linux-kernel - Re: [PATCH] Convert filldir[64]() from __put_user() to unsafe_put

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Message-ID: <20191007173432.GM26530@ZenIV.linux.org.uk>
Date:   Mon, 7 Oct 2019 18:34:32 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Guenter Roeck <linux@...ck-us.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] Convert filldir[64]() from __put_user() to
 unsafe_put_user()

On Sun, Oct 06, 2019 at 08:11:42PM -0700, Linus Torvalds wrote:

> > So do we want to bother with separation between raw_copy_to_user() and
> > unsafe_copy_to_user()?  After all, __copy_to_user() also has only few
> > callers, most of them in arch/*
> 
> No, you're right. Just switch over.
> 
> > I'll take a look into that tomorrow - half-asleep right now...
> 
> Thanks. No huge hurry.

Tangentially related: copy_regster_to_user() and copy_regset_from_user().
That's where we do access_ok(), followed by calls of ->get() and
->set() resp.  Those tend to either use user_regset_copy{out,in}(),
or open-code those.  The former variant tends to lead to few calls
of __copy_{to,from}_user(); the latter...  On x86 it ends up doing
this:
static int genregs_get(struct task_struct *target,
                       const struct user_regset *regset,
                       unsigned int pos, unsigned int count,
                       void *kbuf, void __user *ubuf)
{
        if (kbuf) {
                unsigned long *k = kbuf;
                while (count >= sizeof(*k)) {
                        *k++ = getreg(target, pos);
                        count -= sizeof(*k);
                        pos += sizeof(*k);
                }
        } else {
                unsigned long __user *u = ubuf;
                while (count >= sizeof(*u)) {
                        if (__put_user(getreg(target, pos), u++))
                                return -EFAULT;
                        count -= sizeof(*u);
                        pos += sizeof(*u);
                }
        }

        return 0;
}

Potentially doing arseloads of stac/clac as it goes.  OTOH, getreg()
(and setreg()) in there are not entirely trivial, so blanket
user_access_begin()/user_access_end() over the entire loop might be
a bad idea...

How hot is that codepath?  I know that arch/um used to rely on it
(== PTRACE_[GS]ETREGS) quite a bit...