linux-kernel - Re: [PATCH v6 13/29] arm64/build: Assert for unwanted sections

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Date:   Tue, 27 Oct 2020 14:56:59 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Russell King <linux@...linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Ingo Molnar <mingo@...hat.com>,
        James Morse <james.morse@....com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Borislav Petkov <bp@...e.de>,
        Peter Collingbourne <pcc@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v6 13/29] arm64/build: Assert for unwanted sections

On Tue, 27 Oct 2020 at 12:29, Ard Biesheuvel <ardb@...nel.org> wrote:
>
> On Tue, 27 Oct 2020 at 11:20, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> >
> > Hi Jean-Philippe,
> >
> > On Tue, Oct 27, 2020 at 11:09 AM Jean-Philippe Brucker
> > <jean-philippe@...aro.org> wrote:
> > > On Mon, Oct 26, 2020 at 06:38:46PM +0100, Ard Biesheuvel wrote:
> > > > > > > Note that even on plain be2881824ae9eb92, I get:
> > > > > > >
> > > > > > >     aarch64-linux-gnu-ld: Unexpected GOT/PLT entries detected!
> > > > > > >     aarch64-linux-gnu-ld: Unexpected run-time procedure linkages detected!
> > > > > > >
> > > > > > > The parent commit obviously doesn't show that (but probably still has
> > > > > > > the problem).
> > > > >
> > > > > Reverting both
> > > > > b3e5d80d0c48c0cc ("arm64/build: Warn on orphan section placement")
> > > > > be2881824ae9eb92 ("arm64/build: Assert for unwanted sections")
> > > > > seems to solve my problems, without any ill effects?
> > > > >
> > > >
> > > > I cannot reproduce the issue here with my distro GCC+binutils (Debian 8.3.0)
> > >
> > > I have the same problem with one of my debug configs and Linux v5.10-rc1,
> > > and can reproduce with the Debian 8.3.0 toolchain, by using the arm64
> > > defconfig and disabling CONFIG_MODULES:
> > >
> > > ld -EL -maarch64elf --no-undefined -X -z norelro -shared -Bsymbolic -z notext --no-apply-dynamic-relocs --fix-cortex-a53-843419 --orphan-handling=warn --build-id=sha1 --strip-debug -o .tmp_vmlinux.kallsyms1 -T ./arch/arm64/kernel/vmlinux.lds --whole-archive arch/arm64/kernel/head.o init/built-in.a usr/built-in.a arch/arm64/built-in.a kernel/built-in.a certs/built-in.a mm/built-in.a fs/built-in.a ipc/built-in.a security/built-in.a crypto/built-in.a block/built-in.a arch/arm64/lib/built-in.a lib/built-in.a drivers/built-in.a sound/built-in.a net/built-in.a virt/built-in.a --no-whole-archive --start-group arch/arm64/lib/lib.a lib/lib.a ./drivers/firmware/efi/libstub/lib.a --end-group
> > > ld: Unexpected GOT/PLT entries detected!
> > > ld: Unexpected run-time procedure linkages detected!
> > >
> > > Adding -fno-pie to this command doesn't fix the problem.
> > >
> > > Note that when cross-building with a GCC 10.2 and binutils 2.35.1 I also
> > > get several "aarch64-linux-gnu-ld: warning: -z norelro ignored" in
> > > addition to the error, but I don't get that warning with the 8.3.0
> > > toolchain.
> >
> > Thanks, my config (renesas_defconfig) also had CONFIG_MODULES disabled.
> > Enabling that fixes the link error due to unexpected entries, but the
> > .eh_frame orphan section warning is still there.
> >
>
> Looks like this is caused by the VFIO driver doing nasty things with
> symbol_get(), resulting in weak symbol references being emitted. Since
> taking the address of a weak symbol can yield NULL, the only way for
> the linker to accommodate this is to use GOT indirection for the
> direct symbol reference, so that the GOT entry can be set to NULL if
> the reference is not satisfied at link time.

This seems to do the trick for me.

diff --git a/include/linux/module.h b/include/linux/module.h
index 7ccdf87f376f..6264617bab4d 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -740,7 +740,7 @@ static inline bool within_module(unsigned long
addr, const struct module *mod)
 }

 /* Get/put a kernel symbol (calls should be symmetric) */
-#define symbol_get(x) ({ extern typeof(x) x __attribute__((weak)); &(x); })
+#define symbol_get(x) ({ extern typeof(x) x
__attribute__((weak,visibility("hidden"))); &(x); })
 #define symbol_put(x) do { } while (0)
 #define symbol_put_addr(x) do { } while (0)