fs/binfmt: close symlink TOCTOU and harden setuid/setgid exec hygiene

Perform pseudo-filesystem permission checks inside inode_reserve() and
inode_remove() while the inode tree lock is held, and hold that lock across
pseudorename mutations so symlink swaps cannot bypass directory checks.
Hold a read lock around pseudo-fs open permission checks.

On setuid/setgid exec, update saved set-IDs, mark the task group secure,
sanitize dangerous environment variables, clear debug/dumpable flags, and
add issetugid(), secure_getenv(), and PR_SET/GET_DUMPABLE support.

Signed-off-by: Abhishek Mishra <mishra.abhishek2808@gmail.com>

include/nuttx/sched.h

@@ -118,7 +118,9 @@
 #define GROUP_FLAG_DELETED         (1 << 2)                      /* Bit 2: Group has been deleted but not yet freed */
 #define GROUP_FLAG_EXITING         (1 << 3)                      /* Bit 3: Group exit is in progress */
 #define GROUP_FLAG_FD_BACKTRACE    (1 << 4)                      /* Bit 4: Enable FD backtrace for the group */
-                                                                 /* Bits 5-7: Available */
+#define GROUP_FLAG_SECURE_EXEC     (1 << 5)                      /* Bit 5: Secure (setuid/setgid) executable */
+#define GROUP_FLAG_DUMPABLE        (1 << 6)                      /* Bit 6: Process may be traced / coredumped */
+                                                                 /* Bit 7: Available */
 
 /* Values for struct child_status_s ch_flags */
 

include/stdlib.h

@@ -173,6 +173,7 @@ uint32_t  arc4random(void);
 
 FAR char **get_environ_ptr(void);
 FAR char *getenv(FAR const char *name);
+FAR char *secure_getenv(FAR const char *name);
 int       putenv(FAR const char *string);
 int       clearenv(void);
 int       setenv(FAR const char *name, FAR const char *value, int overwrite);