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

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Message-Id: <16ec2a7a-c469-4732-aeca-e74a9fb88d3e@app.fastmail.com>
Date:   Wed, 30 Nov 2022 16:07:27 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Jason A . Donenfeld" <Jason@...c4.com>,
        "Florian Weimer" <fweimer@...hat.com>
Cc:     linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        "Thomas Gleixner" <tglx@...utronix.de>,
        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 15:59, Jason A. Donenfeld wrote:
> On Wed, Nov 30, 2022 at 03:51:01PM +0100, Jason A. Donenfeld wrote:
>> On Wed, Nov 30, 2022 at 11:44:30AM +0100, Florian Weimer wrote:
>> > 
>> >  * vdso_data will be accessed by 64 bit and compat code at the same time
>> >  * so we should be careful before modifying this structure.
>> > 
>> > So the ABI must be same for 32-bit and 64-bit mode, and long isn't.

> I'll do something like this:
>
> 
> +#ifdef CONFIG_64BIT
> +typedef u64 vdso_kernel_ulong;
> +#else
> +typedef u32 vdso_kernel_ulong;
> +#endif

This does not address the ABI concern: to allow 32-bit and 64-bit
tasks to share the same data page, it has to be the same width on
both, either u32 or 64, but not depending on a configuration
option.

> struct vdso_rng_data {
>	vdso_kernel_ulong	generation;
>	bool			is_ready;
> };

There is another problem with this: you have implicit padding
in the structure because the two members have different size
and alignment requirements. The easiest fix is to make them
both u64, or you could have a u32 is_ready and an explit u32
for the padding.

      Arnd