linux-kernel - Re: [PATCH v3 0/3] preadv & pwritev syscalls.

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Date:	Mon, 15 Dec 2008 21:57:24 +0100
From:	Gerd Hoffmann <kraxel@...hat.com>
To:	Ralf Baechle <ralf@...ux-mips.org>
CC:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCH v3 0/3] preadv & pwritev syscalls.

Ralf Baechle wrote:
> On Mon, Dec 15, 2008 at 12:36:14PM +0100, Gerd Hoffmann wrote:
> 
>> Next revision of the patch series, with the alignment issue fixed by
>> swapping the last two arguments as suggested by arch maintainers.
>>
>> I've dropped the now-obsolete wrappers for mips.  Ralf, please
>> double-check.
> 
> It fixes the alignment issue but still won't work; on MIPS 32-bit userspace
> will pass the 64-bit argument in two registers but the 64-bit kernel code
> will assume it to be passed in a single registers.  It'd be ugly but passing
> a pointer to a 64-bit argument would solve the issue; something like this:
>
> sys_preadv(unsigned long fd, const struct iovec __user *vec,
>                   unsigned long vlen, loff_t __user *pos);
> compat_sys_preadv(unsigned long fd, const struct compat_iovec __user *vec,
>                   unsigned long vlen, loff_t __user *pos);

Suggestion from the s390 front was to explicitly pass high and low part
of pos as two arguments.  A bit ugly too, but should work fine as well
and it avoids the user pointer dereference.  What do you think about this?

> sys_splice uses loff_t __user * arguments as well and that's why it's
> doesn't need any compat wrapper.

Well, avoiding the compat wrapper altogether unfortunately isn't going
to work because struct iovec looks different in 32 and 64 bit.

> I'm surprised this works for x86; does x86-64 code really expect 64-bit
> arguments as 2 32-bit arguments?

Args are passed on the stack, not in registers.

> Patch 1/3 looks like it's a bug fix and you may want to submit this
> separate from the remainder of the series for 2.6.28?

Yep, it is a separate bugfix.  Spotted by Christoph while reviewing v1,
but there are no other dependencies to this patch series.

cheers,
  Gerd

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