linux-kernel - Re: [PATCH v4 0/4] LoongArch: Support new relocation types

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Message-ID: <10751c9af5d45fea741e0bbed6c818ddb9db3ac3.camel@xry111.site>
Date:   Mon, 01 Aug 2022 17:55:26 +0800
From:   Xi Ruoyao <xry111@...111.site>
To:     Huacai Chen <chenhuacai@...nel.org>,
        Youling Tang <tangyouling@...ngson.cn>
Cc:     loongarch@...ts.linux.dev, LKML <linux-kernel@...r.kernel.org>,
        WANG Xuerui <kernel@...0n.name>,
        Jinyang He <hejinyang@...ngson.cn>,
        Lulu Cheng <chenglulu@...ngson.cn>
Subject: Re: [PATCH v4 0/4] LoongArch: Support new relocation types

On Mon, 2022-08-01 at 10:34 +0800, Huacai Chen wrote:
> Hi, all,
> 
> On Mon, Aug 1, 2022 at 10:16 AM Youling Tang <tangyouling@...ngson.cn>
> wrote:
> > 
> > Hi, Ruoyao
> > 
> > On 07/30/2022 10:52 AM, Xi Ruoyao wrote:
> > > On Sat, 2022-07-30 at 10:24 +0800, Xi Ruoyao wrote:
> > > > On Sat, 2022-07-30 at 01:55 +0800, Xi Ruoyao wrote:
> > > > > On Fri, 2022-07-29 at 20:19 +0800, Youling Tang wrote:
> > > > > 
> > > > > > On 07/29/2022 07:45 PM, Xi Ruoyao wrote:
> > > > > > > Hmm... The problem is the "addresses" of per-cpu symbols
> > > > > > > are
> > > > > > > faked: they
> > > > > > > are actually offsets from $r21.  So we can't just load
> > > > > > > such an
> > > > > > > offset
> > > > > > > with PCALA addressing.
> > > > > > > 
> > > > > > > It looks like we'll need to introduce an attribute for GCC
> > > > > > > to
> > > > > > > make
> > > > > > > an
> > > > > > > variable "must be addressed via GOT", and add the
> > > > > > > attribute into
> > > > > > > PER_CPU_ATTRIBUTES.
> > > > > 
> > > > > > Yes, we need a GCC attribute to specify the per-cpu
> > > > > > variable.
> > > > > 
> > > > > GCC patch adding "addr_global" attribute for LoongArch:
> > > > > https://gcc.gnu.org/pipermail/gcc-patches/2022-July/599064.html
> > > > > 
> > > > > An experiment to use it:
> > > > > https://github.com/xry111/linux/commit/c1d5d70
> > > > 
> > > > Correction: https://github.com/xry111/linux/commit/c1d5d708
> > > > 
> > > > It seems 7-bit SHA is not enough for kernel repo.
> > > 
> > > If addr_global is rejected or not implemented (for example,
> > > building the
> > > kernel with GCC 12), *I expect* the following hack to work (I've
> > > not
> > > tested it because I'm AFK now).  Using visibility in kernel seems
> > > strange, but I think it may make some sense because the modules
> > > are some
> > > sort of similar to an ELF shared object being dlopen()'ed, and our
> > > way
> > > to inject per-CPU symbols is analog to ELF interposition.
> > > 
> > > arch/loongarch/include/asm/percpu.h:
> > > 
> > >    #if !__has_attribute(__addr_global__) && defined(MODULE)
> > >    /* Magically remove "static" for per-CPU variables.  */
> > >    # define ARCH_NEEDS_WEAK_PER_CPU
> > >    /* Force GOT-relocation for per-CPU variables.  */
> > >    # define PER_CPU_ATTRIBUTES
> > > __attribute__((__visibility__("default")))
> > >    #endif
> > > 
> > > arch/loongarch/Makefile:
> > > 
> > >    # Hack for per-CPU variables, see PER_CPU_ATTRIBUTES in
> > >    # include/asm/percpu.h
> > >    if (call gcc-does-not-support-addr-global)
> > >      KBUILD_CFLAGS_MODULE += -fPIC -fvisibility=hidden
> > >    endif
> > > 
> > Using the old toolchain (GCC 12) can successfully load the
> > nf_tables.ko
> > module after applying the above patch.
> I don't like such a hack..., can we consider using old relocation
> types when building by old toolchains?


I don't like the hack too.  I only developed it as an intellectual game.

We need to consider multiple combinations:

(1) Old GCC + old Binutils.  We need -mla-local-with-abs for
KBUILD_CFLAGS_MODULE.

(2) Old GCC + new Binutils.  We need -mla-local-with-abs for
KBUILD_CFLAGS_MODULE, *and* adding the support for
R_LARCH_ABS{_HI20,_LO12,64_LO20,64_HI12} in the kernel module loader.

(3) New GCC + old Binutils.  As new GCC should support our new attribute
(I now intend to send V2 patch to gcc-patches using "movable" as the
attribute name), no special action is needed.

Basically, we need:

(1) Handle R_LARCH_ABS{_HI20,_LO12,64_LO20,64_HI12} in the kernel module
loader.
(2) Add -Wa,-mla-local-with-abs into KBUILD_CFLAGS_MODULE if GCC version
is <= 12. 
-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University