1 /* Copyright (C) 1991-2021 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 General Public
6    License as published by the Free Software Foundation; either
7    version 3 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    General Public License for more details.
13 
14    You should have received a copy of the GNU 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 <sys/types.h>
24 #include <assert.h>
25 #include <stdbool.h>
26 
27 #include <errno.h>
28 
29 #include <stdio.h>
30 #ifndef P_tmpdir
31 # define P_tmpdir "/tmp"
32 #endif
33 #ifndef TMP_MAX
34 # define TMP_MAX 238328
35 #endif
36 #ifndef __GT_FILE
37 # define __GT_FILE      0
38 # define __GT_DIR       1
39 # define __GT_NOCREATE  2
40 #endif
41 #if !_LIBC && (GT_FILE != __GT_FILE || GT_DIR != __GT_DIR       \
42                || GT_NOCREATE != __GT_NOCREATE)
43 # error report this to bug-gnulib@gnu.org
44 #endif
45 
46 #include <stddef.h>
47 #include <stdlib.h>
48 #include <string.h>
49 
50 #include <fcntl.h>
51 #include <stdalign.h>
52 #include <stdint.h>
53 #include <sys/random.h>
54 #include <sys/stat.h>
55 #include <time.h>
56 
57 #if _LIBC
58 # define struct_stat64 struct stat64
59 # define __secure_getenv __libc_secure_getenv
60 #else
61 # define struct_stat64 struct stat
62 # define __gen_tempname gen_tempname
63 # define __mkdir mkdir
64 # define __open open
65 # define __lstat64(file, buf) lstat (file, buf)
66 # define __stat64(file, buf) stat (file, buf)
67 # define __getrandom getrandom
68 # define __clock_gettime64 clock_gettime
69 # define __timespec64 timespec
70 #endif
71 
72 /* Use getrandom if it works, falling back on a 64-bit linear
73    congruential generator that starts with Var's value
74    mixed in with a clock's low-order bits if available.  */
75 typedef uint_fast64_t random_value;
76 #define RANDOM_VALUE_MAX UINT_FAST64_MAX
77 #define BASE_62_DIGITS 10 /* 62**10 < UINT_FAST64_MAX */
78 #define BASE_62_POWER (62LL * 62 * 62 * 62 * 62 * 62 * 62 * 62 * 62 * 62)
79 
80 static random_value
random_bits(random_value var,bool use_getrandom)81 random_bits (random_value var, bool use_getrandom)
82 {
83   random_value r;
84   /* Without GRND_NONBLOCK it can be blocked for minutes on some systems.  */
85   if (use_getrandom && __getrandom (&r, sizeof r, GRND_NONBLOCK) == sizeof r)
86     return r;
87 #if _LIBC || (defined CLOCK_MONOTONIC && HAVE_CLOCK_GETTIME)
88   /* Add entropy if getrandom did not work.  */
89   struct __timespec64 tv;
90   __clock_gettime64 (CLOCK_MONOTONIC, &tv);
91   var ^= tv.tv_nsec;
92 #endif
93   return 2862933555777941757 * var + 3037000493;
94 }
95 
96 #if _LIBC
97 /* Return nonzero if DIR is an existent directory.  */
98 static int
direxists(const char * dir)99 direxists (const char *dir)
100 {
101   struct_stat64 buf;
102   return __stat64 (dir, &buf) == 0 && S_ISDIR (buf.st_mode);
103 }
104 
105 /* Path search algorithm, for tmpnam, tmpfile, etc.  If DIR is
106    non-null and exists, uses it; otherwise uses the first of $TMPDIR,
107    P_tmpdir, /tmp that exists.  Copies into TMPL a template suitable
108    for use with mk[s]temp.  Will fail (-1) if DIR is non-null and
109    doesn't exist, none of the searched dirs exists, or there's not
110    enough space in TMPL. */
111 int
__path_search(char * tmpl,size_t tmpl_len,const char * dir,const char * pfx,int try_tmpdir)112 __path_search (char *tmpl, size_t tmpl_len, const char *dir, const char *pfx,
113                int try_tmpdir)
114 {
115   const char *d;
116   size_t dlen, plen;
117 
118   if (!pfx || !pfx[0])
119     {
120       pfx = "file";
121       plen = 4;
122     }
123   else
124     {
125       plen = strlen (pfx);
126       if (plen > 5)
127         plen = 5;
128     }
129 
130   if (try_tmpdir)
131     {
132       d = __secure_getenv ("TMPDIR");
133       if (d != NULL && direxists (d))
134         dir = d;
135       else if (dir != NULL && direxists (dir))
136         /* nothing */ ;
137       else
138         dir = NULL;
139     }
140   if (dir == NULL)
141     {
142       if (direxists (P_tmpdir))
143         dir = P_tmpdir;
144       else if (strcmp (P_tmpdir, "/tmp") != 0 && direxists ("/tmp"))
145         dir = "/tmp";
146       else
147         {
148           __set_errno (ENOENT);
149           return -1;
150         }
151     }
152 
153   dlen = strlen (dir);
154   while (dlen > 1 && dir[dlen - 1] == '/')
155     dlen--;                     /* remove trailing slashes */
156 
157   /* check we have room for "${dir}/${pfx}XXXXXX\0" */
158   if (tmpl_len < dlen + 1 + plen + 6 + 1)
159     {
160       __set_errno (EINVAL);
161       return -1;
162     }
163 
164   sprintf (tmpl, "%.*s/%.*sXXXXXX", (int) dlen, dir, (int) plen, pfx);
165   return 0;
166 }
167 #endif /* _LIBC */
168 
169 #if _LIBC
170 static int try_tempname_len (char *, int, void *, int (*) (char *, void *),
171                              size_t);
172 #endif
173 
174 static int
try_file(char * tmpl,void * flags)175 try_file (char *tmpl, void *flags)
176 {
177   int *openflags = flags;
178   return __open (tmpl,
179                  (*openflags & ~O_ACCMODE)
180                  | O_RDWR | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR);
181 }
182 
183 static int
try_dir(char * tmpl,void * flags _GL_UNUSED)184 try_dir (char *tmpl, void *flags _GL_UNUSED)
185 {
186   return __mkdir (tmpl, S_IRUSR | S_IWUSR | S_IXUSR);
187 }
188 
189 static int
try_nocreate(char * tmpl,void * flags _GL_UNUSED)190 try_nocreate (char *tmpl, void *flags _GL_UNUSED)
191 {
192   struct_stat64 st;
193 
194   if (__lstat64 (tmpl, &st) == 0 || errno == EOVERFLOW)
195     __set_errno (EEXIST);
196   return errno == ENOENT ? 0 : -1;
197 }
198 
199 /* These are the characters used in temporary file names.  */
200 static const char letters[] =
201 "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
202 
203 /* Generate a temporary file name based on TMPL.  TMPL must match the
204    rules for mk[s]temp (i.e., end in at least X_SUFFIX_LEN "X"s,
205    possibly with a suffix).
206    The name constructed does not exist at the time of the call to
207    this function.  TMPL is overwritten with the result.
208 
209    KIND may be one of:
210    __GT_NOCREATE:       simply verify that the name does not exist
211                         at the time of the call.
212    __GT_FILE:           create the file using open(O_CREAT|O_EXCL)
213                         and return a read-write fd.  The file is mode 0600.
214    __GT_DIR:            create a directory, which will be mode 0700.
215 
216    We use a clever algorithm to get hard-to-predict names. */
217 #ifdef _LIBC
218 static
219 #endif
220 int
gen_tempname_len(char * tmpl,int suffixlen,int flags,int kind,size_t x_suffix_len)221 gen_tempname_len (char *tmpl, int suffixlen, int flags, int kind,
222                   size_t x_suffix_len)
223 {
224   static int (*const tryfunc[]) (char *, void *) =
225     {
226       [__GT_FILE] = try_file,
227       [__GT_DIR] = try_dir,
228       [__GT_NOCREATE] = try_nocreate
229     };
230   return try_tempname_len (tmpl, suffixlen, &flags, tryfunc[kind],
231                            x_suffix_len);
232 }
233 
234 #ifdef _LIBC
235 static
236 #endif
237 int
try_tempname_len(char * tmpl,int suffixlen,void * args,int (* tryfunc)(char *,void *),size_t x_suffix_len)238 try_tempname_len (char *tmpl, int suffixlen, void *args,
239                   int (*tryfunc) (char *, void *), size_t x_suffix_len)
240 {
241   size_t len;
242   char *XXXXXX;
243   unsigned int count;
244   int fd = -1;
245   int save_errno = errno;
246 
247   /* A lower bound on the number of temporary files to attempt to
248      generate.  The maximum total number of temporary file names that
249      can exist for a given template is 62**6.  It should never be
250      necessary to try all of these combinations.  Instead if a reasonable
251      number of names is tried (we define reasonable as 62**3) fail to
252      give the system administrator the chance to remove the problems.
253      This value requires that X_SUFFIX_LEN be at least 3.  */
254 #define ATTEMPTS_MIN (62 * 62 * 62)
255 
256   /* The number of times to attempt to generate a temporary file.  To
257      conform to POSIX, this must be no smaller than TMP_MAX.  */
258 #if ATTEMPTS_MIN < TMP_MAX
259   unsigned int attempts = TMP_MAX;
260 #else
261   unsigned int attempts = ATTEMPTS_MIN;
262 #endif
263 
264   /* A random variable.  The initial value is used only the for fallback path
265      on 'random_bits' on 'getrandom' failure.  Its initial value tries to use
266      some entropy from the ASLR and ignore possible bits from the stack
267      alignment.  */
268   random_value v = ((uintptr_t) &v) / alignof (max_align_t);
269 
270   /* How many random base-62 digits can currently be extracted from V.  */
271   int vdigits = 0;
272 
273   /* Whether to consume entropy when acquiring random bits.  On the
274      first try it's worth the entropy cost with __GT_NOCREATE, which
275      is inherently insecure and can use the entropy to make it a bit
276      less secure.  On the (rare) second and later attempts it might
277      help against DoS attacks.  */
278   bool use_getrandom = tryfunc == try_nocreate;
279 
280   /* Least unfair value for V.  If V is less than this, V can generate
281      BASE_62_DIGITS digits fairly.  Otherwise it might be biased.  */
282   random_value const unfair_min
283     = RANDOM_VALUE_MAX - RANDOM_VALUE_MAX % BASE_62_POWER;
284 
285   len = strlen (tmpl);
286   if (len < x_suffix_len + suffixlen
287       || strspn (&tmpl[len - x_suffix_len - suffixlen], "X") < x_suffix_len)
288     {
289       __set_errno (EINVAL);
290       return -1;
291     }
292 
293   /* This is where the Xs start.  */
294   XXXXXX = &tmpl[len - x_suffix_len - suffixlen];
295 
296   for (count = 0; count < attempts; ++count)
297     {
298       for (size_t i = 0; i < x_suffix_len; i++)
299         {
300           if (vdigits == 0)
301             {
302               do
303                 {
304                   v = random_bits (v, use_getrandom);
305                   use_getrandom = true;
306                 }
307               while (unfair_min <= v);
308 
309               vdigits = BASE_62_DIGITS;
310             }
311 
312           XXXXXX[i] = letters[v % 62];
313           v /= 62;
314           vdigits--;
315         }
316 
317       fd = tryfunc (tmpl, args);
318       if (fd >= 0)
319         {
320           __set_errno (save_errno);
321           return fd;
322         }
323       else if (errno != EEXIST)
324         return -1;
325     }
326 
327   /* We got out of the loop because we ran out of combinations to try.  */
328   __set_errno (EEXIST);
329   return -1;
330 }
331 
332 int
__gen_tempname(char * tmpl,int suffixlen,int flags,int kind)333 __gen_tempname (char *tmpl, int suffixlen, int flags, int kind)
334 {
335   return gen_tempname_len (tmpl, suffixlen, flags, kind, 6);
336 }
337 
338 #if !_LIBC
339 int
try_tempname(char * tmpl,int suffixlen,void * args,int (* tryfunc)(char *,void *))340 try_tempname (char *tmpl, int suffixlen, void *args,
341               int (*tryfunc) (char *, void *))
342 {
343   return try_tempname_len (tmpl, suffixlen, args, tryfunc, 6);
344 }
345 #endif
346