linux-kernel - Re: [PATCH] riscv: Allow PROT

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Date:   Fri, 9 Sep 2022 01:21:18 +0800
From:   SS JieJi <c141028@...il.com>
To:     Andrew Bresticker <abrestic@...osinc.com>
Cc:     Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Atish Patra <atishp@...shpatra.org>,
        Celeste Liu <coelacanthus@...look.com>,
        dram <dramforever@...e.com>, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: Allow PROT_WRITE-only mmap()

> is unnecessary since RISC-V defines its protection_map such that PROT_WRITE
> maps to the same PTE permissions as PROT_WRITE|PROT_READ, and it is
> inconsistent with other architectures that don't support write-only PTEs,
> creating a potential software portability issue.

I don't believe that the check is unnecessary. The missing check is
discovered in realworld scenario, while we are fixing libaio's test
failure on RISC-V [1]. A minimum reproducible example is uploaded to
https://fars.ee/1sPb, showing *inconsistent* read results on -r- pages
before/after a write attempt performed by the kernel.

[1]: https://pagure.io/libaio/blob/1b18bfafc6a2f7b9fa2c6be77a95afed8b7be448/f/harness/cases/5.t

> -       if (unlikely((prot & PROT_WRITE) && !(prot & PROT_READ)))
> -               return -EINVAL;
> -

Just to mention, this revert patch is removing the check of exec
without read (--x), too.