linux-kernel - Re: [PATCH v3 2/2] riscv: Allow PROT_WRITE-only mmap()

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Message-ID: <aba1be51-6951-6251-0802-9db573ea2aef@microchip.com>
Date:   Thu, 15 Sep 2022 17:40:23 +0000
From:   <Conor.Dooley@...rochip.com>
To:     <abrestic@...osinc.com>, <Conor.Dooley@...rochip.com>
CC:     <palmer@...belt.com>, <paul.walmsley@...ive.com>,
        <coelacanthus@...look.com>, <dramforever@...e.com>,
        <c141028@...il.com>, <linux-riscv@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <atishp@...osinc.com>
Subject: Re: [PATCH v3 2/2] riscv: Allow PROT_WRITE-only mmap()

On 15/09/2022 18:27, Andrew Bresticker wrote:
> On Thu, Sep 15, 2022 at 12:56 PM <Conor.Dooley@...rochip.com> wrote:
>>
>> On 09/09/2022 22:27, Andrew Bresticker wrote:
>>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
>>>
>>> Commit 2139619bcad7 ("riscv: mmap with PROT_WRITE but no PROT_READ is
>>> invalid") made mmap() return EINVAL if PROT_WRITE was set wihtout
>>> PROT_READ with the justification that a write-only PTE is considered a
>>> reserved PTE permission bit pattern in the privileged spec. This check
>>> is unnecessary since we let VM_WRITE imply VM_READ on RISC-V, and it is
>>> inconsistent with other architectures that don't support write-only PTEs,
>>> creating a potential software portability issue. Just remove the check
>>> altogether and let PROT_WRITE imply PROT_READ as is the case on other
>>> architectures.
>>>
>>> Note that this also allows PROT_WRITE|PROT_EXEC mappings which were
>>> disallowed prior to the aforementioned commit; PROT_READ is implied in
>>> such mappings as well.
>>>
>>> Fixes: 2139619bcad7 ("riscv: mmap with PROT_WRITE but no PROT_READ is invalid")
>>
>> For the naive members of the audience such as myself, this patch
>> came after a non-fixes patch in the series. What is the dependence
>> of this patch on the other one (if any)?
> 
> This patch is dependent on the first. Happy to re-spin with a "Fixes"
> tag on the first patch (or maybe Palmer can add when applying).

If it is a fix, then it should have a fixes tag. If it's cosmetic reorg
to make the fix easier then no & it should be moved after the fix. If
it is neither then you should prob mention it in the cover or under the
--- /shrug

Thanks,
Conor.

>>> Reviewed-by: Atish Patra <atishp@...osinc.com>
>>> Signed-off-by: Andrew Bresticker <abrestic@...osinc.com>
>>> ---
>>> v1 -> v2: Update access_error() to account for write-implies-read
>>> v2 -> v3: Separate into two commits
>>> ---
>>>  arch/riscv/kernel/sys_riscv.c | 3 ---
>>>  1 file changed, 3 deletions(-)
>>>
>>> diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
>>> index 571556bb9261..5d3f2fbeb33c 100644
>>> --- a/arch/riscv/kernel/sys_riscv.c
>>> +++ b/arch/riscv/kernel/sys_riscv.c
>>> @@ -18,9 +18,6 @@ static long riscv_sys_mmap(unsigned long addr, unsigned long len,
>>>         if (unlikely(offset & (~PAGE_MASK >> page_shift_offset)))
>>>                 return -EINVAL;
>>>
>>> -       if (unlikely((prot & PROT_WRITE) && !(prot & PROT_READ)))
>>> -               return -EINVAL;
>>> -
>>>         return ksys_mmap_pgoff(addr, len, prot, flags, fd,
>>>                                offset >> (PAGE_SHIFT - page_shift_offset));
>>>  }
>>> --
>>> 2.25.1
>>>
>>>
>>> _______________________________________________
>>> linux-riscv mailing list
>>> linux-riscv@...ts.infradead.org
>>> http://lists.infradead.org/mailman/listinfo/linux-riscv
>>

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