Re: [PATCH] ppc: add "-z notext" flag to disable diagnostic

[Fangrui Song]

On 2021-08-14, Daniel Axtens wrote:
> Bill Wendling <morbo@xxxxxxxxxx> writes:
>
> > The "-z notext" flag disables reporting an error if DT_TEXTREL is set on
> > PPC with CONFIG=kdump:
> >
> >   ld.lld: error: can't create dynamic relocation R_PPC64_ADDR64 against
> >     local symbol in readonly segment; recompile object files with -fPIC
> >     or pass '-Wl,-z,notext' to allow text relocations in the output
> >   >>> defined in built-in.a(arch/powerpc/kernel/misc.o)
> >   >>> referenced by arch/powerpc/kernel/misc.o:(.text+0x20) in archive
> >       built-in.a
> >
> > The BFD linker disables this by default (though it's configurable in
> > current versions). LLD enables this by default. So we add the flag to
> > keep LLD from emitting the error.
>
> You didn't provide a huge amount of context but I was able to reproduce
> a similar set of errors with pseries_le_defconfig and
>
> make ARCH=powerpc CROSS_COMPILE=powerpc64-linux-gnu- CC="ccache clang-11" LD=ld.lld-11 AR=llvm-ar-11 -j4 vmlinux
>
> I also checked the manpage, and indeed the system ld does not issue this
> warning/error by default.
>
> > ---
> >  arch/powerpc/Makefile | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
> > index 6505d66f1193..17a9fbf9b789 100644
> > --- a/arch/powerpc/Makefile
> > +++ b/arch/powerpc/Makefile
> > @@ -122,6 +122,7 @@ endif
> >
> >  LDFLAGS_vmlinux-y := -Bstatic
> >  LDFLAGS_vmlinux-$(CONFIG_RELOCATABLE) := -pie
> > +LDFLAGS_vmlinux-$(CONFIG_RELOCATABLE) += -z notext
>
> Is there any reason this should be gated on CONFIG_RELOCATABLE? (I tried
> without it and got different but possibly related linker errors...)
>
> Also, is this behaviour new?

This is a longstanding behavior.

https://maskray.me/blog/2020-12-19-lld-and-gnu-linker-incompatibilities
See "Text relocations"

.o files used to link .tmp_vmlinux.kallsyms1 have many R_PPC64_ADDR64
relocations in non-SHF_WRITE sections. There are many text relocations (e.g. in
.rela___ksymtab_gpl+* and .rela__mcount_loc sections) in a -pie link and are
disallowed by LLD:

  ld.lld: error: can't create dynamic relocation R_PPC64_ADDR64 against local symbol in readonly segment; recompile object files with -fPIC or pass '-Wl,-z,notext' to allow text relocations in the output
  >>> defined in arch/powerpc/kernel/head_64.o
  >>> referenced by arch/powerpc/kernel/head_64.o:(__restart_table+0x10)

Newer GNU ld configured with --enable-textrel-check=error will report an error
as well:

  ld/ld-new: read-only segment has dynamic relocations



Text relocations are considered very awful by linker developers.
binutils 2.35 added --enable-textrel-check={no,warn,error}
https://sourceware.org/bugzilla/show_bug.cgi?id=20824

I can imagine that in the future some Linux distributions (especially those
focusing on security) will default their binutils to use
--enable-textrel-check={no,warn,error}.  CONFIG_RELOCATABLE build will break
sooner or later.


In -no-pie links, R_PPC64_ADDR64 relocations are link-time constants.
There are no text relocations, therefore no need for -z notext.

> Kind regards,
> Daniel
>
> >  LDFLAGS_vmlinux	:= $(LDFLAGS_vmlinux-y)
> >
> >  ifdef CONFIG_PPC64
> > --
> > 2.33.0.rc1.237.g0d66db33f3-goog
>
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