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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 29 Mar 2017 09:45:12 -0700
Message-ID: <CA+55aFy1Dayhmw0r8syRPBJo+10EOfPmeCTTTxzcLiQRrG69jg@mail.gmail.com>
Subject: Re: syscall_get_error() && TS_ checks
To: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Denys Vlasenko <dvlasenk@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Ingo Molnar <mingo@redhat.com>,
        Jan Kratochvil <jan.kratochvil@redhat.com>,
        Pedro Alves <palves@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, Mar 29, 2017 at 9:33 AM, Oleg Nesterov <oleg@redhat.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