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

[Huacai Chen <chenhuacai@kernel.org>]

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