lib/tempname.c

   1 /* Copyright (C) 1991-2024 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <https://www.gnu.org/licenses/>.  */
  17
  18 #if !_LIBC
  19 # include <libc-config.h>
  20 # include "tempname.h"
  21 #endif
  22
  23 #include <errno.h>
  24
  25 #include <stdio.h>
  26 #ifndef TMP_MAX
  27 # define TMP_MAX 238328
  28 #endif
  29 #ifndef __GT_FILE
  30 # define __GT_FILE      0
  31 # define __GT_DIR       1
  32 # define __GT_NOCREATE  2
  33 #endif
  34 #if !_LIBC && (GT_FILE != __GT_FILE || GT_DIR != __GT_DIR       \
  35                || GT_NOCREATE != __GT_NOCREATE)
  36 # error report this to bug-gnulib@gnu.org
  37 #endif
  38
  39 #include <stdlib.h>
  40 #include <string.h>
  41
  42 #include <fcntl.h>
  43 #include <stdint.h>
  44 #include <sys/random.h>
  45 #include <sys/stat.h>
  46 #include <time.h>
  47
  48 #if _LIBC
  49 # define struct_stat64 struct __stat64_t64
  50 #else
  51 # define struct_stat64 struct stat
  52 # define __gen_tempname gen_tempname
  53 # define __mkdir mkdir
  54 # define __open open
  55 # define __lstat64_time64(file, buf) lstat (file, buf)
  56 # define __getrandom getrandom
  57 # define __clock_gettime64 clock_gettime
  58 # define __timespec64 timespec
  59 #endif
  60
  61 /* Use getrandom if it works, falling back on a 64-bit linear
  62    congruential generator that starts with Var's value
  63    mixed in with a clock's low-order bits if available.  */
  64 typedef uint_fast64_t random_value;
  65 #define RANDOM_VALUE_MAX UINT_FAST64_MAX
  66 #define BASE_62_DIGITS 10 /* 62**10 < UINT_FAST64_MAX */
  67 #define BASE_62_POWER (62LL * 62 * 62 * 62 * 62 * 62 * 62 * 62 * 62 * 62)
  68
  69 /* Return the result of mixing the entropy from R and S.
  70    Assume that R and S are not particularly random,
  71    and that the result should look randomish to an untrained eye.  */
  72
  73 static random_value
  74 mix_random_values (random_value r, random_value s)
  75 {
  76   /* As this code is used only when high-quality randomness is neither
  77      available nor necessary, there is no need for fancier polynomials
  78      such as those in the Linux kernel's 'random' driver.  */
  79   return (2862933555777941757 * r + 3037000493) ^ s;
  80 }
  81
  82 /* Set *R to a random value.
  83    Return true if *R is set to high-quality value taken from getrandom.
  84    Otherwise return false, falling back to a low-quality *R that might
  85    depend on S.
  86
  87    This function returns false only when getrandom fails.
  88    On GNU systems this should happen only early in the boot process,
  89    when the fallback should be good enough for programs using tempname
  90    because any attacker likely has root privileges already.  */
  91
  92 static bool
  93 random_bits (random_value *r, random_value s)
  94 {
  95   /* Without GRND_NONBLOCK it can be blocked for minutes on some systems.  */
  96   if (__getrandom (r, sizeof *r, GRND_NONBLOCK) == sizeof *r)
  97     return true;
  98
  99   /* If getrandom did not work, use ersatz entropy based on low-order
 100      clock bits.  On GNU systems getrandom should fail only
 101      early in booting, when ersatz should be good enough.
 102      Do not use ASLR-based entropy, as that would leak ASLR info into
 103      the resulting file name which is typically public.
 104
 105      Of course we are in a state of sin here.  */
 106
 107   random_value v = s;
 108
 109 #if _LIBC || (defined CLOCK_REALTIME && HAVE_CLOCK_GETTIME)
 110   struct __timespec64 tv;
 111   __clock_gettime64 (CLOCK_REALTIME, &tv);
 112   v = mix_random_values (v, tv.tv_sec);
 113   v = mix_random_values (v, tv.tv_nsec);
 114 #endif
 115
 116   *r = mix_random_values (v, clock ());
 117   return false;
 118 }
 119
 120 #if _LIBC
 121 static int try_tempname_len (char *, int, void *, int (*) (char *, void *),
 122                              size_t);
 123 #endif
 124
 125 static int
 126 try_file (char *tmpl, void *flags)
 127 {
 128   int *openflags = flags;
 129   return __open (tmpl,
 130                  (*openflags & ~O_ACCMODE)
 131                  | O_RDWR | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR);
 132 }
 133
 134 static int
 135 try_dir (char *tmpl, _GL_UNUSED void *flags)
 136 {
 137   return __mkdir (tmpl, S_IRUSR | S_IWUSR | S_IXUSR);
 138 }
 139
 140 static int
 141 try_nocreate (char *tmpl, _GL_UNUSED void *flags)
 142 {
 143   struct_stat64 st;
 144
 145   if (__lstat64_time64 (tmpl, &st) == 0 || errno == EOVERFLOW)
 146     __set_errno (EEXIST);
 147   return errno == ENOENT ? 0 : -1;
 148 }
 149
 150 /* These are the characters used in temporary file names.  */
 151 static const char letters[] =
 152 "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
 153
 154 /* Generate a temporary file name based on TMPL.  TMPL must match the
 155    rules for mk[s]temp (i.e., end in at least X_SUFFIX_LEN "X"s,
 156    possibly with a suffix).
 157    The name constructed does not exist at the time of the call to
 158    this function.  TMPL is overwritten with the result.
 159
 160    KIND may be one of:
 161    __GT_NOCREATE:       simply verify that the name does not exist
 162                         at the time of the call.
 163    __GT_FILE:           create the file using open(O_CREAT|O_EXCL)
 164                         and return a read-write fd.  The file is mode 0600.
 165    __GT_DIR:            create a directory, which will be mode 0700.
 166
 167    */
 168 #ifdef _LIBC
 169 static
 170 #endif
 171 int
 172 gen_tempname_len (char *tmpl, int suffixlen, int flags, int kind,
 173                   size_t x_suffix_len)
 174 {
 175   static int (*const tryfunc[]) (char *, void *) =
 176     {
 177       [__GT_FILE] = try_file,
 178       [__GT_DIR] = try_dir,
 179       [__GT_NOCREATE] = try_nocreate
 180     };
 181   return try_tempname_len (tmpl, suffixlen, &flags, tryfunc[kind],
 182                            x_suffix_len);
 183 }
 184
 185 #ifdef _LIBC
 186 static
 187 #endif
 188 int
 189 try_tempname_len (char *tmpl, int suffixlen, void *args,
 190                   int (*tryfunc) (char *, void *), size_t x_suffix_len)
 191 {
 192   size_t len;
 193   char *XXXXXX;
 194   unsigned int count;
 195   int fd = -1;
 196   int saved_errno = errno;
 197
 198   /* A lower bound on the number of temporary files to attempt to
 199      generate.  The maximum total number of temporary file names that
 200      can exist for a given template is 62**6.  It should never be
 201      necessary to try all of these combinations.  Instead if a reasonable
 202      number of names is tried (we define reasonable as 62**3) fail to
 203      give the system administrator the chance to remove the problems.
 204      This value requires that X_SUFFIX_LEN be at least 3.  */
 205 #define ATTEMPTS_MIN (62 * 62 * 62)
 206
 207   /* The number of times to attempt to generate a temporary file.  To
 208      conform to POSIX, this must be no smaller than TMP_MAX.  */
 209 #if ATTEMPTS_MIN < TMP_MAX
 210   unsigned int attempts = TMP_MAX;
 211 #else
 212   unsigned int attempts = ATTEMPTS_MIN;
 213 #endif
 214
 215   /* A random variable.  */
 216   random_value v = 0;
 217
 218   /* A value derived from the random variable, and how many random
 219      base-62 digits can currently be extracted from VDIGBUF.  */
 220   random_value vdigbuf;
 221   int vdigits = 0;
 222
 223   /* Least biased value for V.  If V is less than this, V can generate
 224      BASE_62_DIGITS unbiased digits.  Otherwise the digits are biased.  */
 225   random_value const biased_min
 226     = RANDOM_VALUE_MAX - RANDOM_VALUE_MAX % BASE_62_POWER;
 227
 228   len = strlen (tmpl);
 229   if (len < x_suffix_len + suffixlen
 230       || strspn (&tmpl[len - x_suffix_len - suffixlen], "X") < x_suffix_len)
 231     {
 232       __set_errno (EINVAL);
 233       return -1;
 234     }
 235
 236   /* This is where the Xs start.  */
 237   XXXXXX = &tmpl[len - x_suffix_len - suffixlen];
 238
 239   for (count = 0; count < attempts; ++count)
 240     {
 241       for (size_t i = 0; i < x_suffix_len; i++)
 242         {
 243           if (vdigits == 0)
 244             {
 245               /* Worry about bias only if the bits are high quality.  */
 246               while (random_bits (&v, v) && biased_min <= v)
 247                 continue;
 248
 249               vdigbuf = v;
 250               vdigits = BASE_62_DIGITS;
 251             }
 252
 253           XXXXXX[i] = letters[vdigbuf % 62];
 254           vdigbuf /= 62;
 255           vdigits--;
 256         }
 257
 258       fd = tryfunc (tmpl, args);
 259       if (fd >= 0)
 260         {
 261           __set_errno (saved_errno);
 262           return fd;
 263         }
 264       else if (errno != EEXIST)
 265         return -1;
 266     }
 267
 268   /* We got out of the loop because we ran out of combinations to try.  */
 269   __set_errno (EEXIST);
 270   return -1;
 271 }
 272
 273 int
 274 __gen_tempname (char *tmpl, int suffixlen, int flags, int kind)
 275 {
 276   return gen_tempname_len (tmpl, suffixlen, flags, kind, 6);
 277 }
 278
 279 #if !_LIBC
 280 int
 281 try_tempname (char *tmpl, int suffixlen, void *args,
 282               int (*tryfunc) (char *, void *))
 283 {
 284   return try_tempname_len (tmpl, suffixlen, args, tryfunc, 6);
 285 }
 286 #endif