linux-kernel - Re: 64-syscall args on 32-bit vs syscall()

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Date:	Mon, 15 Mar 2010 15:04:01 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	linux-arch@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mark Lord <kernel@...savvy.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Munroe <munroesj@...ux.vnet.ibm.com>
Subject: Re: 64-syscall args on 32-bit vs syscall()

Benjamin Herrenschmidt wrote:
> err = syscall(SYS_fallocate, fd, mode, offset, len);
> 
> With "offset" being a 64-bit argument.
> 
> This will break because the first argument to syscall now shifts
> everything by one register, which breaks the register pair alignment
> (and I suppose archs with stack based calling convention can have
> similar alignment issues even if x86 doesn't).
> 
> Ulrich, Steven, shouldn't we have glibc's syscall() take a long long as
> it's first argument to correct that ? Either that or making it some kind
> of macro wrapper around a __syscall(int dummy, int sysno, ...) ?
> 
> As it is, any 32-bit app using syscall() on any of the syscalls that
> takes 64-bit arguments will be broken, unless the app itself breaks up
> the argument, but the the order of the hi and lo part is different
> between BE and LE architectures ;-)
> 
> So is there a more "correct" solution than another here ? Should powerpc
> glibc be fixed at least so that syscall() keeps the alignment ?

There are several problems with syscall(), not just this - because a
number of system calls in section 2 of the manual don't map directly
to kernel syscalls with the same function prototype.

Even fork() has become something complicated in Glibc that doesn't use
the fork syscall :-(

So anything using syscall() has to be careful on Linux already.
Changing the 64-bit alignment won't fix the other differences.

-- Jamie
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