linux-kernel - RE: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast path with retpolines on

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Message-ID: <b8ea504729db4073abf82ebd5d8a53c6@AcuMS.aculab.com>
Date:   Fri, 26 Jan 2018 11:17:29 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Andy Lutomirski' <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <alan@...ux.intel.com>, Jann Horn <jannh@...gle.com>,
        Samuel Neves <samuel.c.p.neves@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Borislav Petkov <bp@...en8.de>
Subject: RE: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast
 path with retpolines on

From: Andy Lutomirski
> Sent: 25 January 2018 21:31
...
> Another potentially useful if rather minor optimization would be to
> rejigger the SYSCALL_DEFINE macros a bit.  Currently we treat all
> syscalls like this:
> 
> long func(long arg0, long arg1, long arg2, long arg3, long arg4, long arg5);
> 
> I wonder if we'd be better off doing:
> 
> long func(const struct pt_regs *regs);
> 
> and autogenerating:
> 
> static long SyS_read(const struct pt_regs *regs)
> {
>    return sys_reg(regs->di, ...);
> }

Hmmm....
NetBSD (and the other BSD?) defines a structure for the arguments to each syscall.
On systems where all function arguments are put on stack the user stack that
contains the arguments is copied into a kernel buffer.
For amd64 I changed the register save area layout so that the arguments were in
the right order [1]. Then added an extra area for the extra arguments that had to be
read from the user stack.
Just passing a pointer into the save area has to be better than reading
all the values again.

[1] There was some horrid fallout from that :-(

	David