1 /* Retrieve the umask of the process (multithread-safe).
2 Copyright (C) 2020-2021 Free Software Foundation, Inc.
3
4 This program is free software: you can redistribute it and/or modify
5 it under the terms of the GNU General Public License as published by
6 the Free Software Foundation; either version 3 of the License, or
7 (at your option) any later version.
8
9 This program 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
12 GNU General Public License for more details.
13
14 You should have received a copy of the GNU General Public License
15 along with this program. If not, see <https://www.gnu.org/licenses/>. */
16
17 /* Written by Bruno Haible <bruno@clisp.org>, 2020. */
18
19 /* There are three ways to implement a getumask() function on systems that
20 don't have it:
21 (a) Through system calls on the file system.
22 (b) Through a global variable that the main() function has to set,
23 together with an override of the umask() function.
24 (c) Through { mode_t mask = umask (0); umask (mask); }.
25
26 Each has its drawbacks:
27 (a) Causes additional system calls. May fail in some rare cases.
28 (b) Causes globally visible code complexity / maintainer effort.
29 (c) Is not multithread-safe: open() calls in other threads may
30 create files with wrong access permissions.
31
32 Here we implement (a), as the least evil. */
33
34 #include <config.h>
35 /* The package may define ASSUME_UMASK_CONSTANT to 1, to indicate that the
36 program does not call umask(). */
37 /* #define ASSUME_UMASK_CONSTANT 1 */
38
39 /* Specification. */
40 #include <sys/stat.h>
41
42 #include <fcntl.h>
43 #include <stdlib.h>
44 #include <string.h>
45 #include <unistd.h>
46
47 #include "clean-temp.h"
48 #include "tempname.h"
49
50 mode_t
getumask(void)51 getumask (void)
52 {
53 #if 0
54 /* This is not multithread-safe! */
55 mode_t mask = umask (0);
56 umask (mask);
57 return mask;
58 #else
59 # if ASSUME_UMASK_CONSTANT
60 static int cached_umask = -1;
61 if (cached_umask >= 0)
62 return cached_umask;
63 # endif
64
65 int mask = -1;
66 # if defined __linux__
67 {
68 /* In Linux >= 4.7, the umask can be retrieved from an "Umask:" line in the
69 /proc/self/status file. */
70 char buf[4096];
71 int fd = open ("/proc/self/status", O_RDONLY);
72 if (fd >= 0)
73 {
74 ssize_t n = read (fd, buf, sizeof (buf));
75 if (n > 0)
76 {
77 const char *p_end = buf + n;
78 const char *p = buf;
79
80 for (;;)
81 {
82 /* Here we're at the beginning of a line. */
83 if (p_end - p > 8 && memcmp (p, "Umask:\t0", 8) == 0)
84 {
85 unsigned int value = 0;
86 p += 8;
87 for (; p < p_end && *p >= '0' && *p <= '7'; p++)
88 value = 8 * value + (*p - '0');
89 if (p < p_end && *p == '\n')
90 mask = value;
91 break;
92 }
93 /* Search the start of the next line. */
94 for (; p < p_end && *p != '\n'; p++)
95 ;
96 if (p == p_end)
97 break;
98 p++;
99 }
100 }
101 close (fd);
102 }
103 }
104 # endif
105 if (mask < 0)
106 {
107 /* Create a temporary file and inspect its access permissions. */
108 const char *tmpdir = getenv ("TMPDIR");
109 if (tmpdir == NULL || *tmpdir == '\0')
110 tmpdir = "/tmp";
111 size_t tmpdir_length = strlen (tmpdir);
112 char *temp_filename = (char *) malloc (tmpdir_length + 15 + 1);
113 if (temp_filename != NULL)
114 {
115 memcpy (temp_filename, tmpdir, tmpdir_length);
116 strcpy (temp_filename + tmpdir_length, "/gtumask.XXXXXX");
117 int fd = gen_register_open_temp (temp_filename, 0, O_RDWR,
118 S_IRWXU|S_IRWXG|S_IRWXO);
119 if (fd >= 0)
120 {
121 struct stat statbuf;
122 if (fstat (fd, &statbuf) >= 0)
123 mask = (S_IRWXU|S_IRWXG|S_IRWXO) & ~statbuf.st_mode;
124 close_temp (fd);
125 cleanup_temporary_file (temp_filename, false);
126 }
127 free (temp_filename);
128 }
129 }
130 if (mask < 0)
131 {
132 /* We still don't know! Assume a paranoid user. */
133 mask = 077;
134 }
135 # if ASSUME_UMASK_CONSTANT
136 cached_umask = mask;
137 # endif
138 return mask;
139 #endif
140 }
141