
  int fd = entry->fd;

  while (entry)
    {
      struct cached_dirfd *parent = entry->parent;
      if (! parent)
	break;
      list_add (&entry->lru_link, &lru_list);
      entry = parent;
    }

  return fd;
}

static struct cached_dirfd *new_cached_dirfd (struct cached_dirfd *dir, const char *name, int fd)
{
  struct cached_dirfd *entry = xmalloc (sizeof (struct cached_dirfd));

  INIT_LIST_HEAD (&entry->lru_link);
  list_add (&entry->children_link, &dir->children);
  INIT_LIST_HEAD (&entry->children);
  entry->parent = dir;
  entry->name = xstrdup (name);
  entry->fd = fd;
  return entry;
}

static struct cached_dirfd *openat_cached (struct cached_dirfd *dir, const char *name, int keepfd)
{
  int fd;
  struct cached_dirfd *entry = lookup_cached_dirfd (dir, name);

  if (entry)
    {
      list_del_init (&entry->lru_link);
      /* assert (list_empty (&entry->lru_link)); */
      goto out;
    }
  dirfd_cache_misses++;

  /* Actually get the new directory file descriptor. Don't follow
     symbolic links. */
  fd = openat (dir->fd, name, O_DIRECTORY | O_NOFOLLOW);

  /* Don't cache errors. */
  if (fd < 0)
    return NULL;

  /* Store new cache entry */
  entry = new_cached_dirfd (dir, name, fd);
  insert_cached_dirfd (entry, keepfd);

out:
  return entry;
}

_GL_ATTRIBUTE_PURE
static unsigned int count_path_components (const char *path)
{
  unsigned int components;

  while (ISSLASH (*path))
    path++;
  if (! *path)
    return 1;
  for (components = 0; *path; components++)
    {
      while (*path && ! ISSLASH (*path))
	path++;
      while (ISSLASH (*path))
	path++;
    }
  return components;
}

/* A symlink to resolve. */
struct symlink {
  struct symlink *prev;
  const char *path;
};

static void push_symlink (struct symlink **stack, struct symlink *symlink)
{
  symlink->prev = *stack;
  *stack = symlink;
}

static void pop_symlink (struct symlink **stack)
{
  struct symlink *top = *stack;
  *stack = top->prev;
  free (top);
}

int cwd_stat_errno = -1;
struct stat cwd_stat;

static struct symlink *read_symlink(int dirfd, const char *name)
{
  int saved_errno = errno;
  struct stat st;
  struct symlink *symlink;
  char *buffer;
  ssize_t ret;

  if (fstatat (dirfd, name, &st, AT_SYMLINK_NOFOLLOW)
      || ! S_ISLNK (st.st_mode))
    {
      errno = saved_errno;
      return NULL;
    }
  symlink = xmalloc (sizeof (*symlink) + st.st_size + 1);
  buffer = (char *)(symlink + 1);
  ret = readlinkat (dirfd, name, buffer, st.st_size);
  if (ret <= 0)
    goto fail;
  buffer[ret] = 0;
  symlink->path = buffer;
  if (ISSLASH (*buffer))
    {
      char *end;

      if (cwd_stat_errno == -1)
	{
	  cwd_stat_errno = stat (".", &cwd_stat) == 0 ? 0 : errno;
	  if (cwd_stat_errno)
	    goto fail_exdev;
	}
      end = buffer + ret;
      for (;;)
	{
	  char slash;
	  int rv;

	  slash = *end; *end = 0;
	  rv = stat (symlink->path, &st);
	  *end = slash;

	  if (rv == 0
	      && st.st_dev == cwd_stat.st_dev
	      && st.st_ino == cwd_stat.st_ino)
	    {
	      while (ISSLASH (*end))
		end++;
	      symlink->path = end;
	      return symlink;
	    }
	  end--;
	  if (end == symlink->path)
	    break;
	  while (end != symlink->path + 1 && ! ISSLASH (*end))
	    end--;
	  while (end != symlink->path + 1 && ISSLASH (*(end - 1)))
	    end--;
	}
      goto fail_exdev;
    }
  return symlink;

fail_exdev:
  errno = EXDEV;
fail:
  free (symlink);
  return NULL;
}

/* Resolve the next path component in PATH inside DIR.  If it is a symlink,
   read it and returned it in TOP. */
static struct cached_dirfd *
traverse_next (struct cached_dirfd *dir, const char **path, int keepfd,
	       struct symlink **symlink)
{
  const char *p = *path;
  struct cached_dirfd *entry = dir;
  char *name;

  while (*p && ! ISSLASH (*p))
    p++;
  if (**path == '.' && *path + 1 == p)
    goto skip;
  if (**path == '.' && *(*path + 1) == '.' && *path + 2 == p)
    {
      entry = dir->parent;
      if (! entry)
	{
	  /* Must not leave the working tree. */
	  errno = EXDEV;
	  goto out;
	}
      assert (list_empty (&dir->lru_link));
      list_add (&dir->lru_link, &lru_list);
      goto skip;
    }
  name = alloca (p - *path + 1);
  memcpy(name, *path, p - *path);
  name[p - *path] = 0;

  entry = openat_cached (dir, name, keepfd);
  if (! entry)
    {
      if (errno == ELOOP
	  || errno == EMLINK  /* FreeBSD 10.1: Too many links */
	  || errno == EFTYPE  /* NetBSD 6.1: Inappropriate file type or format */
	  || errno == ENOTDIR)
	{
	  if ((*symlink = read_symlink (dir->fd, name)))
	    {
	      entry = dir;
	      goto skip;
	    }
	  errno = ELOOP;
	}
      goto out;
    }
skip:
  while (ISSLASH (*p))
    p++;
out:
  *path = p;
  return entry;
}

/* Traverse PATHNAME.  Updates PATHNAME to point to the last path component and
   returns a file descriptor to its parent directory (which can be AT_FDCWD).
   When KEEPFD is given, make sure that the cache entry for DIRFD is not
   removed from the cache (and KEEPFD remains open).

   When this function is not running, all cache entries are on the lru list,
   and all cache entries which still have a parent are also in the hash table.
   While this function is running, all cache entries on the path being looked
   up are off the lru list but in the hash table.
    */
static int traverse_another_path (const char **pathname, int keepfd)
{
  static struct cached_dirfd cwd = {
    .fd = AT_FDCWD,
  };

  unsigned int misses = dirfd_cache_misses;
  const char *path = *pathname, *last;
  struct cached_dirfd *dir = &cwd;
  struct symlink *stack = NULL;
  unsigned int steps = count_path_components (path);
  struct cached_dirfd *traversed_symlink = NULL;

  INIT_LIST_HEAD (&cwd.children);

  if (steps > MAX_PATH_COMPONENTS)
    {
      errno = ELOOP;
      return -1;
    }

  if (! *path || IS_ABSOLUTE_FILE_NAME (path))
    return AT_FDCWD;

  /* Find the last pathname component */
  last = strrchr (path, 0) - 1;
  if (ISSLASH (*last))
    {
      while (last != path)
	if (! ISSLASH (*--last))
	  break;
    }
  while (last != path && ! ISSLASH (*(last - 1)))
    last--;
  if (last == path)
    return AT_FDCWD;

  if (debug & 32)
    printf ("Resolving path \"%.*s\"", (int) (last - path), path);

  while (stack || path != last)
    {
      struct cached_dirfd *entry;
      struct symlink *symlink = NULL;
      const char *prev = path;

      entry = traverse_next (dir, stack ? &stack->path : &path, keepfd, &symlink);
      if (! entry)
	{
	  if (debug & 32)
	    {
	      printf (" (failed)\n");
	      fflush (stdout);
	    }
	  goto fail;
	}
      dir = entry;
      if (! stack && symlink)
	{
	  const char *p = prev;
	  char *name;

	  while (*p && ! ISSLASH (*p))
	    p++;
	  name = alloca (p - prev + 1);
	  memcpy (name, prev, p - prev);
	  name[p - prev] = 0;

	  traversed_symlink = new_cached_dirfd (dir, name, -1);
	}
      if (stack && ! *stack->path)
	pop_symlink (&stack);
      if (symlink && *symlink->path)
	{
	  push_symlink (&stack, symlink);
	  steps += count_path_components (symlink->path);
	  if (steps > MAX_PATH_COMPONENTS)
	    {
	      errno = ELOOP;
	      goto fail;
	    }
	}
      else if (symlink)
	pop_symlink (&symlink);
      if (traversed_symlink && ! stack)
	{
	  traversed_symlink->fd =
	    entry->fd == AT_FDCWD ? AT_FDCWD : dup (entry->fd);
	  if (traversed_symlink->fd != -1)
	    {
	      insert_cached_dirfd (traversed_symlink, keepfd);
	      list_add (&traversed_symlink->lru_link, &lru_list);
	    }
	  else
	    free_cached_dirfd (traversed_symlink);
	  traversed_symlink = NULL;
	}
    }
  *pathname = last;
  if (debug & 32)
    {
      misses = (signed int) dirfd_cache_misses - (signed int) misses;
      if (! misses)
	printf(" (cached)\n");
      else
	printf (" (%u miss%s)\n", misses, misses == 1 ? "" : "es");
      fflush (stdout);
    }
  return put_path (dir);

fail:
  if (traversed_symlink)
    free_cached_dirfd (traversed_symlink);
  put_path (dir);
  while (stack)
    pop_symlink (&stack);
  return -1;
}

/* Just traverse PATHNAME; see traverse_another_path(). */
static int traverse_path (const char **pathname)
{
  return traverse_another_path (pathname, -1);
}

static int safe_xstat (const char *pathname, struct stat *buf, int flags)
{
  int dirfd;

  if (unsafe)
    return fstatat (AT_FDCWD, pathname, buf, flags);

  dirfd = traverse_path (&pathname);
  if (dirfd < 0 && dirfd != AT_FDCWD)
    return dirfd;
  return fstatat (dirfd, pathname, buf, flags);
}

/* Replacement for stat() */
int safe_stat (const char *pathname, struct stat *buf)
{
  return safe_xstat (pathname, buf, 0);
}

/* Replacement for lstat() */
int safe_lstat (const char *pathname, struct stat *buf)
{
  return safe_xstat (pathname, buf, AT_SYMLINK_NOFOLLOW);
}

/* Replacement for open() */
int safe_open (const char *pathname, int flags, mode_t mode)
{
  int dirfd;

  if (unsafe)
    return open (pathname, flags, mode);

  dirfd = traverse_path (&pathname);
  if (dirfd < 0 && dirfd != AT_FDCWD)
    return dirfd;
  return openat (dirfd, pathname, flags, mode);
}

/* Replacement for rename() */
int safe_rename (const char *oldpath, const char *newpath)
{
  int olddirfd, newdirfd;
  int ret;

  if (unsafe)
    return rename (oldpath, newpath);

  olddirfd = traverse_path (&oldpath);
  if (olddirfd < 0 && olddirfd != AT_FDCWD)
    return olddirfd;

  newdirfd = traverse_another_path (&newpath, olddirfd);
  if (newdirfd < 0 && newdirfd != AT_FDCWD)
    return newdirfd;

  ret = renameat (olddirfd, oldpath, newdirfd, newpath);
  if (! ret)
    {
      invalidate_cached_dirfd (olddirfd, oldpath);
      invalidate_cached_dirfd (newdirfd, newpath);
    }
  return ret;
}

/* Replacement for mkdir() */
int safe_mkdir (const char *pathname, mode_t mode)
{
  int dirfd;

  if (unsafe)
    return mkdir (pathname, mode);

  dirfd = traverse_path (&pathname);
  if (dirfd < 0 && dirfd != AT_FDCWD)
    return dirfd;
  return mkdirat (dirfd, pathname, mode);
}

/* Replacement for rmdir() */
int safe_rmdir (const char *pathname)
{
  int dirfd;
  int ret;

  if (unsafe)
    return rmdir (pathname);

  dirfd = traverse_path (&pathname);
  if (dirfd < 0 && dirfd != AT_FDCWD)
    return dirfd;

  ret = unlinkat (dirfd, pathname, AT_REMOVEDIR);
  if (! ret)
    invalidate_cached_dirfd (dirfd, pathname);
  return ret;
}

/* Replacement for unlink() */
int safe_unlink (const char *pathname)
{
  int dirfd;

  if (unsafe)
    return unlink (pathname);

  dirfd = traverse_path (&pathname);
  if (dirfd < 0 && dirfd != AT_FDCWD)
    return dirfd;
  return unlinkat (dirfd, pathname, 0);
}

/* Replacement for symlink() */
int safe_symlink (const char *target, const char *linkpath)
{
  int dirfd;

  if (unsafe)
    return symlink (target, linkpath);

  dirfd = traverse_path (&linkpath);
  if (dirfd < 0 && dirfd != AT_FDCWD)
    return dirfd;
  return symlinkat (target, dirfd, linkpath);
}

/* Replacement for chmod() */
int safe_chmod (const char *pathname, mode_t mode)
{
  int dirfd;

  if (unsafe)
    return chmod (pathname, mode);

  dirfd = traverse_path (&pathname);
  if (dirfd < 0 && dirfd != AT_FDCWD)
    return dirfd;
  return fchmodat (dirfd, pathname, mode, 0);
}

/* Replacement for lchown() */
int safe_lchown (const char *pathname, uid_t owner, gid_t group)
{
  int dirfd;

  if (unsafe)
    return lchown (pathname, owner, group);

  dirfd = traverse_path (&pathname);
  if (dirfd < 0 && dirfd != AT_FDCWD)
    return dirfd;
  return fchownat (dirfd, pathname, owner, group, AT_SYMLINK_NOFOLLOW);
}

/* Replacement for lutimens() */
int safe_lutimens (const char *pathname, struct timespec const times[2])
{
  int dirfd;

  if (unsafe)
    return utimensat (AT_FDCWD, pathname, times, AT_SYMLINK_NOFOLLOW);

  dirfd = traverse_path (&pathname);
  if (dirfd < 0 && dirfd != AT_FDCWD)
    return dirfd;
  return utimensat (dirfd, pathname, times, AT_SYMLINK_NOFOLLOW);
}

/* Replacement for readlink() */
ssize_t safe_readlink (const char *pathname, char *buf, size_t bufsiz)
{
  int dirfd;

  if (unsafe)
    return readlink (pathname, buf, bufsiz);

  dirfd = traverse_path (&pathname);
  if (dirfd < 0 && dirfd != AT_FDCWD)
    return dirfd;
  return readlinkat (dirfd, pathname, buf, bufsiz);
}

/* Replacement for access() */
int safe_access (const char *pathname, int mode)
{
  int dirfd;

  if (unsafe)
    return access (pathname, mode);

  dirfd = traverse_path (&pathname);
  if (dirfd < 0 && dirfd != AT_FDCWD)
    return dirfd;
  return faccessat (dirfd, pathname, mode, 0);
}