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

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Message-ID: <20210813200508.7bqehxgd6ruerds5@google.com>
Date:   Fri, 13 Aug 2021 13:05:08 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Daniel Axtens <dja@...ens.net>
Cc:     Bill Wendling <morbo@...gle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] ppc: add "-z notext" flag to disable diagnostic

On 2021-08-14, Daniel Axtens wrote:
>Bill Wendling <morbo@...gle.com> 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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