linux-kernel - Re: [PATCH] Syscall arguments are unsigned long (full registers)

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Date:	Mon, 4 Jul 2016 20:13:21 +0000
From:	"Tautschnig, Michael" <tautschn@...zon.co.uk>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	"x86@...nel.org" <x86@...nel.org>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Ingo Molnar" <mingo@...hat.com>,
	Jaswinder Singh <jaswinder@...radead.org>,
	"Andi Kleen" <ak@...ux.intel.com>
Subject: Re: [PATCH] Syscall arguments are unsigned long (full registers)


> On 4 Jul 2016, at 20:27, H. Peter Anvin <hpa@...or.com> wrote:
> 
> On July 4, 2016 6:52:58 AM PDT, "Tautschnig, Michael" <tautschn@...zon.co.uk> wrote:
>> All syscall arguments are passed in as types of the same byte size as
>> unsigned long (width of full registers). Using a smaller type without a
>> cast may result in losing bits of information. In all other instances
>> apart from the ones fixed by the patch the code explicitly introduces
>> type casts (using, e.g., SYSCALL_DEFINE1).
>> 
>> While goto-cc reported these problems at build time, it is noteworthy
>> that the calling conventions specified in the System V AMD64 ABI do
>> ensure that parameters 1-6 are passed via registers, thus there is no
>> implied risk of misaligned stack access.
>> 
>> 
[...]
> 
> Wrong.  Syscall arguments aren't necessarily full registers, and on x86 truncation is already done by the callee, so we don't need any special handing.  Some other architectures have other constraints. 

Ok - I'm assuming I have thus misunderstood eb974c62565072e10c1422eb3205f5b611dd99a1 ? Supposedly all those SYSCALL_DEFINEx are required for other architectures only?

Best,
Michael