mkdir_path(): tolerate mkdir(2) failures on directories that already exist.

OpenSSH 10.6 creates the agent socket directory (default user:.ssh/agent)
with mkdir_path(), which calls mkdirat(2) for every path component including
the home directory itself. On illumos, mkdir(2) of an entry in an autofs
indirect map directory such as /home returns ENOSYS ("Operation not
applicable") instead of EEXIST (auto_mkdir in uts/common/fs/autofs), so
sshd agent forwarding and ssh-agent fail for every user with an automounted
home directory:

  sshd-session[954451]: error: mkdir_path: mkdir("/home/klausz"): Operation not applicable
  sshd-session[954451]: error: agent_listener_user: failed to create agent socket parent directory

Linux and FreeBSD return EEXIST for an already mounted autofs entry, which
is why upstream did not notice. OpenSSH 10.5 only created ~/.ssh and
~/.ssh/agent and was not affected.

Remember the mkdir error, open the component as a directory anyway and only
fail when that does not work either. Also fixes sftp "lmkdir -p" and
ssh-agent's default socket directory on autofs homes.

To be submitted upstream (bugzilla.mindrot.org).

diff -wpruN --no-dereference '--exclude=*.orig' a~/misc.c a/misc.c
--- a~/misc.c	1970-01-01 00:00:00
+++ a/misc.c	1970-01-01 00:00:00
@@ -3270,7 +3270,7 @@ mkdir_path(const char *target, mode_t mode)
 {
 #if defined(HAVE_OPENAT) && defined(O_DIRECTORY)
 	char *dir, *odir = NULL, *next;
-	int fd = AT_FDCWD, fd2, subpath_len, ret = -1;
+	int fd = AT_FDCWD, fd2, subpath_len, mkdir_errno, ret = -1;
 
 	dir = odir = xstrdup(target);
 
@@ -3287,20 +3287,36 @@ mkdir_path(const char *target, mode_t mode)
 		if (*dir == '\0')
 			continue;
 		subpath_len = (next == NULL) ? INT_MAX : next - odir - 1;
+		mkdir_errno = 0;
 		if (mkdirat(fd, dir, mode) == 0)
 			debug_f("created directory %.*s", subpath_len, target);
 		else if (errno != EEXIST) {
-			error_f("mkdir(\"%.*s\"): %s",
-			    subpath_len, target, strerror(errno));
-			goto out;
+			/*
+			 * Some filesystems refuse mkdir(2) for directories
+			 * that already exist without returning EEXIST, e.g.
+			 * automounter (autofs) directories yield ENOSYS on
+			 * illumos/Solaris, EPERM on FreeBSD and EACCES on
+			 * Linux. Remember the error and only fail if the
+			 * directory can't be opened below.
+			 */
+			mkdir_errno = errno;
 		}
 
 		/* descend */
 		if ((fd2 = openat(fd, dir, O_RDONLY|O_DIRECTORY)) == -1) {
-			error_f("open(\"%.*s\"): %s",
-			    subpath_len, target, strerror(errno));
+			if (mkdir_errno != 0) {
+				error_f("mkdir(\"%.*s\"): %s", subpath_len,
+				    target, strerror(mkdir_errno));
+			} else {
+				error_f("open(\"%.*s\"): %s",
+				    subpath_len, target, strerror(errno));
+			}
 			goto out;
 		}
+		if (mkdir_errno != 0) {
+			debug2_f("mkdir(\"%.*s\"): %s, but directory exists",
+			    subpath_len, target, strerror(mkdir_errno));
+		}
 		if (fd != AT_FDCWD)
 			close(fd);
 		fd = fd2;
@@ -3318,7 +3334,8 @@ mkdir_path(const char *target, mode_t mode)
 	 * to the possibly racy way.
 	 */
 	char *dir, *odir, *p, path[PATH_MAX] = "";
-	int ret = -1;
+	struct stat st;
+	int mkdir_errno, ret = -1;
 	size_t plen = sizeof(path);
 
 	dir = odir = xstrdup(target);
@@ -3337,8 +3354,15 @@ mkdir_path(const char *target, mode_t mode)
 		if (mkdir(path, mode) == 0)
 			debug_f("created directory %s", path);
 		else if (errno != EEXIST && errno != EISDIR) {
-			error_f("mkdir(\"%s\"): %s", path, strerror(errno));
-			goto out;
+			/* See above: tolerate failures on existing dirs */
+			mkdir_errno = errno;
+			if (stat(path, &st) != 0 || !S_ISDIR(st.st_mode)) {
+				error_f("mkdir(\"%s\"): %s", path,
+				    strerror(mkdir_errno));
+				goto out;
+			}
+			debug2_f("mkdir(\"%s\"): %s, but directory exists",
+			    path, strerror(mkdir_errno));
 		}
 	}
 	/* success */