linux-kernel - Re: [PATCH] riscv: Define TASK_SIZE_MAX for __access_ok()

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Date: Mon, 25 Mar 2024 08:30:37 +0100
From: Alexandre Ghiti <alex@...ti.fr>
To: David Laight <David.Laight@...LAB.COM>,
 Samuel Holland <samuel.holland@...ive.com>,
 Alexandre Ghiti <alexghiti@...osinc.com>
Cc: Palmer Dabbelt <palmer@...belt.com>,
 "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
 Albert Ou <aou@...s.berkeley.edu>, Andrew Morton
 <akpm@...ux-foundation.org>, Charlie Jenkins <charlie@...osinc.com>,
 Guo Ren <guoren@...nel.org>, Jisheng Zhang <jszhang@...nel.org>,
 Kemeng Shi <shikemeng@...weicloud.com>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 "Mike Rapoport (IBM)" <rppt@...nel.org>,
 Paul Walmsley <paul.walmsley@...ive.com>, Xiao Wang <xiao.w.wang@...el.com>,
 Yangyu Chen <cyy@...self.name>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] riscv: Define TASK_SIZE_MAX for __access_ok()

Hi David,

On 24/03/2024 20:42, David Laight wrote:
> ...
>> The use of alternatives allows to return right away if the buffer is
>> beyond the usable user address space, and it's not just "slightly
>> faster" for some cases (a very large buffer with only a few bytes being
>> beyond the limit or someone could fault-in all the user pages and fail
>> very late...etc). access_ok() is here to guarantee that such situations
>> don't happen, so actually it makes more sense to use an alternative to
>> avoid that.
> Is it really worth doing ANY optimisations for the -EFAULT path?
> They really don't happen.
>
> The only fault path that matters is the one that has to page in
> data from somewhere.

Which is completely avoided with a strict definition of access_ok(). I 
see access_ok() as an already existing optimization of fault paths by 
avoiding them entirely when they are bound to happen.

Thanks,

Alex

>
> Provided there is a gap between the highest valid user address and the
> lowest valid kernel address (may not be true on some 32bit systems)
> and copy_to/from_user() do 'increasing address' copies then the
> access_ok() check they do can almost certainly ignore the length.
>
> This may be true for pretty much all access_ok() tests?
> It would certainly simplify the test.
>
> 	David
>
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