linux-kernel - Re: syscall_get_error() && TS

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Message-ID: <20170329165554.GA24250@redhat.com>
Date:   Wed, 29 Mar 2017 18:55:54 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jan Kratochvil <jan.kratochvil@...hat.com>,
        Pedro Alves <palves@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: syscall_get_error() && TS_ checks

On 03/29, Linus Torvalds wrote:
>
> On Wed, Mar 29, 2017 at 9:33 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > Firstly, why do we need the IS_ERR_VALUE() check? This is only used by
> > do_signal/handle_signal, we do not care if it returns non-zero as long
> > as the value can't be confused with -ERESTART.* codes.
>
> There are system calls that can return "negative" values that aren't errors.
>
> Notably mmap() can return a valid pointer with the high bit set.
>
> So syscall_get_error() should return 0 for not just positive return
> values, but for those kinds of negative non-error values.

Once again, it is only used in arch/x86/kernel/signal.c by do_signal() and
handle_signal(). We do not care if mmap() returns a valid pointer with the
high bit set, regs-ax can't be confused with -ERESTART code.

> > And why do we need the TS_ checks?
>
> Those may be bogus.
>
> > So why we can't simply change putreg32() to always sign-extend regs->ax
> > regs->orig_ax and just do
> >
> >         static inline long syscall_get_error(struct task_struct *task,
> >                                              struct pt_regs *regs)
> >         {
> >                 return regs-ax;
> >         }
>
> That would be *complete* garbage. Lots of system calls return positive
> values that sure as hell aren't errors.

See above. And please note that I actually suggest to kill this helper and
just use syscall_get_return_value() in arch/x86/kernel/signal.c.

Oleg.