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

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Message-ID: <CA+55aFy1Dayhmw0r8syRPBJo+10EOfPmeCTTTxzcLiQRrG69jg@mail.gmail.com>
Date:   Wed, 29 Mar 2017 09:45:12 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Oleg Nesterov <oleg@...hat.com>
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 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.

> 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.

                    Linus