fdpic: ignore the loader's PT_GNU_STACK when calculating the stack size

Ignore the loader's PT_GNU_STACK when calculating the stack size, and only
consider the executable's PT_GNU_STACK, assuming the executable has one.

Currently the behaviour is to take the largest stack size and use that,
but that means you can't reduce the stack size in the executable.  The
loader's stack size should probably only be used when executing the loader
directly.

WARNING: This patch is slightly dangerous - it may render a system
inoperable if the loader's stack size is larger than that of important
executables, and the system relies unknowingly on this increasing the size
of the stack.
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Paul Mundt <lethal@linux-sh.org>
Cc: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/binfmt_elf_fdpic.c

@@ -283,19 +283,22 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm,
 	}
 
 	stack_size = exec_params.stack_size;
-	if (stack_size < interp_params.stack_size)
-		stack_size = interp_params.stack_size;
-
 	if (exec_params.flags & ELF_FDPIC_FLAG_EXEC_STACK)
 		executable_stack = EXSTACK_ENABLE_X;
 	else if (exec_params.flags & ELF_FDPIC_FLAG_NOEXEC_STACK)
 		executable_stack = EXSTACK_DISABLE_X;
-	else if (interp_params.flags & ELF_FDPIC_FLAG_EXEC_STACK)
+	else
+		executable_stack = EXSTACK_DEFAULT;
+
+	if (stack_size == 0) {
+		stack_size = interp_params.stack_size;
+		if (interp_params.flags & ELF_FDPIC_FLAG_EXEC_STACK)
 			executable_stack = EXSTACK_ENABLE_X;
 		else if (interp_params.flags & ELF_FDPIC_FLAG_NOEXEC_STACK)
 			executable_stack = EXSTACK_DISABLE_X;
 		else
 			executable_stack = EXSTACK_DEFAULT;
+	}
 
 	retval = -ENOEXEC;
 	if (stack_size == 0)