exec: do not call request_module() twice from search_binary_handler()

Currently, search_binary_handler() tries to load binary loader module
using request_module() if a loader for the requested program is not yet
loaded.  But second attempt of request_module() does not affect the result
of search_binary_handler().

If request_module() triggered recursion, calling request_module() twice
causes 2 to the power of MAX_KMOD_CONCURRENT (= 50) repetitions.  It is
not an infinite loop but is sufficient for users to consider as a hang up.

Therefore, this patch changes not to call request_module() twice, making 1
to the power of MAX_KMOD_CONCURRENT repetitions in case of recursion.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: Richard Weinberger <richard@nod.at>
Tested-by: Richard Weinberger <richard@nod.at>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -1440,6 +1440,8 @@ int search_binary_handler(struct linux_binprm *bprm,struct pt_regs *regs)
 			    printable(bprm->buf[2]) &&
 			    printable(bprm->buf[3]))
 				break; /* -ENOEXEC */
+			if (try)
+				break; /* -ENOEXEC */
 			request_module("binfmt-%04x", *(unsigned short *)(&bprm->buf[2]));
 #endif
 		}