linux-kernel - Re: [PATCH v14 1/4] asm-generic,arm64: create task variant of access

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Date:   Tue, 28 Mar 2023 23:56:01 -0400
From:   Gregory Price <gregory.price@...verge.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Gregory Price <gourry.memverge@...il.com>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Linux-Arch <linux-arch@...r.kernel.org>, avagin@...il.com,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>, krisman@...labora.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Jonathan Corbet <corbet@....net>, shuah <shuah@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>, tongtiangen@...wei.com,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH v14 1/4] asm-generic,arm64: create task variant of
 access_ok

On Wed, Mar 29, 2023 at 06:03:23PM +0200, Oleg Nesterov wrote:
> On 03/29, Arnd Bergmann wrote:
> >
> > On Wed, Mar 29, 2023, at 17:15, Oleg Nesterov wrote:
> > >
> > > This look as if access_ok() or __access_ok() doesn't depend on task, but
> > > this is not true in general. Say, TASK_SIZE_MAX can check is_32bit_task()
> > > test_thread_flag(TIF_32BIT...) and this uses "current".
> > >
> > > Again, we probably do not care, but I don't like the fact task_access_ok()
> > > looks as if task_access_ok(task) returns the same result as "task" calling
> > > access_ok().
> >
> > I think the idea of TASK_SIZE_MAX is that it is a compile-time constant and in fact independent of current, while TASK_SIZE
> > takes TIF_32BIT into account.
> 
> Say, arch/loongarch defines TASK_SIZE which depends on test_thread_flag(TIF_32BIT_ADDR)
> but it doesn't define TASK_SIZE_MAX, so __access_ok() will use TASK_SIZE.
> 
> Oleg.
> 

I did not notice this at first.  Thinking of solutions, I'd originally
considered writing a similar change in asm-generic that I made in arm64,
but that would have ultimately resulted in "(void) task;" because task
appears unused.

Now it seems like TASK_SIZE/_MAX seems like a dangerous define
combination that hides relevant functionality. Fixing this seeems to
naturally want a "TASK_TASK_SIZE(task)" which is... uh... annoying.

Not sure how I should proceed here, but this makes me wonder if there
are oversights like this elsewhere, as this seems like a pretty easy
thing to overlook.

~Gregory