Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Huacai Chen <chenhuacai@gmail.com>
Cc:     Davidlohr Bueso <dave@stgolabs.net>,
        Huacai Chen <chenhuacai@loongson.cn>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Darren Hart <dvhart@infradead.org>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        "open list:MIPS" <linux-mips@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Xuefeng Li <lixuefeng@loongson.cn>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        Hongchen Zhang <zhanghongchen@loongson.cn>
Subject: Re: [PATCH] futex: Fix fault_in_user_writeable()
In-Reply-To: <CAAhV-H6=MbBFwgxT75Bqr5SY0e5E6gtpCUCZhrVgFE_h1A0q2A@mail.gmail.com>
References: <20210816065417.3987596-1-chenhuacai@loongson.cn>
 <20210816182750.26i535ilc6nef5k6@offworld> <874kbpp5zd.ffs@tglx>
 <CAAhV-H5TKLMi0GSQmh9RFK_k5eNRwx8AE8MjMKjJfbnyVYP-+w@mail.gmail.com>
 <871r6spn0r.ffs@tglx>
 <CAAhV-H6=MbBFwgxT75Bqr5SY0e5E6gtpCUCZhrVgFE_h1A0q2A@mail.gmail.com>
Date:   Tue, 17 Aug 2021 11:05:15 +0200
Message-ID: <87y290o304.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Huacai,

On Tue, Aug 17 2021 at 15:38, Huacai Chen wrote:
> On Tue, Aug 17, 2021 at 3:07 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> On X86, it returns 0; on MIPS64 without patch, it hangs in kernel; on
> MIPS64 with this patch, it returns -1.

As expected.

> Then, I want to know, on "W implies R" archs (such as X86), should it
> return 0? Maybe return -1 is more reasonable? (because the VMA is
> marked as write-only). If this program should return -1, then I don't
> think this is a MIPS-specific problem.

No. mmap(.., PROT_WRITE...) is simply impossible on x86 and implies
PROT_READ as documented in mmap(2).

So why should this fail and only fail in the fault case, but succeed
when the PTE is already established?

Thanks,

        tglx