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

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Message-ID: <20191007012437.GK26530@ZenIV.linux.org.uk>
Date:   Mon, 7 Oct 2019 02:24:37 +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 06:17:02PM -0700, Linus Torvalds wrote:
> On Sun, Oct 6, 2019 at 5:04 PM Guenter Roeck <linux@...ck-us.net> wrote:
> >
> > All my alpha, sparc64, and xtensa tests pass with the attached patch
> > applied on top of v5.4-rc2. I didn't test any others.
> 
> Okay... I really wish my guess had been wrong.
> 
> Because fixing filldir64 isn't the problem. I can come up with
> multiple ways to avoid the unaligned issues if that was the problem.
> 
> But it does look to me like the fundamental problem is that unaligned
> __put_user() calls might just be broken on alpha (and likely sparc
> too). Because that looks to be the only difference between the
> __copy_to_user() approach and using unsafe_put_user() in a loop.
> 
> Now, I should have handled unaligned things differently in the first
> place, and in that sense I think commit 9f79b78ef744 ("Convert
> filldir[64]() from __put_user() to unsafe_put_user()") really is
> non-optimal on architectures with alignment issues.
> 
> And I'll fix it.

Ugh...  I wonder if it would be better to lift STAC/CLAC out of
raw_copy_to_user(), rather than trying to reinvent its guts
in readdir.c...