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

H. Peter Anvin
2014-05-09T20:54:05Z

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