1/* Determine directory for shm/sem files. Linux version.
2 Copyright (C) 2000-2021 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4
5 The GNU C Library is free software; you can redistribute it and/or
6 modify it under the terms of the GNU Lesser General Public
7 License as published by the Free Software Foundation; either
8 version 2.1 of the License, or (at your option) any later version.
9
10 The GNU C Library is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 Lesser General Public License for more details.
14
15 You should have received a copy of the GNU Lesser General Public
16 License along with the GNU C Library; if not, see
17 <https://www.gnu.org/licenses/>. */
18
19#include "shm-directory.h"
20
21#include <errno.h>
22#include <mntent.h>
23#include <paths.h>
24#include <stdio.h>
25#include <string.h>
26#include <sys/statfs.h>
27#include <libc-lock.h>
28#include "linux_fsinfo.h"
29
30
31/* Mount point of the shared memory filesystem. */
32static struct
33{
34 char *dir;
35 size_t dirlen;
36} mountpoint;
37
38/* This is the default directory. */
39static const char defaultdir[] = "/dev/shm/";
40
41/* Protect the `mountpoint' variable above. */
42__libc_once_define (static, once);
43
44
45/* Determine where the shmfs is mounted (if at all). */
46static void
47where_is_shmfs (void)
48{
49 char buf[512];
50 struct statfs f;
51 struct mntent resmem;
52 struct mntent *mp;
53 FILE *fp;
54
55 /* The canonical place is /dev/shm. This is at least what the
56 documentation tells everybody to do. */
57 if (__statfs (defaultdir, &f) == 0 && (f.f_type == SHMFS_SUPER_MAGIC
58 || f.f_type == RAMFS_MAGIC))
59 {
60 /* It is in the normal place. */
61 mountpoint.dir = (char *) defaultdir;
62 mountpoint.dirlen = sizeof (defaultdir) - 1;
63
64 return;
65 }
66
67 /* OK, do it the hard way. Look through the /proc/mounts file and if
68 this does not exist through /etc/fstab to find the mount point. */
69 fp = __setmntent ("/proc/mounts", "r");
70 if (__glibc_unlikely (fp == NULL))
71 {
72 fp = __setmntent (_PATH_MNTTAB, "r");
73 if (__glibc_unlikely (fp == NULL))
74 /* There is nothing we can do. Blind guesses are not helpful. */
75 return;
76 }
77
78 /* Now read the entries. */
79 while ((mp = __getmntent_r (fp, &resmem, buf, sizeof buf)) != NULL)
80 /* The original name is "shm" but this got changed in early Linux
81 2.4.x to "tmpfs". */
82 if (strcmp (mp->mnt_type, "tmpfs") == 0
83 || strcmp (mp->mnt_type, "shm") == 0)
84 {
85 /* Found it. There might be more than one place where the
86 filesystem is mounted but one is enough for us. */
87 size_t namelen;
88
89 /* First make sure this really is the correct entry. At least
90 some versions of the kernel give wrong information because
91 of the implicit mount of the shmfs for SysV IPC. */
92 if (__statfs (mp->mnt_dir, &f) != 0 || (f.f_type != SHMFS_SUPER_MAGIC
93 && f.f_type != RAMFS_MAGIC))
94 continue;
95
96 namelen = strlen (mp->mnt_dir);
97
98 if (namelen == 0)
99 /* Hum, maybe some crippled entry. Keep on searching. */
100 continue;
101
102 mountpoint.dir = (char *) malloc (namelen + 2);
103 if (mountpoint.dir != NULL)
104 {
105 char *cp = __mempcpy (mountpoint.dir, mp->mnt_dir, namelen);
106 if (cp[-1] != '/')
107 *cp++ = '/';
108 *cp = '\0';
109 mountpoint.dirlen = cp - mountpoint.dir;
110 }
111
112 break;
113 }
114
115 /* Close the stream. */
116 __endmntent (fp);
117}
118
119
120const char *
121__shm_directory (size_t *len)
122{
123 /* Determine where the shmfs is mounted. */
124 __libc_once (once, where_is_shmfs);
125
126 /* If we don't know the mount points there is nothing we can do. Ever. */
127 if (__glibc_unlikely (mountpoint.dir == NULL))
128 {
129 __set_errno (ENOSYS);
130 return NULL;
131 }
132
133 *len = mountpoint.dirlen;
134 return mountpoint.dir;
135}
136#if IS_IN (libpthread)
137hidden_def (__shm_directory)
138
139/* Make sure the table is freed if we want to free everything before
140 exiting. */
141void
142__shm_directory_freeres (void)
143{
144 if (mountpoint.dir != defaultdir)
145 free (mountpoint.dir);
146}
147#endif
148