1 /* SPDX-License-Identifier: LGPL-2.1-or-later */
2 
3 #include <errno.h>
4 #include <fcntl.h>
5 #include <stdio.h>
6 #include <string.h>
7 #include <sys/file.h>
8 #include <sys/stat.h>
9 
10 #include "alloc-util.h"
11 #include "fd-util.h"
12 #include "fs-util.h"
13 #include "lockfile-util.h"
14 #include "macro.h"
15 #include "missing_fcntl.h"
16 #include "path-util.h"
17 
make_lock_file(const char * p,int operation,LockFile * ret)18 int make_lock_file(const char *p, int operation, LockFile *ret) {
19         _cleanup_close_ int fd = -1;
20         _cleanup_free_ char *t = NULL;
21         int r;
22 
23         /*
24          * We use UNPOSIX locks if they are available. They have nice
25          * semantics, and are mostly compatible with NFS. However,
26          * they are only available on new kernels. When we detect we
27          * are running on an older kernel, then we fall back to good
28          * old BSD locks. They also have nice semantics, but are
29          * slightly problematic on NFS, where they are upgraded to
30          * POSIX locks, even though locally they are orthogonal to
31          * POSIX locks.
32          */
33 
34         t = strdup(p);
35         if (!t)
36                 return -ENOMEM;
37 
38         for (;;) {
39                 struct flock fl = {
40                         .l_type = (operation & ~LOCK_NB) == LOCK_EX ? F_WRLCK : F_RDLCK,
41                         .l_whence = SEEK_SET,
42                 };
43                 struct stat st;
44 
45                 fd = open(p, O_CREAT|O_RDWR|O_NOFOLLOW|O_CLOEXEC|O_NOCTTY, 0600);
46                 if (fd < 0)
47                         return -errno;
48 
49                 r = fcntl(fd, (operation & LOCK_NB) ? F_OFD_SETLK : F_OFD_SETLKW, &fl);
50                 if (r < 0) {
51 
52                         /* If the kernel is too old, use good old BSD locks */
53                         if (errno == EINVAL)
54                                 r = flock(fd, operation);
55 
56                         if (r < 0)
57                                 return errno == EAGAIN ? -EBUSY : -errno;
58                 }
59 
60                 /* If we acquired the lock, let's check if the file
61                  * still exists in the file system. If not, then the
62                  * previous exclusive owner removed it and then closed
63                  * it. In such a case our acquired lock is worthless,
64                  * hence try again. */
65 
66                 r = fstat(fd, &st);
67                 if (r < 0)
68                         return -errno;
69                 if (st.st_nlink > 0)
70                         break;
71 
72                 fd = safe_close(fd);
73         }
74 
75         ret->path = t;
76         ret->fd = fd;
77         ret->operation = operation;
78 
79         fd = -1;
80         t = NULL;
81 
82         return r;
83 }
84 
make_lock_file_for(const char * p,int operation,LockFile * ret)85 int make_lock_file_for(const char *p, int operation, LockFile *ret) {
86         const char *fn;
87         char *t;
88 
89         assert(p);
90         assert(ret);
91 
92         fn = basename(p);
93         if (!filename_is_valid(fn))
94                 return -EINVAL;
95 
96         t = newa(char, strlen(p) + 2 + 4 + 1);
97         stpcpy(stpcpy(stpcpy(mempcpy(t, p, fn - p), ".#"), fn), ".lck");
98 
99         return make_lock_file(t, operation, ret);
100 }
101 
release_lock_file(LockFile * f)102 void release_lock_file(LockFile *f) {
103         int r;
104 
105         if (!f)
106                 return;
107 
108         if (f->path) {
109 
110                 /* If we are the exclusive owner we can safely delete
111                  * the lock file itself. If we are not the exclusive
112                  * owner, we can try becoming it. */
113 
114                 if (f->fd >= 0 &&
115                     (f->operation & ~LOCK_NB) == LOCK_SH) {
116                         static const struct flock fl = {
117                                 .l_type = F_WRLCK,
118                                 .l_whence = SEEK_SET,
119                         };
120 
121                         r = fcntl(f->fd, F_OFD_SETLK, &fl);
122                         if (r < 0 && errno == EINVAL)
123                                 r = flock(f->fd, LOCK_EX|LOCK_NB);
124 
125                         if (r >= 0)
126                                 f->operation = LOCK_EX|LOCK_NB;
127                 }
128 
129                 if ((f->operation & ~LOCK_NB) == LOCK_EX)
130                         unlink_noerrno(f->path);
131 
132                 f->path = mfree(f->path);
133         }
134 
135         f->fd = safe_close(f->fd);
136         f->operation = 0;
137 }
138