Re: linux-next: build warning after merge of the bpf-next tree

[Alexandre Ghiti]

On 1/10/20 7:20 PM, Palmer Dabbelt wrote:
> On Fri, 10 Jan 2020 14:28:17 PST (-0800), alexandre@xxxxxxxx wrote:
> > Hi guys,
> >
> > On 10/27/19 8:02 PM, Stephen Rothwell wrote:
> > > Hi all,
> > >
> > > On Fri, 18 Oct 2019 10:56:57 +1100 Stephen Rothwell 
> > > <sfr@xxxxxxxxxxxxxxxx> wrote:
> > > > Hi all,
> > > >
> > > > After merging the bpf-next tree, today's linux-next build (powerpc
> > > > ppc64_defconfig) produced this warning:
> > > >
> > > > WARNING: 2 bad relocations
> > > > c000000001998a48 R_PPC64_ADDR64 _binary__btf_vmlinux_bin_start
> > > > c000000001998a50 R_PPC64_ADDR64 _binary__btf_vmlinux_bin_end
> > > >
> > > > Introduced by commit
> > > >
> > > > ÂÂ 8580ac9404f6 ("bpf: Process in-kernel BTF")
> > > This warning now appears in the net-next tree build.
> > I bump that thread up because Zong also noticed that 2 new 
> > relocations for
> > those symbols appeared in my riscv relocatable kernel branch following
> > that commit.
> >
> > I also noticed 2 new relocations R_AARCH64_ABS64 appearing in arm64 
> > kernel.
> >
> > Those 2 weak undefined symbols have existed since commit
> > 341dfcf8d78e ("btf: expose BTF info through sysfs") but this is the fact
> > to declare those symbols into btf.c that produced those relocations.
> >
> > I'm not sure what this all means, but this is not something I expected
> > for riscv for
> > a kernel linked with -shared/-fpie. Maybe should we just leave them to
> > zero ?
> >
> > I think that deserves a deeper look if someone understands all this
> > better than I do.
>
> Can you give me a pointer to your tree and how to build a relocatable 
> kernel?
> Weak undefined symbols have the absolute value 0,


So according to you the 2 new relocations R_RISCV_64 are normal and 
should not
be modified at runtime right ?


> but the kernel is linked at
> an address such that 0 can't be reached by normal means. When I added 
> support
> to binutils for this I did it in a way that required almost no code --
> essetially I just stopped dissallowing x0 as a possible base register 
> for PCREL
> relocations, which results in 0 always being accessible. I just 
> wanted to get
> the kernel to build again, so I didn't worry about chasing around all the
> addressing modes. The PIC/PIE support generates different relocations 
> and I
> wouldn't be surprised if I just missed one (or more likely all) of them.
>
> It's probably a simple fix, though I feel like every time I say that 
> about the
> linker I end up spending a month in there...

You can find it here:

https://github.com/AlexGhiti/riscv-linux/tree/int/alex/riscv_relocatable_v1

Zong fixed the bug introduced by those 2 new relocations and everything 
works
like a charm, so I'm not sure you have to dig in the linker :)

Alex