Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <32a74a218c76611f897fd1df1ad0059068621133.camel@xry111.site>
 <ec52fd49-4a30-15d9-3d32-fd7bc6d8b3f0@loongson.cn> <0179679b736aea7258981dec2d83107cce74dfc1.camel@xry111.site>
 <a139a8475fe295ac9f17064269cd0312dca6f96e.camel@xry111.site>
 <6b5d2188f93ed72b67a4bbb7116113f833fe1ee5.camel@xry111.site>
 <d7670b60-2782-4642-995b-7baa01779a66@loongson.cn> <7cad6e78014168a8906e130e1cf3809077d2bda7.camel@xry111.site>
 <1d0783b87bda3e454a111862fcc5b5faffcb16b0.camel@xry111.site>
 <00eede4b1380888a500f74b1e818bb25a550632b.camel@xry111.site>
 <CAAhV-H7WxRp4u1iBs47LN1Sj3rDBdbLv1u7EpSt0Bt4QuhFSXQ@mail.gmail.com>
 <674cb3e9-d820-016b-a210-afd37ed6e25e@loongson.cn> <5233653cceab9c2baf6510bd712cd53ef63e3aac.camel@xry111.site>
In-Reply-To: <5233653cceab9c2baf6510bd712cd53ef63e3aac.camel@xry111.site>
From:   Huacai Chen <chenhuacai@kernel.org>
Date:   Sat, 30 Jul 2022 18:38:53 +0800
Message-ID: <CAAhV-H6jAxOx1xfa6N8F5L0Oc-sp9KimQN-Sen6YPDjnRVEMuQ@mail.gmail.com>
Subject: Re: [PATCH v4 0/4] LoongArch: Support new relocation types
To:     Xi Ruoyao <xry111@xry111.site>
Cc:     Lulu Cheng <chenglulu@loongson.cn>,
        Youling Tang <tangyouling@loongson.cn>,
        loongarch@lists.linux.dev, LKML <linux-kernel@vger.kernel.org>,
        WANG Xuerui <kernel@xen0n.name>,
        Jinyang He <hejinyang@loongson.cn>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Sat, Jul 30, 2022 at 5:51 PM Xi Ruoyao <xry111@xry111.site> wrote:
>
> On Sat, 2022-07-30 at 14:44 +0800, Lulu Cheng wrote:
> > > > 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.
> > > >
> > > Sadly, I don't know what visibility is, does it have something to do
> > > with __visible in include/linux/compiler_attributes.h?
>
> They are different definitions of visibility and mostly unrelated.
> Unfortunately humans do not have enough words in the language to
> disambiguate those different concepts :).
>
> -fvisibility and __attribute__((visibility)) are for ELF shared objects.
> Kernel developers usually do not need to take care of them (unless
> working on VDSO).
>
> -fvisibility=default (yes, it's the default) makes the symbol "possible
> to be interposed" while -fPIC.  Say
>
> $ cat main.c
> extern int f(void);
> extern int printf(const char *, ...);
> int x = 1;
> int main() { printf("%d\n", f()); }
> $ cat shared.c
> int x = 42;
> int f(void) { return x; }
> $ cc shared.c -fPIC -shared -o libshared.so
> $ cc main.c -L. -Wl,-rpath,. -lshared
> $ ./a.out
> 1
>
> You may think it strange but it's so-called "symbol interposition"
> mandated by ELF spec.  To make it work, the compiler has to use GOT
> access for "x" instead of PC-relative access.
>
> OTOH, a "hidden" visibility disallows interposition:
>
> $ cat shared-1.c
> __attribute__((visbility("hidden"))) int x = 42;
> int f(void) { return x; }
> $ cc shared-1.c -fPIC -shared -o libshared.so
> $ ./a.out
> 42
>
> Now the compiler will use PC-relative access for "x" in "f".
>
> In my hack the combination of "-fPIC" and
> "__attribute__((visibility("default")))" for per-CPU symbols makes per-
> CPU symbols accessed via GOT, and "-fvisibility=hidden" keeps normal
> symbols accessed via PC-relative within a TU.
>
> Note that the visibility of a symbol is also recorded in the symtab, and
> ld.so will refuse to access a hidden symbol in one shared object from
> another.  But the kernel module loader just doesn't care the visibility
> field in symtab so it won't affect us.
>
> Basically the hack just uses visibility options & attributes *in a way
> they are not designed for* to trick the compiler to emit GOT accesses
> for per-CPU symbols.  A new attribute ("get_through_got"/"movable" or
> whatever) is definitely wanted to avoid such a tricky approach, but the
> hack can be used if we want modular kernel able to be built with GCC 12.
So it has nothing to do with __visible in include/linux/compiler_attributes.h?
Or __visible is a similar thing that used by Linux kernel?

Huacai
> --
> Xi Ruoyao <xry111@xry111.site>
> School of Aerospace Science and Technology, Xidian University