linux-kernel - Re: [PATCH] drivers/char/mem.c: Add /dev/ioports, supporting 16-bit and 32-bit ports

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Date:	Fri, 09 May 2014 13:54:05 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Arnd Bergmann <arnd@...db.de>,
	Josh Triplett <josh@...htriplett.org>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCH] drivers/char/mem.c: Add /dev/ioports, supporting 16-bit
 and 32-bit ports

On 05/09/2014 12:58 PM, Arnd Bergmann wrote:
> On Friday 09 May 2014 12:19:16 Josh Triplett wrote:
> 
>> +	if (!access_ok(VERIFY_WRITE, buf, count))
>> +		return -EFAULT;
>> +	if (port > 65535)
>> +		return 0;
> 
> This should probably test against IO_SPACE_LIMIT, which may be zero,
> something larger than 65536 or even ULONG_MAX, depending on the
> architecture.
> 
> In cases where this IO_SPACE_LIMIT is zero or ULONG_MAX, we should
> probably disallow access completely. The former case is for architectures
> that don't have any I/O ports, the other is either a mistake, or is
> used when inb is defined as readb, and the port numbers are just virtual
> addresses.
> 

PCI supports a 32-bit I/O address space, so if the architecture permits
it, having a 32-bit I/O space is perfectly legitimate.

It is worth noting that /dev/port has the same problem.

However, if we're going to have these devices I'm wondering if having
/dev/portw and /dev/portl (or something like that) might not make sense,
rather than requiring a system call per transaction.

Also, x86 supports unaligned I/O port references, but not all
architectures do.  On the other hand, x86 also supports ioperm().

	-hpa



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