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-rw-r--r--src/shared/util.c48
1 files changed, 44 insertions, 4 deletions
diff --git a/src/shared/util.c b/src/shared/util.c
index 409ccc7eed..64059065d8 100644
--- a/src/shared/util.c
+++ b/src/shared/util.c
@@ -7678,13 +7678,35 @@ int fd_setcrtime(int fd, usec_t usec) {
int same_fd(int a, int b) {
struct stat sta, stb;
+ pid_t pid;
+ int r, fa, fb;
assert(a >= 0);
assert(b >= 0);
+ /* Compares two file descriptors. Note that semantics are
+ * quite different depending on whether we have kcmp() or we
+ * don't. If we have kcmp() this will only return true for
+ * dup()ed file descriptors, but not otherwise. If we don't
+ * have kcmp() this will also return true for two fds of the same
+ * file, created by separate open() calls. Since we use this
+ * call mostly for filtering out duplicates in the fd store
+ * this difference hopefully doesn't matter too much. */
+
if (a == b)
return true;
+ /* Try to use kcmp() if we have it. */
+ pid = getpid();
+ r = kcmp(pid, pid, KCMP_FILE, a, b);
+ if (r == 0)
+ return true;
+ if (r > 0)
+ return false;
+ if (errno != ENOSYS)
+ return -errno;
+
+ /* We don't have kcmp(), use fstat() instead. */
if (fstat(a, &sta) < 0)
return -errno;
@@ -7694,9 +7716,27 @@ int same_fd(int a, int b) {
if ((sta.st_mode & S_IFMT) != (stb.st_mode & S_IFMT))
return false;
- if (S_ISREG(sta.st_mode) || S_ISDIR(sta.st_mode) || S_ISFIFO(sta.st_mode) || S_ISSOCK(sta.st_mode) || S_ISLNK(sta.st_mode))
- return (sta.st_dev == stb.st_dev) && (sta.st_ino == stb.st_ino);
+ /* We consider all device fds different, since two device fds
+ * might refer to quite different device contexts even though
+ * they share the same inode and backing dev_t. */
- /* We consider all device fds different... */
- return false;
+ if (S_ISCHR(sta.st_mode) || S_ISBLK(sta.st_mode))
+ return false;
+
+ if (sta.st_dev != stb.st_dev || sta.st_ino != stb.st_ino)
+ return false;
+
+ /* The fds refer to the same inode on disk, let's also check
+ * if they have the same fd flags. This is useful to
+ * distuingish the read and write side of a pipe created with
+ * pipe(). */
+ fa = fcntl(a, F_GETFL);
+ if (fa < 0)
+ return -errno;
+
+ fb = fcntl(b, F_GETFL);
+ if (fb < 0)
+ return -errno;
+
+ return fa == fb;
}