linux-kernel - Re: [PATCH] x86: work around clang IAS bug referencing __force

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Message-ID: <CA+icZUUV-vXQFkhDrLTCtnCpuKR5BpZ_SG+9376aK2g+BcE-hA@mail.gmail.com>
Date:   Sun, 16 Aug 2020 11:37:37 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Peter Collingbourne <pcc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Zhenzhong Duan <zhenzhong.duan@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juergen Gross <jgross@...e.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order

GCC toolchain simply ignores if kaslr_64.o has __force_order means the
build ends up successfully whereas LLVM toolchain and IAS breaks and
the build stops and needs explicitly commit
df6d4f9db79c1a5d6f48b59db35ccd1e9ff9adfc ("x86/boot/compressed: Don't
declare __force_order in kaslr_64.c") reverted to fix this.
With the revert GCC toolchain is also fine.

Maybe it is good to revert that commit?

This is with [1]:

diff --git a/arch/x86/include/asm/special_insns.h
b/arch/x86/include/asm/special_insns.h
index 59a3e13204c3..e1c19c5ecd5e 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -17,7 +17,7 @@
  * all loads stores around it, which can hurt performance. Solution is to
  * use a variable and mimic reads and writes to it to enforce serialization
  */
-extern unsigned long __force_order;
+extern unsigned long __force_order __weak;

 void native_write_cr0(unsigned long val);

...and the patchset of "x86/boot: Remove run-time relocations from
compressed kernel" applied [3].

More details in [4].

- Sedat -

References:
[1] https://github.com/ClangBuiltLinux/linux/issues/1120#issuecomment-674182703
[2] https://git.kernel.org/linus/df6d4f9db79c1a5d6f48b59db35ccd1e9ff9adfc
[3] https://lore.kernel.org/patchwork/project/lkml/list/?series=456251
[4] https://github.com/ClangBuiltLinux/linux/issues/1120#issuecomment-674502114