linux-kernel - Re: [PATCH] iov_iter: separate direction from flavour

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Message-ID: <03a15dbd-bdb9-1c72-a5cd-2e6a6d49af2b@roeck-us.net>
Date:   Sun, 4 Jul 2021 14:47:24 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Christoph Hellwig <hch@....de>, Al Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Sterba <dsterba@...e.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Anton Altaparmakov <anton@...era.com>,
        David Howells <dhowells@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Pavel Begunkov <asml.silence@...il.com>
Subject: Re: [PATCH] iov_iter: separate direction from flavour

On 7/4/21 1:41 PM, Linus Torvalds wrote:
> On Sun, Jul 4, 2021 at 1:28 PM Guenter Roeck <linux@...ck-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