linux-kernel - Re: [PATCH v2 1/6] fs/readdir: Fix filldir() and filldir64() use of user_access

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Date:   Thu, 23 Jan 2020 13:43:06 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 1/6] fs/readdir: Fix filldir() and filldir64() use of
 user_access_begin()

Le 23/01/2020 à 13:00, Michael Ellerman a écrit :
> Michael Ellerman <mpe@...erman.id.au> writes:
>> Hi Christophe,
>>
>> This patch is independent of the rest of the series AFAICS
> 
> And of course having hit send I immediately realise that's not true.

Without this, book3s/32 fails booting. (And without patch 2, it even 
hangs, looping forever in do_page_fault()).

> 
>> So I'll take patches 2-6 via powerpc and assume this patch will go via
>> Linus or Al or elsewhere.
> 
> So I guess I'll wait and see what happens with patch 1.

We could eventually opt out user_access_begin() for 
CONFIG_PPC_BOOK3S_32, then you could take patches 3 and 6. That's enough 
to have user_access_begin() and stuff for 8xx and RADIX.

Patch 2 should be taken as well as a fix, and can be kept independant of 
the series (once we have patch 1, we normally don't hit the problem 
fixed by patch 2).

Won't don't need patch 4 until we want user_access_begin() supported by 
book3s/32

However, I'm about to send out a v3 with a different approach. It 
modifies the core part where user_access_begin() is returning an opaque 
value used by user_access_end(). And it also tells user_access_begin() 
whether it's a read or a write, so that we can limit unlocking to write 
acccesses on book3s/32, and fine grain rights on book3s/64.

Maybe you would prefer this change on top of first step, in which case 
I'll be able to make a v4 rebasing all this on top of patch 3 and 6 of 
v3 series. Tell me what you prefer.

Christophe