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

[Guenter Roeck]

On 01/23/2016 10:10 PM, Ard Biesheuvel wrote:

On 24 jan. 2016, at 03:35, Guenter Roeck <li...@roeck-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