iov_iter: Mark copy_compat_iovec_from_user() noinline

After commit 6376ce56feb6 ("iov_iter: import single vector iovecs as
ITER_UBUF"), GCC does an inter-procedural compiler optimization which
moves the user_access_begin() out of copy_compat_iovec_from_user() and
into its callers:

  lib/iov_iter.o: warning: objtool: .altinstr_replacement+0x0: redundant UACCESS disable
  lib/iov_iter.o: warning: objtool: iovec_from_user.part.0+0xc7: call to copy_compat_iovec_from_user.part.0() with UACCESS enabled
  lib/iov_iter.o: warning: objtool: __import_iovec+0x21d: call to copy_compat_iovec_from_user.part.0() with UACCESS enabled

Enforce the "no UACCESS enable across function boundaries" rule by
disabling cloning for copy_compat_iovec_from_user().

Fixes: 6376ce56feb6 ("iov_iter: import single vector iovecs as ITER_UBUF")
Reported-by: Stephen Rothwell <[email protected]>
https://lkml.kernel.org/lkml/[email protected]
Signed-off-by: Josh Poimboeuf <[email protected]>
Tested-by: Jens Axboe <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

Author: jpoimboe

lib/iov_iter.c

@@ -1702,7 +1702,7 @@ const void *dup_iter(struct iov_iter *new, struct iov_iter *old, gfp_t flags)
 }
 EXPORT_SYMBOL(dup_iter);
 
-static int copy_compat_iovec_from_user(struct iovec *iov,
+static __noclone int copy_compat_iovec_from_user(struct iovec *iov,
 		const struct iovec __user *uvec, unsigned long nr_segs)
 {
 	const struct compat_iovec __user *uiov =