open.2: Note use of O_PATH to provide O_EXEC functionality

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/open.2

@@ -683,6 +683,24 @@ descriptor referring to the automount directory without triggering a mount.
 can then be used to determine if it is, in fact, an untriggered
 automount point
 .RB ( ".f_type == AUTOFS_SUPER_MAGIC" ).
+.IP
+One use of
+.B O_PATH
+for regular files is to provide the equivalent of POSIX.1's
+.B O_EXEC
+functionality.
+This permits us to open a file for which we have execute
+permission but not read permission, and then executing that file,
+with steps something like the following:
+.IP
+.in +4n
+.EX
+char buf[PATH_MAX];
+fd = open("some_prog", O_PATH);
+snprintf(buf, "/proc/self/fd/%d", fd);
+execl(buf, "some_prog", (char *) NULL);
+.EE
+.in
 .TP
 .B O_SYNC
 Write operations on the file will complete according to the requirements of