Access to data of struct msghdr in C

[VLeshka]

Hi!
Linux RT (am3359).
C (not C++).
I can't access the data of struct msghdr msg.

I have received the message from socket into struct msghdr msg:

struct msghdr msg;
int err
err = sock_recvmsg(sock, &msg, flags);

Then (in the same function) I finely read the length of data:
msg->msg_iter.iov->iov_len.

But I can't read the data of msg:
msg->msg_iter.iov->iov_base.

The compiler compiles finely, but errors pop up.

Why?

P.S. structures:

struct msghdr {
	void		*msg_name;	/* ptr to socket address structure */
	int		msg_namelen;	/* size of socket address structure */
	struct iov_iter	msg_iter;	/* data */
	void		*msg_control;	/* ancillary data */
	__kernel_size_t	msg_controllen;	/* ancillary data buffer length */
	unsigned int	msg_flags;	/* flags on received message */
	struct kiocb	*msg_iocb;	/* ptr to iocb for async requests */
};

As you can see, the data must be pulled from the struct iov_iter msg_iter.

struct iov_iter {
	int type;
	size_t iov_offset;
	size_t count;
	union {
		const struct iovec *iov;
		const struct kvec *kvec;
		const struct bio_vec *bvec;
		struct pipe_inode_info *pipe;
	};
	union {
		unsigned long nr_segs;
		struct {
			int idx;
			int start_idx;
		};
	};
};

struct iovec
{
	void __user *iov_base;	/* BSD uses caddr_t (1003.1g requires void *) */
	__kernel_size_t iov_len; /* Must be size_t (1003.1g) */
};

"void __user *iov_base" - pointer to the array of 8-bit data.

P.S.2 I tried:

(unsigned char*)msg->msg_iter.iov->iov_base
(unsigned char*)((caddr_t)(*(msg->msg_iter.iov)).iov_base)

[kbw]

It seems you're reading junk, possibly by not reading everything you need.

You need to verify that the read is actually happening for all your data. Try putting trace or debug sock_recvmsg to see what's going on.

[VLeshka]

The result of sock_recvmsg is checking.
The code (as is, and where I try to view the data of struct msghdr):

/*
 *	Receive a frame from the socket and optionally record the address of the
 *	sender. We verify the buffers are writable and if needed move the
 *	sender address from kernel to user space.
 */

SYSCALL_DEFINE6(recvfrom, int, fd, void __user *, ubuf, size_t, size,
		unsigned int, flags, struct sockaddr __user *, addr,
		int __user *, addr_len)
{
	struct socket *sock;
	struct iovec iov;
	struct msghdr msg;
	struct sockaddr_storage address;
	int err, err2;
	int fput_needed;

	err = import_single_range(READ, ubuf, size, &iov, &msg.msg_iter);
	if (unlikely(err))
		return err;
	sock = sockfd_lookup_light(fd, &err, &fput_needed);
	if (!sock)
		goto out;

	msg.msg_control = NULL;
	msg.msg_controllen = 0;
	/* Save some cycles and don't copy the address if not needed */
	msg.msg_name = addr ? (struct sockaddr *)&address : NULL;
	/* We assume all kernel code knows the size of sockaddr_storage */
	msg.msg_namelen = 0;
	msg.msg_iocb = NULL;
	msg.msg_flags = 0;
	if (sock->file->f_flags & O_NONBLOCK)
		flags |= MSG_DONTWAIT;
	err = sock_recvmsg(sock, &msg, flags);

	if (err >= 0 && addr != NULL) {
		err2 = move_addr_to_user(&address,
					 msg.msg_namelen, addr, addr_len);

//and here I try to view the data of struct msghdr by printk

		if (err2 < 0)
			err = err2;
	}

	fput_light(sock->file, fput_needed);
out:
	return err;
} 

[VLeshka]

About sock_recvmsg:

\board-support\linux-rt-4.14.67+gitAUTOINC+cfb76a2032-gcfb76a2032\net\socket.c:
int sock_recvmsg(struct socket *sock, struct msghdr *msg, int flags)
{
	int err = security_socket_recvmsg(sock, msg, msg_data_left(msg), flags);

	return err ?: sock_recvmsg_nosec(sock, msg, flags);
}
EXPORT_SYMBOL(sock_recvmsg);

\board-support\linux-rt-4.14.67+gitAUTOINC+cfb76a2032-gcfb76a2032\security\security.c:
int security_socket_recvmsg(struct socket *sock, struct msghdr *msg,
			    int size, int flags)
{
	return call_int_hook(socket_recvmsg, 0, sock, msg, size, flags);
}

-//-:
#define call_int_hook(FUNC, IRC, ...) ({			\
	int RC = IRC;						\
	do {							\
		struct security_hook_list *P;			\
								\
		list_for_each_entry(P, &security_hook_heads.FUNC, list) { \
			RC = P->hook.FUNC(__VA_ARGS__);		\
			if (RC != 0)				\
				break;				\
		}						\
	} while (0);						\
	RC;							\
})