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From: Guenter Roeck <linux@roeck-us.net>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Christoph Hellwig <hch@lst.de>, Al Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	David Sterba <dsterba@suse.com>,
	Miklos Szeredi <miklos@szeredi.hu>,
	Anton Altaparmakov <anton@tuxera.com>,
	David Howells <dhowells@redhat.com>,
	Matthew Wilcox <willy@infradead.org>,
	Pavel Begunkov <asml.silence@gmail.com>
Subject: Re: [PATCH] iov_iter: separate direction from flavour
Date: Sun, 4 Jul 2021 14:47:24 -0700	[thread overview]
Message-ID: <03a15dbd-bdb9-1c72-a5cd-2e6a6d49af2b@roeck-us.net> (raw)
In-Reply-To: <CAHk-=wj0Q8R_3AxZO-34Gp2sEQAGUKhw7t6g4QtsnSxJTxb7WA@mail.gmail.com>

On 7/4/21 1:41 PM, Linus Torvalds wrote:
> On Sun, Jul 4, 2021 at 1:28 PM Guenter Roeck <linux@roeck-us.net> wrote:
>>
>> Turns out that, at least on m68k/nommu, USER_DS and KERNEL_DS are the same.
>>
>> #define USER_DS         MAKE_MM_SEG(TASK_SIZE)
>> #define KERNEL_DS       MAKE_MM_SEG(0xFFFFFFFF)
> 
> Ahh. So the code is fine, it's just that "uaccess_kernel()" isn't
> something that can be reliably even tested for, and it will always
> return true on those nommu platforms.
> 
> And we don't have a "uaccess_user()" macro that would test if it
> matches USER_DS (and that also would always return true on those
> configurations), so we can't just change the
> 
>          WARN_ON_ONCE(uaccess_kernel());
> 
> into a
> 
>          WARN_ON_ONCE(!uaccess_user());
> 
> instead.
> 
> Very annoying. Basically, every single use of "uaccess_kernel()" is unreliable.
> 
> There aren't all that many of them, and most of them are irrelevant
> for no-mmu anyway (like the bpf tracing ones, or mm/memory.c). So this
> iov_iter.c case is likely the only one that would be an issue.
> 
> That warning is something that should go away eventually anyway, but I
> _like_ that warning for now, just to get coverage. But apparently it's
> just not going to be the case for these situations.
> 
> My inclination is to keep it around for a while - to see if it catches
> anything else - but remove it for the final 5.14 release because of
> these nommu issues.
> 
> Of course, I will almost certainly not remember to do that unless
> somebody reminds me...
> 
> The other alternative would be to just make nommu platforms that have
> KERNEL_DS==USER_DS simply do
> 
>      #define uaccess_kernel() (false)
> 
> and avoid it that way, since that's closer to what the modern
> non-CONFIG_SET_FS world view is, and is what include/linux/uaccess.h
> does for that case..
> 

Theoretically, but arm defines it as true with !CONFIG_MMU and then
uses it in user_addr_max():

#define user_addr_max() \
         (uaccess_kernel() ? ~0UL : get_fs())

with !CONFIG_MMU:

#define KERNEL_DS	0x00000000
#define get_fs()	(KERNEL_DS)

How about the following ?

	WARN_ON_ONCE(IS_ENABLED(CONFIG_MMU) && uaccess_kernel());

Guenter

  reply	other threads:[~2021-07-04 21:47 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-04 17:29 Guenter Roeck
2021-07-04 18:31 ` Linus Torvalds
2021-07-04 18:54   ` Guenter Roeck
2021-07-04 19:04     ` Linus Torvalds
2021-07-04 20:28       ` Guenter Roeck
2021-07-04 20:41         ` Linus Torvalds
2021-07-04 21:47           ` Guenter Roeck [this message]
2021-07-04 22:44             ` Linus Torvalds
2021-07-04 22:47               ` Matthew Wilcox
2021-07-04 22:53                 ` Linus Torvalds
2021-07-04 23:34                   ` Linus Torvalds
2021-07-05  5:17           ` Christoph Hellwig

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