linux-kernel - Re: Problems with commit 'kallsyms: add support for relative offsets in kallsyms address table' (in mmotm)

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Date:	Sat, 23 Jan 2016 23:06:09 -0800
From:	Guenter Roeck <linux@...ck-us.net>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Problems with commit 'kallsyms: add support for relative offsets
 in kallsyms address table' (in mmotm)

On 01/23/2016 10:10 PM, Ard Biesheuvel wrote:
>
>
>> On 24 jan. 2016, at 03:35, Guenter Roeck <linux@...ck-us.net> wrote:
>>
>>> On 01/23/2016 06:06 PM, Guenter Roeck wrote:
>>> Hi,
>>>
>>> I see runtime problems with the current mmotm branch. All qemu mips targets
>>> (32 and 64 bit, big and little endian) are stuck in boot after this commit.
>>>
>>> Bisect points to commit d13682e4d9d2 ("kallsyms: add support for relative offsets
>>> in kallsyms address table". Disabling CONFIG_KALLSYMS_BASE_RELATIVE fixes the problem,
>>> ie I can boot the image with qemu.
>>>
>>> Bisect log is attached.
>>>
>>> Playing with the problem, I found the following:
>>>
>>> 1) The problem is only seen with a toolchain using binutils 2.22, but not
>>>     with a toolchain using binutils 2.25. The compiler configuration may be
>>>     different for both toolchains.
>>> 2) Message "kallsyms failure: absolute symbol value 0xffffffff807afd14 out of range
>>>     in relative mode" (twice) when using the toolchain with binutils 2.22.
>>>     This does not cause the build to fail, though.
>>> 3) kallsyms_sym_address() parameter variable type is "int". In the calling code,
>>>     the variable type used is "unsigned long". That has no impact on the problem,
>>>     though.
>>
>> An additional data point: When using the older toolchain, many symbols in System.map
>> are marked "A".
>>     ffffffff80100000 A _text
>> With the more recent toolchain, the same symbols are marked "T".
>>     ffffffff80100000 T _text
>>
>
> Thanks for the analysis. It is surprising that the build does not fail when this occurs, and the subsequent hangs themselves are probably caused by missing kallsyms data.
>
Yes, I wondered why the build doesn't fail. Seems odd.

> scripts/kallsyms.c ignores all A symbols except _text, which is actually a relative symbol by nature so we can simply assume it is relative (i.e., override it as T)
>
> Re x86_64 !SMP, any build time errors there as well? Likewise for sparc32?
>

Yes, same kind of errors for both. For x86_64/nosmp I also get the error message
when using the Ubuntu native toolchain, so it doesn't seem to be (directly)
related to binutils 2.22 vs. 2.25 for that architecture.

Runtime behavior is a bit different for the different architectures.
x86_64 dies silently without any console output, mips just hangs,
and sparc32 gets a panic with NULL pointer access.
Of course, with missing kallsyms data all bets are off.

>
> Thanks again, and sorry for the trouble,

No worries. Hope you'll get this sorted out.

Guenter