Re: [PATCH] kallsyms: fix absolute addresses for kASLR

Kees Cook
2014-02-26T06:15:27Z

On Mon, Feb 24, 2014 at 5:29 PM, Rusty Russell <rusty@rustcorp.com.au> wrote:
> Kees Cook <keescook@chromium.org> writes:
>> From: Andy Honig <ahonig@google.com>
>>
>> Currently symbols that are absolute addresses are incorrectly
>> displayed in /proc/kallsyms if the kernel is loaded with kASLR.
>>
>> The problem was that the scripts/kallsyms.c file which generates
>> the array of symbol names and addresses uses an relocatable value
>> for all symbols, even absolute symbols. This patch fixes that.
>
> Hi Andy, Kees,
>
> This is not a good patch. See the commit where this was
> introduced:
>
> [PATCH] relocatable kernel: Fix kallsyms on avr32 after relocatable kernel changes
>
> o On some platforms like avr32, section init comes before .text and
> not necessarily a symbol's relative position w.r.t _text is positive.
> In such cases assembler detects the overflow and emits warning. This
> patch fixes it.
>
> Did you just break avr32?
>
> And absolute symbols are supposed to be handled in the other branch:
>
> for (i = 0; i < table_cnt; i++) {
> if (toupper(table[i].sym[0]) != 'A') {
> if (_text <= table[i].addr)
> printf("\tPTR\t_text + %#llx\n",
> table[i].addr - _text);
> else
> printf("\tPTR\t_text - %#llx\n",
> _text - table[i].addr);
> } else {
> printf("\tPTR\t%#llx\n", table[i].addr);
> }
> }
>
> __per_cpu_start is not an absolute symbol anyway.
>
> You need to fix this properly.
> Rusty.

Hm, yeah, it seems we need another class of variable. The per_cpu
stuff is technically relative, but it's not relocated, since it's not
relative to the text location. We'll see how to do this more sanely.

Thanks!

-Kees

--
Kees Cook
Chrome OS Security