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

From: "Arnd Bergmann" <arnd@arndb.de>
To: "Alexandre Ghiti" <alex@ghiti.fr>,
	"Samuel Holland" <samuel.holland@sifive.com>,
	"Alexandre Ghiti" <alexghiti@rivosinc.com>
Cc: "Palmer Dabbelt" <palmer@dabbelt.com>,
	linux-riscv@lists.infradead.org,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Charlie Jenkins" <charlie@rivosinc.com>,
	guoren <guoren@kernel.org>, "Jisheng Zhang" <jszhang@kernel.org>,
	"Kemeng Shi" <shikemeng@huaweicloud.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Mike Rapoport" <rppt@kernel.org>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Xiao W Wang" <xiao.w.wang@intel.com>,
	"Yangyu Chen" <cyy@cyyself.name>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] riscv: Define TASK_SIZE_MAX for __access_ok()
Date: Sun, 24 Mar 2024 23:05:52 +0100	[thread overview]
Message-ID: <b0a07878-a9f1-40aa-b177-423b05137d2e@app.fastmail.com> (raw)
In-Reply-To: <b5624bba-9917-421b-8ef0-4515d442f80b@ghiti.fr>

On Tue, Mar 19, 2024, at 17:51, Alexandre Ghiti wrote:
> On 18/03/2024 22:29, Samuel Holland wrote:
>> On 2024-03-18 3:50 PM, Alexandre Ghiti wrote:
>>> On Wed, Mar 13, 2024 at 7:00 PM Samuel Holland
>> It looks like the call to fixup_exception() [added
>> in 416721ff05fd ("riscv, mm: Perform BPF exhandler fixup on page fault")] is
>> only intended to catch null pointer dereferences. So making the change wouldn't
>> have any functional impact, but it would still be a valid optimization.
>>
>>> Or I was wondering if it would not be better to do like x86 and use an
>>> alternative, it would be more correct (even though I believe your
>>> solution works)
>>> https://elixir.bootlin.com/linux/latest/source/arch/x86/include/asm/page_64.h#L82.
>> What would be the benefit of using an alternative? Any access to an address
>> between TASK_SIZE and TASK_SIZE_MAX is guaranteed to generate a page fault, so
>> the only benefit I see is returning -EFAULT slightly faster at the cost of
>> applying a few hundred alternatives at boot. But it's possible I'm missing
>> something.
>
>
> 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.

The access_ok() function really wants a compile-time constant
value for TASK_SIZE_MAX so it can do constant folding for
repeated calls inside of one function, so for configurations
with a boot-time selected TASK_SIZE_64 it's already not ideal,
with or without alternatives.

If I read the current code correctly, riscv doesn't even
have a way to build with a compile-time selected
VA_BITS/PGDIR_SIZE, which is probably a better place to
start optimizing, since this rarely needs to be selected
dynamically.

      Arnd

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