linux-kernel - Re: [PATCH] vdso: Improve cmd_vdso_check to check all dynamic relocations

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Date:   Mon, 14 Nov 2022 16:46:25 -0800
From:   Fangrui Song <maskray@...gle.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Vincenzo Frascino <vincenzo.frascino@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-csky@...r.kernel.org" <linux-csky@...r.kernel.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "loongarch@...ts.linux.dev" <loongarch@...ts.linux.dev>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] vdso: Improve cmd_vdso_check to check all dynamic
 relocations

On 2022-11-14, Christophe Leroy wrote:
>
>
>Le 28/09/2022 à 07:25, Fangrui Song a écrit :
>> On Sat, Sep 10, 2022 at 12:53 AM Fangrui Song <maskray@...gle.com> wrote:
>>>
>>> On 2022-08-30, Fangrui Song wrote:
>>>> The actual intention is that no dynamic relocation exists. However, some
>>>> GNU ld ports produce unneeded R_*_NONE. (If a port is not care enough to
>>>> determine the exact .rel[a].dyn size, the trailing zeros become R_*_NONE
>>>> relocations. E.g. powerpc64le ld as of 2.38 has the issue with
>>>> defconfig.) R_*_NONE are generally no-op in the dynamic loaders. So just
>>>> ignore them.
>>>>
>>>> With the change, we can remove ARCH_REL_TYPE_ABS. ARCH_REL_TYPE_ABS is a
>>>> bit misnomer as ports may check RELAVETIVE/GLOB_DAT/JUMP_SLOT which are
>>>> not called "absolute relocations". (The patch is motivated by the arm64
>>>> port missing R_AARCH64_RELATIVE.)
>>>>
>>>> While here, replace "egrep" with "grep" as "egrep" is deprecated in GNU
>>>> grep 3.7.
>>>>
>>>> Signed-off-by: Fangrui Song <maskray@...gle.com>
>>>> ---
>>>> [...]
>>>>
>>>
>>> Ping.
>>
>> Ping^2 :)
>
>Can you explain which ARCH_REL_TYPE_ABS can be removed with this change ?
>How is the verification done if ARCH_REL_TYPE_ABS is removed ?

All ARCH_REL_TYPE_ABS relocation types can be removed. As explained, the
real intention is to check no dynamic relocation, and this is done by
the new

   cmd_vdso_check = if $(READELF) -rW $@ | grep -v _NONE | grep -q "R_\w*_"; \

in this patch.

grep -v _NONE is to work around some GNU ld ports (and likely older releases
of some ports even if their latest versions are fixed) which produce
unneeded R_*_NONE dynamic relocations.