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

From: Kees Cook
Date: Mon Jan 24 2011 - 21:04:17 EST

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 â%pâ 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 Cook
Ubuntu Security Team
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