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Subject: Re: [PATCH] Syscall arguments are unsigned long (full registers)
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Mon, 04 Jul 2016 13:23:58 -0700
To: "Tautschnig, Michael" <tautschn@amazon.co.uk>
CC: "x86@kernel.org" <x86@kernel.org>,
        "linux-api@vger.kernel.org" <linux-api@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Jaswinder Singh <jaswinder@infradead.org>,
        Andi Kleen <ak@linux.intel.com>
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On July 4, 2016 1:13:21 PM PDT, "Tautschnig, Michael" <tautschn@amazon.co.uk> wrote:
>
>> On 4 Jul 2016, at 20:27, H. Peter Anvin <hpa@zytor.com> wrote:
>> 
>> On July 4, 2016 6:52:58 AM PDT, "Tautschnig, Michael"
><tautschn@amazon.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

That, and tracing.
-- 
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