linux-kernel - Re: [PATCH 2/2] arm64: Avoid memcpy() for syscall_get

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Message-ID: <aS1uOGm7cQf1euIw@J2N7QTR9R3>
Date: Mon, 1 Dec 2025 10:30:16 +0000
From: Mark Rutland <mark.rutland@....com>
To: david laight <david.laight@...box.com>
Cc: Jinjie Ruan <ruanjinjie@...wei.com>, linux@...linux.org.uk,
	catalin.marinas@....com, will@...nel.org, chris@...kel.net,
	jcmvbkbc@...il.com, akpm@...ux-foundation.org, macro@...am.me.uk,
	charlie@...osinc.com, deller@....de, ldv@...ace.io,
	rostedt@...dmis.org, tglx@...utronix.de,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] arm64: Avoid memcpy() for syscall_get_arguments()

On Mon, Dec 01, 2025 at 10:26:33AM +0000, david laight wrote:
> On Mon, 1 Dec 2025 10:13:54 +0000
> Mark Rutland <mark.rutland@....com> wrote:
> > On Thu, Nov 27, 2025 at 08:36:30PM +0800, Jinjie Ruan wrote:
> > > Before:
> > > <syscall_get_arguments.constprop.0>:
> > >        aa0103e2        mov     x2, x1
> > >        91002003        add     x3, x0, #0x8
> > >        f9408804        ldr     x4, [x0, #272]
> > >        f8008444        str     x4, [x2], #8
> > >        a9409404        ldp     x4, x5, [x0, #8]
> > >        a9009424        stp     x4, x5, [x1, #8]
> > >        a9418400        ldp     x0, x1, [x0, #24]
> > >        a9010440        stp     x0, x1, [x2, #16]
> > >        f9401060        ldr     x0, [x3, #32]
> > >        f9001040        str     x0, [x2, #32]
> > >        d65f03c0        ret
> > >        d503201f        nop
> > > 
> > > After:
> > >        a9408e82        ldp     x2, x3, [x20, #8]
> > >        2a1603e0        mov     w0, w22
> > >        f9400e84        ldr     x4, [x20, #24]
> > >        f9408a81        ldr     x1, [x20, #272]
> > >        9401c4ba        bl      ffff800080215ca8 <__audit_syscall_entry>  
> > 
> > It's probably worth noting that __audit_syscall_entry() only takes 4
> > syscall arguments, and hence the compiler has elided the copy of
> > regs->regs[4] and regs->regs[5], which it apparently couldn't manage
> > before.
> 
> Hasn't it actually inlined it and completely optimised away the regs[] array?
> It looks (from the asm) as though syscall_get_arguments() is followed by:
> 	fn(regs[0], regs[1], regs[2], regs[3])

Yes; I was assuming that people could infer that.

I was poining out that the elision of copies/loads of regs->regs[4] and
regs->regs[5] in particular was not a bug.

Mark.