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 <linux@xxxxxxxxxxxx> 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