linux-kernel - Re: [RFC][PATCH v2 0/7] printk/ia64/ppc64/parisc64: let's deprecate %pF/%pf printk specifiers

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Date:   Wed, 27 Sep 2017 16:26:44 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Santosh Sivaraj <santosh@...six.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        James Bottomley <jejb@...isc-linux.org>,
        Helge Deller <deller@....de>, Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jessica Yu <jeyu@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        linux-ia64@...r.kernel.org, linux-parisc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH v2 0/7] printk/ia64/ppc64/parisc64: let's deprecate %pF/%pf printk specifiers

Santosh Sivaraj <santosh@...six.org> writes:

> * Sergey Senozhatsky <sergey.senozhatsky@...il.com> wrote (on 2017-09-20 16:29:02 +0000):
>
>>         Hello
>> 
>>         RFC
>> 
>>         On some arches C function pointers are indirect and point to
>> a function descriptor, which contains the actual pointer to the code.
>> This mostly doesn't matter, except for cases when people want to print
>> out function pointers in symbolic format, because the usual '%pS/%ps'
>> does not work on those arches as expected. That's the reason why we
>> have '%pF/%pf', but since it's here because of a subtle ABI detail
>> specific to some arches (ppc64/ia64/parisc64) it's easy to misuse
>> '%pF/%pf' and '%pS/%ps' (see [1], for example).
>> 
>>         This patch set attempts to move ia64/ppc64/parisc64 C function
>> pointer ABI details out of printk() to arch code. Function dereference
>> code now checks if a pointer belongs to a .opd ELF section and dereferences
>> that pointer only if it does. The kernel and modules have their own .opd
>> sections that's why I use two different ARCH functions: for kernel and
>> for module pointer dereference.
>> 
>>         I planned to remove dereference_function_descriptor() entirely,
>> but then I discovered a bunch other uses cases (kgdbts, init/main.c,
>> extable, etc.), so I decided to keep dereference_function_descriptor()
>> around because the main point of this patch set is to deprecate %pF/%pf.
>> But at the same time, I think I can go further and handle both kernel
>> and module descriptor dereference in dereference_function_descriptor().
>> We need a module pointer for module .opd check, so that will come at an
>> extra cost of module lookup (may be there will some other issues along
>> the way, haven't checked it).
>> 
>> Right now we've got:
>> 
>> - dereference_function_descriptor(addr)
>>         a generic (old) function. it simply attempts to dereference
>>         whatever pointer we give it.
>> 
>> - dereference_kernel_function_descriptor(addr)
>>         dereferences a kernel pointer if it's within the kernel's .opd
>>         section.
>> 
>> - dereference_module_function_descriptor(module, addr)
>>         dereference a module pointer if it's within the module's .opd
>>         section.
>> 
>> 
>> *** A BIG NOTE ***
>>         I don't own ia64/ppc64/parisc64 hardware, so the patches are not
>>         tested. Sorry about that!
>
> Tested patch series on ppc64 sucessfully.
>
> You may add tested by to the series.
>
> Tested-by: Santosh Sivaraj <santosh@...six.org>

Thanks Santosh.

I also gave it a quick spin. I'll give you an ack for the powerpc changes.

Acked-by: Michael Ellerman <mpe@...erman.id.au> (powerpc)


Thanks for cleaning this up Sergey.

cheers