linux-kernel - Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq

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Date:   Mon, 2 Jul 2018 15:45:01 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King - ARM Linux <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>, Peter Anvin <hpa@...or.com>,
        Andi Kleen <andi@...stfloor.org>,
        Christoph Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields, validate
 user inputs

On Mon, Jul 2, 2018 at 3:31 PM Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
> Change the rseq ABI so rseq_cs start_ip, post_commit_offset and abort_ip
> fields are seen as 64-bit fields by both 32-bit and 64-bit kernels rather
> that ignoring the 32 upper bits on 32-bit kernels. This ensures we have a
> consistent behavior for a 32-bit binary executed on 32-bit kernels and in
> compat mode on 64-bit kernels.

Actually, now that I see this again, I react to:


> +static int check_rseq_cs_padding(struct task_struct *t)
> +{
> +       u32 pad;
> +       int ret;
> +
> +       ret = __get_user(pad, &t->rseq->rseq_cs_padding);
> +       if (ret)
> +               return ret;
> +       if (pad)
> +               return -EINVAL;
> +       return 0;
> +}

This is all wrong.

Just make "rseq_cs" be an __u64" too. That will clean up everything,
and user space will have a much easier time filling it in too, since
it's just one field. Instead of having to remember about the "let's
fill in padding for 32-bit cases".

Then the rseq_get_rseq_cs() will be

        __u64 rseq_cs;

        ret = get_user(rseq_cs, &t->rseq->rseq_cs);
        if (ret)
                return ret;
        ptr = (void *)rseq_cs;
        if (rseq_cs != (unsigned long)ptr)
                return -EINVAL;

and it's all good, no #ifdef's etc needed.

Hmm?

Sorry for the bike-shedding, but this is now the last remaining user
of that LINUX_FIELD_u32_u64, so let's just get rid of it entirely, ok?

Then we can also get rid of that silly uapi/linux/types_32_64.h header
file entirely.

That would be *lovely*. Simpler code, simpler and less error-prone
interfaces, and one less specialized header file.

              Linus