linux-kernel - Re: linux-next: build warnings from Linus' tree

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Message-ID: <20181118112244.GD21617@bubble.grove.modra.org>
Date:   Sun, 18 Nov 2018 21:52:44 +1030
From:   Alan Modra <amodra@...il.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Joel Stanley <joel@....id.au>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        linuxppc-dev@...ts.ozlabs.org,
        Linux-Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: linux-next: build warnings from Linus' tree

On Wed, Nov 14, 2018 at 09:20:23PM +1100, Michael Ellerman wrote:
> Joel Stanley <joel@....id.au> writes:
> > Hello Alan,
> >
> > On Tue, 12 Jun 2018 at 07:44, Stephen Rothwell <sfr@...b.auug.org.au> wrote:
> >
> >> Building Linus' tree, today's linux-next build (powerpc ppc64_defconfig)
> >> produced these warning:
> >>
> >> ld: warning: orphan section `.gnu.hash' from `linker stubs' being placed in section `.gnu.hash'.
> >> ld: warning: orphan section `.gnu.hash' from `linker stubs' being placed in section `.gnu.hash'.
> >> ld: warning: orphan section `.gnu.hash' from `linker stubs' being placed in section `.gnu.hash'.
> >>
> >> This may just be because I have started building using the native Debian
> >> gcc for the powerpc builds ...
> >
> > Do you know why we started creating these?
> 
> It's controlled by the ld option --hash-style, which AFAICS still
> defaults to sysv (generating .hash).
> 
> But it seems gcc can be configured to have a different default, and at
> least my native ppc64le toolchains are passing gnu, eg:
> 
>  /usr/lib/gcc/powerpc64le-linux-gnu/6/collect2 -plugin
>  /usr/lib/gcc/powerpc64le-linux-gnu/6/liblto_plugin.so
>  -plugin-opt=/usr/lib/gcc/powerpc64le-linux-gnu/6/lto-wrapper
>  -plugin-opt=-fresolution=/tmp/ccw1U2fF.res
>  -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s
>  -plugin-opt=-pass-through=-lc -plugin-opt=-pass-through=-lgcc
>  -plugin-opt=-pass-through=-lgcc_s --sysroot=/ --build-id --eh-frame-hdr
>  -V -shared -m elf64lppc
>  --hash-style=gnu
>  ^^^^^^^^^^^^^^^^
> 
> So that's presumably why we're seeing it, some GCCs are configured to
> use it.
> 
> > If it's intentional, should we be putting including them in the same
> > way as .hash sections?
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/kernel/vmlinux.lds.S#n282
> >
> >   .hash : AT(ADDR(.hash) - LOAD_OFFSET) { *(.hash) }
> 
> That would presumably work.
> 
> My question though is do we even need it?
> 
> >From what I can see for it to be useful you need the section as well as
> an entry in the dynamic section pointing at it, and we don't have a
> dynamic section at all:
> 
>   $ readelf -S vmlinux | grep gnu.hash
>     [ 4] .gnu.hash         GNU_HASH         c000000000dbbdb0  00dcbdb0
>   $ readelf -d vmlinux
>   
>   There is no dynamic section in this file.
> 
> Compare to the vdso:
> 
> $ readelf -d arch/powerpc/kernel/vdso64/vdso64.so
> 
> Dynamic section at offset 0x868 contains 12 entries:
>   Tag        Type                         Name/Value
>  0x000000000000000e (SONAME)             Library soname: [linux-vdso64.so.1]
>  0x0000000000000004 (HASH)               0x120
>  0x000000006ffffef5 (GNU_HASH)           0x170
>  0x0000000000000005 (STRTAB)             0x320
>  0x0000000000000006 (SYMTAB)             0x1d0
>  0x000000000000000a (STRSZ)              269 (bytes)
>  0x000000000000000b (SYMENT)             24 (bytes)
>  0x0000000070000003 (PPC64_OPT)          0x0
>  0x000000006ffffffc (VERDEF)             0x450
>  0x000000006ffffffd (VERDEFNUM)          2
>  0x000000006ffffff0 (VERSYM)             0x42e
>  0x0000000000000000 (NULL)               0x0
> 
> 
> So can't we just discard .gnu.hash? And in fact do we need .hash either?
> 
> Actually arm64 discards the latter, and parisc discards both.
> 
> Would still be good to hear from Alan or someone else who knows anything
> about toolchain stuff, ie. not me :)

.gnu.hash, like .hash, is used by glibc ld.so for dynamic symbol
lookup.  I imagine you don't need either section in a kernel, so
discarding both sounds reasonable.  Likely you could discard .interp
and .dynstr too, and .dynsym when !CONFIG_PPC32.

-- 
Alan Modra
Australia Development Lab, IBM