linux-kernel - Re: [PATCH] riscv: Define TASK_SIZE_MAX for __access

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Date: Mon, 25 Mar 2024 16:39:12 +0000
From: Mark Rutland <mark.rutland@....com>
To: Alexandre Ghiti <alex@...ti.fr>
Cc: David Laight <David.Laight@...lab.com>,
	Samuel Holland <samuel.holland@...ive.com>,
	Alexandre Ghiti <alexghiti@...osinc.com>,
	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()

On Mon, Mar 25, 2024 at 08:30:37AM +0100, Alexandre Ghiti wrote:
> 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.

I think the point that David is making is that address+size pairs that'd fail
access_ok() *should* be rare, and hence it's a better trade-off to occasionally
handle faults for those if it makes the common case of successful access_ok()
smaller or faster. For any well-behaved userspace applications, access_ok()
should practically never fail, since userspace should be passing good
address+size pairs as arguments to syscalls.

Using a compile-time constant TASK_SIZE_MAX allows the compiler to generate
much better code for access_ok(), and on arm64 we use a compile-time constant
even when our page table depth can change at runtime (and when native/compat
task sizes differ). The only abosolute boundary that needs to be maintained is
that access_ok() fails for kernel addresses.

Mark.