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

Luck, Tony
2017-09-18T17:44:32Z

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