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From:   Andy Lutomirski <luto@kernel.org>
Date:   Fri, 30 Nov 2018 08:42:26 -0800
Message-ID: <CALCETrW1ju40hFrM3sr21tDTZ-m1=swHfD2cMiyYFJkgbb3cug@mail.gmail.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call
 implementation for x86-64
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Andrew Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>, julia@ni.com, jeyu@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>
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On Thu, Nov 29, 2018 at 12:24 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> > Alternatively, we could actually emulate call instructions like this:
> >
> > void __noreturn jump_to_kernel_pt_regs(struct pt_regs *regs, ...)
> > {
> >   struct pt_regs ptregs_copy = *regs;
> >   barrier();
> >   *(unsigned long *)(regs->sp - 8) = whatever;  /* may clobber old
> > regs, but so what? */
> >   asm volatile ("jmp return_to_alternate_ptregs");
> > }
> >
> > where return_to_alternate_ptregs points rsp to the ptregs and goes
> > through the normal return path.  It's ugly, but we could have a test
> > case for it, and it should work fine.
>
> Is that really any better than my patch to create a gap in the stack
> (modified for kernel space #BP only)?
>

I tend to prefer a nice local hack like mine over a hack that further
complicates the entry in general.  This is not to say I'm thrilled by
my idea either.