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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 10 Oct 2019 15:12:49 -0700
Message-ID: <CAHk-=wgWRQo0m7TUCK4T_J-3Vqte+p-FWzvT3CB1jJHgX-KctA@mail.gmail.com>
Subject: Re: [PATCH] Convert filldir[64]() from __put_user() to unsafe_put_user()
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Guenter Roeck <linux@roeck-us.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
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On Thu, Oct 10, 2019 at 12:55 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> Anyway, another question you way: what do you think of try/catch approaches
> to __get_user() blocks, like e.g. restore_sigcontext() is doing?

I'd rather have them converted to our unsafe_get/put_user() instead.

We don't generate great code for the "get" case (because of how gcc
doesn't allow us to mix "asm goto" and outputs), but I really despise
the x86-specific "{get,put}_user_ex()" machinery. It's not actually
doing a real try/catch at all, and will just keep taking faults if one
happens.

But I've not gotten around to rewriting those disgusting sequences to
the unsafe_get/put_user() model. I did look at it, and it requires
some changes exactly *because* the _ex() functions are broken and
continue, but also because the current code ends up also doing other
things inside the try/catch region that you're not supposed to do in a
user_access_begin/end() region .

> Should that be available outside of arch/*?  For that matter, would
> it be a good idea to convert get_user_ex() users in arch/x86 to
> unsafe_get_user()?

See above: yes, it would be a good idea to convert to
unsafe_get/put_user(), and no, we don't want to expose the horrid
*_ex() model to other architectures.

          Linus