linux-kernel - Re: [PATCH RFC v3 1/2] mm: Add personality flag to limit address to 47 bits

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Message-Id: <8130e50c-01e2-45c3-a516-45f5499311f2@app.fastmail.com>
Date: Wed, 11 Sep 2024 07:25:08 +0000
From: "Arnd Bergmann" <arnd@...db.de>
To: "Charlie Jenkins" <charlie@...osinc.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 "Catalin Marinas" <catalin.marinas@....com>, guoren <guoren@...nel.org>,
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Subject: Re: [PATCH RFC v3 1/2] mm: Add personality flag to limit address to 47 bits

On Wed, Sep 11, 2024, at 00:45, Charlie Jenkins wrote:
> On Tue, Sep 10, 2024 at 03:08:14PM -0400, Liam R. Howlett wrote:
>
> I responded to Arnd in the other thread, but I am still not convinced
> that the solution that x86 and arm64 have selected is the best solution.
> The solution of defaulting to 47 bits does allow applications the
> ability to get addresses that are below 47 bits. However, due to
> differences across architectures it doesn't seem possible to have all
> architectures default to the same value. Additionally, this flag will be
> able to help users avoid potential bugs where a hint address is passed
> that causes upper bits of a VA to be used.
>
> The other issue I have with this is that if there is not a hint address
> specified to be greater than 47 bits on x86, then mmap() may return an
> address that is greater than 47-bits. The documentation in
> Documentation/arch/x86/x86_64/5level-paging.rst says:
>
> "If hint address set above 47-bit, but MAP_FIXED is not specified, we try
> to look for unmapped area by specified address. If it's already
> occupied, we look for unmapped area in *full* address space, rather than
> from 47-bit window."

This is also in the commit message of b569bab78d8d ("x86/mm: Prepare
to expose larger address space to userspace"), which introduced it.
However, I don't actually see the fallback to the full address space,
instead the actual behavior seems to be the same as arm64.

Am I missing something in the x86 implementation, or do we just
need to update the documentation?

      Arnd