linux-kernel - Re: [PATCH 3/9] x86/entry/32: fix the end of the stack for newly forked tasks

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Date:   Tue, 20 Sep 2016 23:39:35 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Nilay Vaish <nilayvaish@...il.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 3/9] x86/entry/32: fix the end of the stack for newly
 forked tasks

On Sep 20, 2016 5:25 PM, "Josh Poimboeuf" <jpoimboe@...hat.com> wrote:
>
> On Tue, Sep 20, 2016 at 09:10:55PM -0400, Brian Gerst wrote:
> > Dropping asmlinkage from schedule_tail() would be a better option if possible.
>
> My understanding is that it's still needed for ia64.  AFAICT, ia64
> relies on schedule_tail() having the syscall_linkage function attribute.
> From the gcc manual:
>
>   This attribute is used to modify the IA64 calling convention by
>   marking all input registers as live at all function exits. This makes
>   it possible to restart a system call after an interrupt without having
>   to save/restore the input registers. This also prevents kernel data
>   from leaking into application code.

/me needs to excise this from i386.  The amount of BS code involved to
avoid a whopping *six* register saves per syscall was absurd.

>
> And the ia64 entry code has some similar language:
>
>                 /*
>                  * Invoke schedule_tail(task) while preserving in0-in7, which may be needed
>                  * in case a system call gets restarted.
>                  */
>         GLOBAL_ENTRY(ia64_invoke_schedule_tail)
>                 ...

That comment has to be wrong.  What syscall could possibly be
restarted across schedule_tail()?  It's a brand new thread and has
literally never done a syscall.

There may be another reason that the registers are live there, but I
generally do my best to never look at ia64 asm code.