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Date:	Sat, 17 Mar 2012 09:05:55 +0800
From:	Alek Du <alek.du@...el.com>
To:	Jiri Slaby <jslaby@...e.cz>
CC:	Jiri Slaby <jirislaby@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"Tu, Xiaobing" <xiaobing.tu@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"Zhang, Yanmin" <yanmin.zhang@...el.com>,
	"Zuo, Jiao" <jiao.zuo@...el.com>
Subject: Re: [PATCH] tty: hold lock across tty buffer finding and buffer filling



> On 03/16/2012 02:50 PM, Du, Alek wrote:
>> For #1, as I said, we already held the tty refcount. The tty refcount cannot help this crash.
> 
> Where exactly? Do you have some local changes?
> $ grep tty_port_tty_ drivers/tty/serial/mfd.c
> $

Yes, we do have local changes not upstream yet. But you should know now, TTY recount is not the key of the spin lock hole.


> 
>> For #2, this patch will avoid memcpy to a NULL pointer and avoid a kernel crash.
> 
> Ok, could you explain how? Linearized code from moxa.c with your patch
> applied follows:
> ofs = baseAddr + DynPage_addr + bufhead + head;
> size = (tail >= head) ? (tail - head) : (rx_mask + 1 - head);
> size = min(size, count);
> // call to tty_prepare_flip_string
>    spin_lock_irqsave(&tty->buf.lock, flags);
>    space = __tty_buffer_request_room(tty, size);
> 
>    tb = tty->buf.tail;
>    if (likely(space)) {
>        *chars = tb->char_buf_ptr + tb->used;
>        memset(tb->flag_buf_ptr + tb->used, TTY_NORMAL, space);
>        tb->used += space;
>    }
>    spin_unlock_irqrestore(&tty->buf.lock, flags);
> // return to MoxaPortReadData
> memcpy_fromio(*chars, ofs, space); <- memcpy without buf.lock

Ok. This is true. Seems we need another patch to remove this prepare interface. And let callers use the protected interface.

Thanks,
Alek


> head = (head + len) & rx_mask;
> ...
> 
> thanks,
> -- 
> js
> suse labs
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