[RFC] call_with_creds()

[Al Viro]

On Wed, Jul 18, 2018 at 05:46:09PM +0200, Miklos Szeredi wrote:

> I like the call_with_creds() idea.  I didn't realize that
> override_creds()/revert_creds() can be quite heavyweight due to doing
> (unnecessary in this case) refcounting.   Could use call_with_creds()
> in overlayfs too, since we hold ref on ofs->creator_cred for the
> lifetime of the filesystem.

OK...

/* expr is non-void here */
#define __call_with_creds(__cred, expr) ({			\
	const struct cred *____old = current->cred;		\
	const struct cred *____new = __cred;			\
	__typeof__(expr) __res;					\
	rcu_assign_pointer(current->cred, ____new);		\
	__res = (expr);						\
	BUG_ON(current->cred != ____new);			\
	rcu_assign_pointer(current->cred, ____old);		\
	__res; })

/* expr for non-void, expr,0 for void */
#define __wrap_void(expr) __builtin_choose_expr(		\
	__builtin_types_compatible_p(__typeof__(expr), void),	\
		((expr),0), (expr))

/*
 * Evaluate an expression with current->cred temporary set to __cred.
 * NOTE: expr must not result in changed ->cred - any changes during
 * its evaluation must be undone.
 */
#define call_with_creds(__cred, expr)	\
	((__typeof__(expr))__call_with_creds(__cred, __wrap_void(expr)))

Linus, David - do you have any objections to the above?  I would like
to do some of the file method calls via that - e.g. have callers of
->write() (both __vfs_write() and do_loop_readv_writev()) use
	nr = call_with_creds(file->f_cred,
			file->f_op->write(file, iovec.iov_base,
					  iovec.iov_len, ppos));
instead of
	nr = file->f_op->write(file, iovec.iov_base,
				  iovec.iov_len, ppos));
Ditto for call of open() in do_dentry_open(), etc.