RFC: introduce "K" flag for printf, similar to %pK

[Kees Cook <kees.cook@canonical.com>]

In the interests of hiding kernel addresses from userspace (without
messing with file permissions), I want to use %pK for /proc/kallsyms and
/proc/modules, but this results in changing several %x's to %p's. The
primary side-effects is that some legitimately "0" value things in
/proc/kallsyms turn into "(null)".

For example in kernel/kallsyms.c:
-               seq_printf(m, "%0*lx %c %s\t[%s]\n",
+               seq_printf(m, "%0*pK %c %s\t[%s]\n",

This results in /proc/kallsyms looking like this:
          (null) D irq_stack_union
          (null) D __per_cpu_start
0000000000004000 D gdt_page
...

(Secondary effect is building with -Wformat results in harmless warnings
"warning: '0' flag used with ‘%p’ gnu_printf format".)


If, on the other hand, I introduce a printf flag "K" for numbers, the
original behavior is left, and kernel/kallsyms.c changes like this:
-               seq_printf(m, "%0*lx %c %s\t[%s]\n",
+               seq_printf(m, "%K0*lx %c %s\t[%s]\n",

The only side-effect from this is when compiling with -Wformat, now we get
these harmless warnings "warning: unknown conversion type character 'K' in
format", as well as breaking the warning parser, so it can't count arguments
correctly any more "warning: format '%s' expects type 'char *', but
argument 4 has type 'long unsigned int'" etc.

I'm not very happy with either situation, but I'll reply to this email
with both versions of the potential patch...

-Kees

-- 
Kees Cook
Ubuntu Security Team