linux-kernel - Re: [PATCH 01/23] all: syscall wrappers: add documentation

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Message-ID: <57470D19.2000501@arm.com>
Date:	Thu, 26 May 2016 15:50:01 +0100
From:	Szabolcs Nagy <szabolcs.nagy@....com>
To:	Catalin Marinas <catalin.marinas@....com>,
	David Miller <davem@...emloft.net>
CC:	<nd@....com>, <arnd@...db.de>, <ynorov@...iumnetworks.com>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, <linux-doc@...r.kernel.org>,
	<linux-arch@...r.kernel.org>, <linux-s390@...r.kernel.org>,
	<libc-alpha@...rceware.org>, <schwidefsky@...ibm.com>,
	<heiko.carstens@...ibm.com>, <pinskia@...il.com>,
	<broonie@...nel.org>, <joseph@...esourcery.com>,
	<christoph.muellner@...obroma-systems.com>,
	<bamvor.zhangjian@...wei.com>, <klimov.linux@...il.com>,
	<Nathan_Lynch@...tor.com>, <agraf@...e.de>,
	<Prasun.Kapoor@...iumnetworks.com>, <kilobyte@...band.pl>,
	<geert@...ux-m68k.org>, <philipp.tomsich@...obroma-systems.com>
Subject: Re: [PATCH 01/23] all: syscall wrappers: add documentation

On 26/05/16 15:20, Catalin Marinas wrote:
> While writing the above, I realised the current ILP32 patches still miss
> on converting pointers passed from user space (unless I got myself
> confused in macros). The new __SC_WRAP() and COMPAT_SYSCALL_WRAPx()
> macros take care of zero or sign extension via __SC_COMPAT_CAST().
> However, we have two more existing cases which I don't see covered:
> 
> a) Native syscalls taking a pointer argument and invoked directly from
>    ILP32. For example, sys_read() takes a pointer but I don't see any
>    __SC_WRAP added by patch 5
> 
> b) Current compat syscalls taking a pointer argument. For example,
>    compat_sys_vmsplice() gets the iov32 pointer and the compiler assumes
>    it is a 64-bit variable. I don't see where the upper half is zeroed
> 

on x32 sign/zero extension is currently left to userspace,
which is difficult to deal with, (long long)arg does the
wrong thing for pointer args.

> We can solve (a) by adding more __SC_WRAP annotations in the generic
> unistd.h. For (b), we would need an __SC_DELOUSE with a bit of penalty
> on AArch32/compat support where it isn't needed. So maybe davem has a
> point on the overall impact of always zeroing the upper half of the
> arguments ;) (both from a performance and maintainability perspective).
> I guess this part of the ABI is still up for discussion.
>