* Adrian Hunter <adrian.hun...@intel.com> wrote:
> On 30/01/14 10:59, Ingo Molnar wrote:
> >
> > * Adrian Hunter <adrian.hun...@intel.com> wrote:
> >
> >> On 29/01/14 20:57, Arnaldo Carvalho de Melo wrote:
> >>> Em Wed, Jan 29, 2014 at 04:14:36PM +0200, Adrian Hunter escreveu:
> >>>> Kernel maps map memory addresses to file offsets.
> >>>> For symbol annotation, objdump needs the object VMA
> >>>> addresses. For an unrelocated kernel, that is the
> >>>> same as the memory address.
> >>>>
> >>>> The addresses passed to objdump for symbol annotation
> >>>> did not take into account kernel relocation. This
> >>>> patch fixes that.
> >>>
> >>> Question: To fix the problem reported by Linus, i.e. the very minimal
> >>> fix, we only need this patch, right?
> >>
> >> Yes but the other fixes are needed too.
> >
> > So, for the specific case of kernel address layout randomization, how
> > does this fix Linus's bug with KASLR enabled? How does the code
> > recover the random, runtime offset of the relocated kernel, which
> > varies from boot to boot?
>
> By comparing the address of a symbol ("_text" or "_stext")
> in /proc/kallsyms (or perf.data - see below) with the same
> symbol in vmlinux.
>
> perf tools call this the ref_reloc_sym and stores it in
> perf.data hidden in the synthesized kernel mmap record.
> e.g.
>
> 0xd8 [0x50]: event: 1
> .
> . ... raw event: size 80 bytes
> . 0000: 01 00 00 00 01 00 50 00 ff ff ff ff 00 00 00 00 ......P.........
> . 0010: 00 00 00 17 00 00 00 00 ff ff ff a8 ff ff ff ff ................
> . 0020: c8 01 00 98 ff ff ff ff 5b 6b 65 72 6e 65 6c 2e ........[kernel.
> . 0030: 6b 61 6c 6c 73 79 6d 73 5d 5f 73 74 65 78 74 00 kallsyms]_stext.
> . 0040: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
> .
> 0 0xd8 [0x50]: PERF_RECORD_MMAP -1/0: [0x17000000(0xffffffffa8ffffff) @
> 0xffffffff980001c8]: x [kernel.kallsyms]_stext
>
> That tells perf tools that _stext was 0xffffffff980001c8.
> Compare to vmlinux:
>
> $ objdump -t vmlinux | grep _stext
> ffffffff810001c8 g .text 0000000000000000 _stext
>
> So the relocation is 0xffffffff980001c8 - 0xffffffff810001c8
> = 0x17000000
Ok, cool, thanks!
I'd suggest the whole fix series if perf/urgent material.
Thanks,
Ingo
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