Re: [PATCH 1/3] x86: record relocation offset

H. Peter Anvin
2009-12-30T23:31:58Z

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Modules are a completely separate thing - they are linked (not even just relocated) at insertion time, so they need to be tracked separately.

The statement that a _text-based relocation is insufficient is false. The entire x86-32 monolithic kernel is relocated as a unit. The x86-64 kernel, too, is relocated as a unit, but using the page tables, which means it always runs at the compile-time-selected virtual address.

-hpa

"James Bottomley" <James.Bottomley@suse.de> wrote:

>On Wed, 2009-12-30 at 19:58 -0200, Arnaldo Carvalho de Melo wrote:
>> Em Wed, Dec 30, 2009 at 06:39:36PM -0200, Arnaldo Carvalho de Melo escreveu:
>> > Em Wed, Dec 30, 2009 at 11:45:30AM -0800, H. Peter Anvin escreveu:
>> > > The kernel already knows where it is loaded -- obviously, by sheer
>> > > necessity -- and knows how it was itself configured, and as such we
>> > > can do this calculation in C code without modifying boot_params or
>> > > the early bootstrap.
>> >
>> > Problem is that at 'perf record' time we may not have access to the
>> > vmlinux file, and thus not be able to figure out the relocation applied
>> > in that boot.
>> >
>> > Then, at a later time, and possibly on another machine, on another arch,
>> > we try to map back IPs to symbols, the /proc/kallsyms is completely
>> > unrelated and we now have a vmlinux unrelocated...
>> >
>> > So we need a way to get the relocation applied at 'perf record' time and
>> > encode it in the perf.data header. Ideas about how to do that?
>>
>> Well, I guess we could do the _stext trick again, storing its value,
>> taken from /proc/kallsyms, into the perf.data header, then figuring out
>> the relocation by looking at its value in the vmlinux symtab.
>
>So reading the thread, I think the problem only exists for x86 compiled
>as a relocateable kernel.
>
>> There were concerns in the past about relying on _stext, IIRC, James?
>
>Well, the original concerns were that _text relative relocation
>resolution only works for the core kernel, not for modules.
>Additionally, the kernel is in several sections, most notably init and
>runtime ... these may get loaded at different locations so _text
>relative symbol resolution won't work in init sections.
>
>Right at the moment, only x86 and ppc do a relocatable kernel, and, as I
>understand the process, the whole kernel image gets a relative offset
>applied, so all sections get the same offset. Thus, for this case it
>looks like computing the offset from any known symbol would work
>(including _text).
>
>James
>
>
>
>

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