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Date:   Tue, 8 Oct 2019 20:58:58 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Guenter Roeck <linux@roeck-us.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH] Convert filldir[64]() from __put_user() to
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On Mon, Oct 07, 2019 at 11:26:35AM -0700, Linus Torvalds wrote:

> The good news is that right now x86 is the only architecture that does
> that user_access_begin(), so we don't need to worry about anything
> else. Apparently the ARM people haven't had enough performance
> problems with the PAN bit for them to care.

Take a look at this:
static inline unsigned long raw_copy_from_user(void *to,
                const void __user *from, unsigned long n)
{
        unsigned long ret;
        if (__builtin_constant_p(n) && (n <= 8)) {
                ret = 1;

                switch (n) {
                case 1:
                        barrier_nospec();
                        __get_user_size(*(u8 *)to, from, 1, ret);
                        break;
                case 2:
                        barrier_nospec();
                        __get_user_size(*(u16 *)to, from, 2, ret);
                        break;
                case 4:
                        barrier_nospec();
                        __get_user_size(*(u32 *)to, from, 4, ret);
                        break;
                case 8:
                        barrier_nospec();
                        __get_user_size(*(u64 *)to, from, 8, ret);
                        break;
                }
                if (ret == 0)
                        return 0;
        }

        barrier_nospec();
        allow_read_from_user(from, n);
        ret = __copy_tofrom_user((__force void __user *)to, from, n);
        prevent_read_from_user(from, n);
        return ret;
}

That's powerpc.  And while the constant-sized bits are probably pretty
useless there as well, note the allow_read_from_user()/prevent_read_from_user()
part.  Looks suspiciously similar to user_access_begin()/user_access_end()...

The difference is, they have separate "for read" and "for write" primitives
and they want the range in their user_access_end() analogue.  Separating
the read and write isn't a problem for callers (we want them close to
the actual memory accesses).  Passing the range to user_access_end() just
might be tolerable, unless it makes you throw up...