linux-kernel - Re: [PATCH 0/5] [RFC] printk/ia64/ppc64/parisc64: let's deprecate %pF/%pf printk specifiers

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Date:   Mon, 18 Sep 2017 10:44:32 -0700
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        "James E . J . 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: [PATCH 0/5] [RFC] printk/ia64/ppc64/parisc64: let's deprecate
 %pF/%pf printk specifiers

On Sat, Sep 16, 2017 at 12:53:42PM +0900, Sergey Senozhatsky wrote:
> 	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).

A few new warnings when building on ia64:

arch/ia64/kernel/module.c:931: warning: passing argument 1 of 'dereference_function_descriptor' makes pointer from integer without a cast
arch/ia64/kernel/module.c:931: warning: return makes integer from pointer without a cast
kernel/kallsyms.c:325: warning: assignment makes integer from pointer without a cast
kernel/kallsyms.c:325: warning: passing argument 1 of 'dereference_kernel_function_descriptor' makes pointer from integer without a cast

Tried out the module case with a simple Hello-world test case.
This code:

char buf[1];

int init_module(void)
{
	printk(KERN_INFO "Hello world 1.\n");

	printk("using %%p  my init_module is at %p\n", init_module);
	printk("using %%pF my init_module is at %pF\n", init_module);
	printk("using %%pS my init_module is at %pS\n", init_module);

	printk("using %%p  my buf is at %p\n", buf);
	printk("using %%pF my buf is at %pF\n", buf);
	printk("using %%pS my buf is at %pS\n", buf);

	return 0;
}

Gave this console output:

Hello world 1.
using %p  my init_module is at a000000203bf0328
using %pF my init_module is at init_module+0x0/0x140 [hello_1]
using %pS my init_module is at init_module+0x0/0x140 [hello_1]
using %p  my buf is at a000000203bf0648
using %pF my buf is at buf+0x0/0xfffffffffffffb58 [hello_1]
using %pS my buf is at buf+0x0/0xfffffffffffffb58 [hello_1]


Which looks like what you wanted. People unaware of the vagaries
of ppc64/ia64/parisc64 can use the wrong %p[SF] variant, but still
get the right output.

-Tony