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