linux-kernel - Re: [PATCH] kallsyms: fix nonconverging kallsyms table with lld

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Date:   Thu, 10 Jun 2021 05:05:31 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        David Brazdil <dbrazdil@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Mikhail Petrov <Mikhail.Petrov@....dev>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        mfaltesek@...gle.com
Subject: Re: [PATCH] kallsyms: fix nonconverging kallsyms table with lld

On Wed, Jun 09, 2021 at 10:30:23PM +0200, Arnd Bergmann wrote:
> On Wed, Jun 9, 2021 at 9:15 PM Guenter Roeck <linux@...ck-us.net> wrote:
> > On Wed, Jun 09, 2021 at 08:16:11AM -0700, Guenter Roeck wrote:
> > > > I suppose we could improve the situation if scripts/link-vmlinux.sh was able
> > > > to do that automatically, and compare the kallsyms output .S file between
> > > > steps 1 and 2.
> > >
> > > Comparing the .S files doesn't result in useful data; turns out there are
> > > always irrelevant differences. We'll try to run a diff on the output of
> > > "objdump --syms". Hopefully that will generate something useful.
> > >
> >
> > Turns out it wasn't that useful.
> >
> > chromeos-kernel-5_10-5.10.42-r406: Symbol file differences:
> > chromeos-kernel-5_10-5.10.42-r406: 7c7
> > chromeos-kernel-5_10-5.10.42-r406: < 00000000000325c8 g       .rodata   0000000000000000 kallsyms_relative_base
> > chromeos-kernel-5_10-5.10.42-r406: ---
> > chromeos-kernel-5_10-5.10.42-r406: > 00000000000325c0 g       .rodata   0000000000000000 kallsyms_relative_base
> > chromeos-kernel-5_10-5.10.42-r406: 9,13c9,13
> > chromeos-kernel-5_10-5.10.42-r406: < 00000000000325d0 g       .rodata   0000000000000000 kallsyms_num_syms
> > chromeos-kernel-5_10-5.10.42-r406: < 00000000000325d8 g       .rodata   0000000000000000 kallsyms_names
> > chromeos-kernel-5_10-5.10.42-r406: < 00000000000cd7f0 g       .rodata   0000000000000000 kallsyms_markers
> > chromeos-kernel-5_10-5.10.42-r406: < 00000000000cdb18 g       .rodata   0000000000000000 kallsyms_token_table
> > chromeos-kernel-5_10-5.10.42-r406: < 00000000000cde78 g       .rodata   0000000000000000 kallsyms_token_index
> > chromeos-kernel-5_10-5.10.42-r406: ---
> > chromeos-kernel-5_10-5.10.42-r406: > 00000000000325c8 g       .rodata   0000000000000000 kallsyms_num_syms
> > chromeos-kernel-5_10-5.10.42-r406: > 00000000000325d0 g       .rodata   0000000000000000 kallsyms_names
> > chromeos-kernel-5_10-5.10.42-r406: > 00000000000cd7d8 g       .rodata   0000000000000000 kallsyms_markers
> > chromeos-kernel-5_10-5.10.42-r406: > 00000000000cdb00 g       .rodata   0000000000000000 kallsyms_token_table
> > chromeos-kernel-5_10-5.10.42-r406: > 00000000000cde60 g       .rodata   0000000000000000 kallsyms_token_index
> >
> > I thought I'd see the added symbols, but it looks like the only difference
> > between the two files is the addresses.
> >
> > What am I missing ?
> 
> I probably misremembered the part about 'objdump --syms' and there was
> something more to it.
> 
> Maybe this was the last version before converging? It looks like the '<' version
> has one extra symbol ompared to the '>' version. The diff has no context, but I

It is the difference between step 1 and 2. Why would diff on objdump not
show the additional symbol ? Is it possible that the symbol is not added
to the object file ?

> assume the first symbol that has a different size is 'kallsyms_offsets', which
> is generated by kallsyms.

I'll give it another try and run diff -u.

> 
> I see that link-vmlinux.sh already compares the System.map files, using
> "cmp -s System.map .tmp_System.map", which is roughly the same as the
> objdump --syms diff you got, so comparing these files probably doesn't
> help either. However, comparing the .tmp_System.map file with the previous
> version might reveal the problem. This might need another step to filter out
> the address and only compare the symbol names.

I'll do that as well.

Thanks!
Guenter