summary refs log tree commit diff
path: root/sysdeps/posix/tempname.c
blob: 5f804b38d789572e269ff4f80ba13b236fa37360 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
/* Copyright (C) 1991-2021 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/>.  */

#if !_LIBC
# include <libc-config.h>
# include "tempname.h"
#endif

#include <sys/types.h>
#include <assert.h>
#include <stdbool.h>

#include <errno.h>

#include <stdio.h>
#ifndef P_tmpdir
# define P_tmpdir "/tmp"
#endif
#ifndef TMP_MAX
# define TMP_MAX 238328
#endif
#ifndef __GT_FILE
# define __GT_FILE      0
# define __GT_DIR       1
# define __GT_NOCREATE  2
#endif
#if !_LIBC && (GT_FILE != __GT_FILE || GT_DIR != __GT_DIR       \
               || GT_NOCREATE != __GT_NOCREATE)
# error report this to bug-gnulib@gnu.org
#endif

#include <stddef.h>
#include <stdlib.h>
#include <string.h>

#include <fcntl.h>
#include <stdalign.h>
#include <stdint.h>
#include <sys/random.h>
#include <sys/stat.h>
#include <time.h>

#if _LIBC
# define struct_stat64 struct stat64
# define __secure_getenv __libc_secure_getenv
#else
# define struct_stat64 struct stat
# define __gen_tempname gen_tempname
# define __mkdir mkdir
# define __open open
# define __lstat64(file, buf) lstat (file, buf)
# define __stat64(file, buf) stat (file, buf)
# define __getrandom getrandom
# define __clock_gettime64 clock_gettime
# define __timespec64 timespec
#endif

/* Use getrandom if it works, falling back on a 64-bit linear
   congruential generator that starts with Var's value
   mixed in with a clock's low-order bits if available.  */
typedef uint_fast64_t random_value;
#define RANDOM_VALUE_MAX UINT_FAST64_MAX
#define BASE_62_DIGITS 10 /* 62**10 < UINT_FAST64_MAX */
#define BASE_62_POWER (62LL * 62 * 62 * 62 * 62 * 62 * 62 * 62 * 62 * 62)

static random_value
random_bits (random_value var, bool use_getrandom)
{
  random_value r;
  /* Without GRND_NONBLOCK it can be blocked for minutes on some systems.  */
  if (use_getrandom && __getrandom (&r, sizeof r, GRND_NONBLOCK) == sizeof r)
    return r;
#if _LIBC || (defined CLOCK_MONOTONIC && HAVE_CLOCK_GETTIME)
  /* Add entropy if getrandom did not work.  */
  struct __timespec64 tv;
  __clock_gettime64 (CLOCK_MONOTONIC, &tv);
  var ^= tv.tv_nsec;
#endif
  return 2862933555777941757 * var + 3037000493;
}

#if _LIBC
/* Return nonzero if DIR is an existent directory.  */
static int
direxists (const char *dir)
{
  struct_stat64 buf;
  return __stat64 (dir, &buf) == 0 && S_ISDIR (buf.st_mode);
}

/* Path search algorithm, for tmpnam, tmpfile, etc.  If DIR is
   non-null and exists, uses it; otherwise uses the first of $TMPDIR,
   P_tmpdir, /tmp that exists.  Copies into TMPL a template suitable
   for use with mk[s]temp.  Will fail (-1) if DIR is non-null and
   doesn't exist, none of the searched dirs exists, or there's not
   enough space in TMPL. */
int
__path_search (char *tmpl, size_t tmpl_len, const char *dir, const char *pfx,
               int try_tmpdir)
{
  const char *d;
  size_t dlen, plen;

  if (!pfx || !pfx[0])
    {
      pfx = "file";
      plen = 4;
    }
  else
    {
      plen = strlen (pfx);
      if (plen > 5)
        plen = 5;
    }

  if (try_tmpdir)
    {
      d = __secure_getenv ("TMPDIR");
      if (d != NULL && direxists (d))
        dir = d;
      else if (dir != NULL && direxists (dir))
        /* nothing */ ;
      else
        dir = NULL;
    }
  if (dir == NULL)
    {
      if (direxists (P_tmpdir))
        dir = P_tmpdir;
      else if (strcmp (P_tmpdir, "/tmp") != 0 && direxists ("/tmp"))
        dir = "/tmp";
      else
        {
          __set_errno (ENOENT);
          return -1;
        }
    }

  dlen = strlen (dir);
  while (dlen > 1 && dir[dlen - 1] == '/')
    dlen--;                     /* remove trailing slashes */

  /* check we have room for "${dir}/${pfx}XXXXXX\0" */
  if (tmpl_len < dlen + 1 + plen + 6 + 1)
    {
      __set_errno (EINVAL);
      return -1;
    }

  sprintf (tmpl, "%.*s/%.*sXXXXXX", (int) dlen, dir, (int) plen, pfx);
  return 0;
}
#endif /* _LIBC */

#if _LIBC
static int try_tempname_len (char *, int, void *, int (*) (char *, void *),
                             size_t);
#endif

static int
try_file (char *tmpl, void *flags)
{
  int *openflags = flags;
  return __open (tmpl,
                 (*openflags & ~O_ACCMODE)
                 | O_RDWR | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR);
}

static int
try_dir (char *tmpl, void *flags _GL_UNUSED)
{
  return __mkdir (tmpl, S_IRUSR | S_IWUSR | S_IXUSR);
}

static int
try_nocreate (char *tmpl, void *flags _GL_UNUSED)
{
  struct_stat64 st;

  if (__lstat64 (tmpl, &st) == 0 || errno == EOVERFLOW)
    __set_errno (EEXIST);
  return errno == ENOENT ? 0 : -1;
}

/* These are the characters used in temporary file names.  */
static const char letters[] =
"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

/* Generate a temporary file name based on TMPL.  TMPL must match the
   rules for mk[s]temp (i.e., end in at least X_SUFFIX_LEN "X"s,
   possibly with a suffix).
   The name constructed does not exist at the time of the call to
   this function.  TMPL is overwritten with the result.

   KIND may be one of:
   __GT_NOCREATE:       simply verify that the name does not exist
                        at the time of the call.
   __GT_FILE:           create the file using open(O_CREAT|O_EXCL)
                        and return a read-write fd.  The file is mode 0600.
   __GT_DIR:            create a directory, which will be mode 0700.

   We use a clever algorithm to get hard-to-predict names. */
#ifdef _LIBC
static
#endif
int
gen_tempname_len (char *tmpl, int suffixlen, int flags, int kind,
                  size_t x_suffix_len)
{
  static int (*const tryfunc[]) (char *, void *) =
    {
      [__GT_FILE] = try_file,
      [__GT_DIR] = try_dir,
      [__GT_NOCREATE] = try_nocreate
    };
  return try_tempname_len (tmpl, suffixlen, &flags, tryfunc[kind],
                           x_suffix_len);
}

#ifdef _LIBC
static
#endif
int
try_tempname_len (char *tmpl, int suffixlen, void *args,
                  int (*tryfunc) (char *, void *), size_t x_suffix_len)
{
  size_t len;
  char *XXXXXX;
  unsigned int count;
  int fd = -1;
  int save_errno = errno;

  /* A lower bound on the number of temporary files to attempt to
     generate.  The maximum total number of temporary file names that
     can exist for a given template is 62**6.  It should never be
     necessary to try all of these combinations.  Instead if a reasonable
     number of names is tried (we define reasonable as 62**3) fail to
     give the system administrator the chance to remove the problems.
     This value requires that X_SUFFIX_LEN be at least 3.  */
#define ATTEMPTS_MIN (62 * 62 * 62)

  /* The number of times to attempt to generate a temporary file.  To
     conform to POSIX, this must be no smaller than TMP_MAX.  */
#if ATTEMPTS_MIN < TMP_MAX
  unsigned int attempts = TMP_MAX;
#else
  unsigned int attempts = ATTEMPTS_MIN;
#endif

  /* A random variable.  The initial value is used only the for fallback path
     on 'random_bits' on 'getrandom' failure.  Its initial value tries to use
     some entropy from the ASLR and ignore possible bits from the stack
     alignment.  */
  random_value v = ((uintptr_t) &v) / alignof (max_align_t);

  /* How many random base-62 digits can currently be extracted from V.  */
  int vdigits = 0;

  /* Whether to consume entropy when acquiring random bits.  On the
     first try it's worth the entropy cost with __GT_NOCREATE, which
     is inherently insecure and can use the entropy to make it a bit
     less secure.  On the (rare) second and later attempts it might
     help against DoS attacks.  */
  bool use_getrandom = tryfunc == try_nocreate;

  /* Least unfair value for V.  If V is less than this, V can generate
     BASE_62_DIGITS digits fairly.  Otherwise it might be biased.  */
  random_value const unfair_min
    = RANDOM_VALUE_MAX - RANDOM_VALUE_MAX % BASE_62_POWER;

  len = strlen (tmpl);
  if (len < x_suffix_len + suffixlen
      || strspn (&tmpl[len - x_suffix_len - suffixlen], "X") < x_suffix_len)
    {
      __set_errno (EINVAL);
      return -1;
    }

  /* This is where the Xs start.  */
  XXXXXX = &tmpl[len - x_suffix_len - suffixlen];

  for (count = 0; count < attempts; ++count)
    {
      for (size_t i = 0; i < x_suffix_len; i++)
        {
          if (vdigits == 0)
            {
              do
                {
                  v = random_bits (v, use_getrandom);
                  use_getrandom = true;
                }
              while (unfair_min <= v);

              vdigits = BASE_62_DIGITS;
            }

          XXXXXX[i] = letters[v % 62];
          v /= 62;
          vdigits--;
        }

      fd = tryfunc (tmpl, args);
      if (fd >= 0)
        {
          __set_errno (save_errno);
          return fd;
        }
      else if (errno != EEXIST)
        return -1;
    }

  /* We got out of the loop because we ran out of combinations to try.  */
  __set_errno (EEXIST);
  return -1;
}

int
__gen_tempname (char *tmpl, int suffixlen, int flags, int kind)
{
  return gen_tempname_len (tmpl, suffixlen, flags, kind, 6);
}

#if !_LIBC
int
try_tempname (char *tmpl, int suffixlen, void *args,
              int (*tryfunc) (char *, void *))
{
  return try_tempname_len (tmpl, suffixlen, args, tryfunc, 6);
}
#endif