linux-kernel - RE: [PATCH] vsprintf: Do not break early boot with probing addresses

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Message-ID: <8ad8bb83b7034f7e92df12040fb8c2c2@AcuMS.aculab.com>
Date:   Thu, 9 May 2019 13:46:59 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Michal Suchánek' <msuchanek@...e.de>,
        Petr Mladek <pmladek@...e.com>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Michal Hocko <mhocko@...e.cz>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Stephen Rothwell <sfr@...abs.org>,
        "Andy Shevchenko" <andriy.shevchenko@...ux.intel.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "Martin Schwidefsky" <schwidefsky@...ibm.com>,
        "Tobin C . Harding" <me@...in.cc>
Subject: RE: [PATCH] vsprintf: Do not break early boot with probing addresses

From: Michal Suchánek
> Sent: 09 May 2019 14:38
...
> > The problem is the combination of some new code called via printk(),
> > check_pointer() which calls probe_kernel_read(). That then calls
> > allow_user_access() (PPC_KUAP) and that uses mmu_has_feature() too early
> > (before we've patched features).
> 
> There is early_mmu_has_feature for this case. mmu_has_feature does not
> work before patching so parts of kernel that can run before patching
> must use the early_ variant which actually runs code reading the
> feature bitmap to determine the answer.

Does the early_ variant get patched so the it is reasonably
efficient after the 'patching' is done?
Or should there be a third version which gets patched across?

	David

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