linux-kernel - Re: [PATCH 4/9] ARM: syscall: always store thread

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Date:   Mon, 28 Sep 2020 11:41:24 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Christoph Hellwig <hch@....de>,
        Russell King <rmk@....linux.org.uk>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "linux-kernel@...r.kernel.org" <kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Russell King <linux@...linux.org.uk>,
        Oleg Nesterov <oleg@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/9] ARM: syscall: always store thread_info->syscall

Hi Arnd,

help me out here because I feel vaguely stupid...

On Mon, Sep 7, 2020 at 5:38 PM Arnd Bergmann <arnd@...db.de> wrote:

>  {
> +       if (IS_ENABLED(CONFIG_OABI_COMPAT))
> +               return task_thread_info(task)->syscall & ~__NR_OABI_SYSCALL_BASE;

Where __NR_OABI_SYSCALL_BASE is
#define __NR_OABI_SYSCALL_BASE       0x900000

So you will end up with sycall number & FF6FFFFF
masking off bits 20 and 23.

I suppose this is based on this:

>         bics    r10, r10, #0xff000000
> +       str     r10, [tsk, #TI_SYSCALL]

OK we mask off bits 24-31 before we store this.

>         bic     scno, scno, #0xff000000         @ mask off SWI op-code
> +       str     scno, [tsk, #TI_SYSCALL]

And here too.

>         eor     scno, scno, #__NR_SYSCALL_BASE  @ check OS number

And then happens that which will ... I don't know really.
Exclusive or with 0x9000000 is not immediately intuitive
evident to me, I suppose it is for everyone else... :/

I need some idea how this numberspace is managed in order to
understand the code so I can review it, I guess it all makes perfect
sense but I need some background here.

Thanks,
Linus Walleij