linux-kernel - Re: [PATCH v10 3/4] random: introduce generic vDSO getrandom() implementation

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Date:   Wed, 30 Nov 2022 18:00:07 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Arnd Bergmann <arnd@...db.de>
Cc:     Florian Weimer <fweimer@...hat.com>, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev, linux-crypto@...r.kernel.org,
        linux-api@...r.kernel.org, x86@...nel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>,
        Carlos O'Donell <carlos@...hat.com>,
        Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v10 3/4] random: introduce generic vDSO getrandom()
 implementation

On Wed, Nov 30 2022 at 16:47, Jason A. Donenfeld wrote:
> On Wed, Nov 30, 2022 at 4:29 PM Arnd Bergmann <arnd@...db.de> wrote:
>> I see what you mean now. However this means your vdso32 copies
>> are different between 32-bit and 64-bit kernels. If you need to
>> access one of the fields from assembler, it even ends up
>> different at source level, which adds a bit of complexity.
>>
>> Making the interface configuration-independent makes it obvious
>> to the reader that none of these problems can happen.
>
> Except ideally, these are word-sized accesses (where only compat code
> has to suffer I suppose).

While I hate it with a passion, there is actually a valid reason to use
this ugly typedef.

On 32bit architectures which have load/store tearing of 64bit variables
into two 32bit accesses due to ISA limitations, that results in
undefined behaviour when write and read are concurrent. Neither
READ_ONCE() nor WRITE_ONCE help there.

Though that begs the question whether we need a 64bit generation counter
for the VDSO at all.

Thanks,

        tglx