linux-kernel - Re: [PATCH] USB: cdc-acm: Prevent data loss when filling tty buffer.

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Date:	Mon, 21 Mar 2011 16:56:12 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Toby Gray <toby.gray@...lvnc.com>
Cc:	Oliver Neukum <oliver@...kum.name>,
	Greg Kroah-Hartman <gregkh@...e.de>, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] USB: cdc-acm: Prevent data loss when filling tty
 buffer.

On Mon, 21 Mar 2011 15:52:25 +0000
Toby Gray <toby.gray@...lvnc.com> wrote:

> When sending large quantities of data through a CDC ACM channel it is possible
> for data to be lost when attempting to copy the data to the tty buffer. This
> occurs due to the return value from tty_insert_flip_string not being checked.

For a tty that is normally the right thing to do - no flow control was
asserted and the internal 64K of buffering was overrun so discard.

> This patch adds checking for how many bytes have been inserted into the tty
> buffer and returns any remaining bytes back to the filled read buffer list.

How does ACM handle flow control - is the expectation that it gets flow
controlled in hardware by not having the opportunity to send bits
to the host end ? If so this seems to make sense.


> +	copied = 0;

Surely copied = buf->size is the no tty assumption "we had the lot and
discarded it"

>  	if (tty) {
>  		spin_lock_irqsave(&acm->throttle_lock, flags);
>  		throttled = acm->throttle;
>  		spin_unlock_irqrestore(&acm->throttle_lock, flags);
>  		if (!throttled) {
> -			tty_insert_flip_string(tty, buf->base, buf->size);
> +			copied = tty_insert_flip_string(tty,
> +							buf->base, buf->size);
>  			tty_flip_buffer_push(tty);
>  		} else {
>  			tty_kref_put(tty);
> @@ -440,9 +443,26 @@ next_buffer:
>  		}
>  	}
>  
> -	spin_lock_irqsave(&acm->read_lock, flags);
> -	list_add(&buf->list, &acm->spare_read_bufs);
> -	spin_unlock_irqrestore(&acm->read_lock, flags);
> +	if (copied == buf->size || !tty) {

Which would simplify this lot

> +		spin_lock_irqsave(&acm->read_lock, flags);
> +		list_add(&buf->list, &acm->spare_read_bufs);
> +		spin_unlock_irqrestore(&acm->read_lock, flags);
> +	} else {
> +		tty_kref_put(tty);
> +		dbg("Partial buffer fill");
> +		if (copied > 0) {
> +			memmove(buf->base,
> +				buf->base + copied,
> +				buf->size - copied);
> +			buf->size -= copied;
> +		}

Would it not be cleaner to add a buf->head pointer that could simply be
advanced so the code would become

	buf->head += copied;
	buf->size -= copied;
	if (buf->size != 0)
		list_add(&buf->list, &acm->filled_read_bufs);

instead of all the memmove uglies ?


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