linux-kernel - Re: [PATCH 3/5] powerpc64: Add .opd based function descriptor dereference

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Message-ID: <878tha9lxm.fsf@concordia.ellerman.id.au>
Date:   Wed, 20 Sep 2017 11:51:49 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        "James E . J . Bottomley" <jejb@...isc-linux.org>,
        Helge Deller <deller@....de>, Petr Mladek <pmladek@...e.com>,
        linux-ia64@...r.kernel.org, linux-parisc@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, Jessica Yu <jeyu@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org, Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH 3/5] powerpc64: Add .opd based function descriptor dereference

Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> writes:

> On (09/19/17 20:22), Michael Ellerman wrote:
>> > On 2017/09/16 12:53PM, Sergey Senozhatsky wrote:
>> >> We are moving towards separate kernel and module function descriptor
>> >> dereference callbacks. This patch enables it for powerpc64.
>> >> 
>> >> For pointers that belong to the kernel
>> >> -  Added __start_opd and __end_opd pointers, to track the kernel
>> >>    .opd section address range;
>> >> 
>> >> -  Added dereference_kernel_function_descriptor(). Now we
>> >>    will dereference only function pointers that are within
>> >>    [__start_opd, __end_opd];
>> >> 
>> >> For pointers that belong to a module
>> >> -  Added dereference_module_function_descriptor() to handle module
>> >>    function descriptor dereference. Now we will dereference only
>> >>    pointers that are within [module->opd.start, module->opd.end].
>> >
>> > Would it be simpler to just use kernel_text_address() and dereference 
>> > everything else? See commit 83e840c770f2c5 ("powerpc64/elfv1: Only 
>> > dereference function descriptor for non-text symbols") for a related 
>> > patch.
>> 
>> Yeah that would be a lot simpler and probably work perfectly well.
>
> unlike ppc_function_entry(), printk() can get called on any symbol,
> not just function pointers.
>
> for example,
>
> cat /proc/kallsyms | grep shrinker_rwsem
> ffffffff81a4b1e0 d shrinker_rwsem

Yep, good point. So your patch is probably good then. Maybe someone
other than me can find time to test it ;)

cheers