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From:   Bill Wendling <morbo@google.com>
Date:   Thu, 12 Nov 2020 17:03:22 -0800
Message-ID: <CAGG=3QVQTQsHhsb5cSEejZ=D4Gq60cz7kMfD8moXWeVU1odTdQ@mail.gmail.com>
Subject: Re: [PATCH] riscv: Explicitly specify the build id style in vDSO
 Makefile again
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Nathan Chancellor <natechancellor@gmail.com>,
        Palmer Dabbelt <palmerdabbelt@google.com>,
        linux-riscv@lists.infradead.org,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Masahiro Yamada <masahiroy@kernel.org>
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On Thu, Nov 12, 2020 at 4:53 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Sun, Nov 8, 2020 at 12:37 PM Nathan Chancellor
> <natechancellor@gmail.com> wrote:
> >
> > Commit a96843372331 ("kbuild: explicitly specify the build id style")
> > explicitly set the build ID style to SHA1. Commit c2c81bb2f691 ("RISC-V:
> > Fix the VDSO symbol generaton for binutils-2.35+") undid this change,
> > likely unintentionally.
> >
> > Restore it so that the build ID style stays consistent across the tree
> > regardless of linker.
> >
> > Fixes: c2c81bb2f691 ("RISC-V: Fix the VDSO symbol generaton for binutils-2.35+")
> > Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
>
> Thanks for the fixup!
>
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
>
> (I'm curious what --build-id linker flag does, and what kind of spooky
> bugs that led to a96843372331?)
>
--build-id generates a unique "build id" for the build. It can use
several different algorithms to do this. The BFD linker uses sha1 by
default while LLD uses a "fast" algorithm. The difference is that the
fast algorithm generates a shorter build id. This shouldn't matter in
general, but there are some tools out there that expect the build id
to be of a certain length, i.e. the BFD style's length, because BFD is
more prevalent. The obvious response "well, why don't they just change
the expected length?" is difficult in all situations. (Once an
assumption is made, it's hard to backtrack.)

You can add this if you like:

Reviewed-by: Bill Wendling <morbo@google.com>

> > ---
> >  arch/riscv/kernel/vdso/Makefile | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
> > index cb8f9e4cfcbf..0cfd6da784f8 100644
> > --- a/arch/riscv/kernel/vdso/Makefile
> > +++ b/arch/riscv/kernel/vdso/Makefile
> > @@ -44,7 +44,7 @@ SYSCFLAGS_vdso.so.dbg = $(c_flags)
> >  $(obj)/vdso.so.dbg: $(src)/vdso.lds $(obj-vdso) FORCE
> >         $(call if_changed,vdsold)
> >  SYSCFLAGS_vdso.so.dbg = -shared -s -Wl,-soname=linux-vdso.so.1 \
> > -       -Wl,--build-id -Wl,--hash-style=both
> > +       -Wl,--build-id=sha1 -Wl,--hash-style=both
> >
> >  # We also create a special relocatable object that should mirror the symbol
> >  # table and layout of the linked DSO. With ld --just-symbols we can then
> >
> > base-commit: c2c81bb2f69138f902e1a58d3bef6ad97fb8a92c
> > --
> > 2.29.2
> >
> > --
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> > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20201108203737.94270-1-natechancellor%40gmail.com.
>
>
>
> --
> Thanks,
> ~Nick Desaulniers