linux-kernel - Re: [PATCH 00/22] add support for Clang LTO

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Message-ID: <2ac9e722-949b-aa92-3553-df1bf69bf9e5@molgen.mpg.de>
Date:   Tue, 14 Jul 2020 14:16:44 +0200
From:   Paul Menzel <pmenzel@...gen.mpg.de>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kbuild <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-pci@...r.kernel.org,
        x86@...nel.org
Subject: Re: [PATCH 00/22] add support for Clang LTO

Dear Sami,


Am 13.07.20 um 01:34 schrieb Sami Tolvanen:
> On Sat, Jul 11, 2020 at 9:32 AM Paul Menzel <pmenzel@...gen.mpg.de> wrote:
>> Thank you very much for sending these changes.
>>
>> Do you have a branch, where your current work can be pulled from? Your
>> branch on GitHub [1] seems 15 months old.
> 
> The clang-lto branch is rebased regularly on top of Linus' tree.
> GitHub just looks at the commit date of the last commit in the tree,
> which isn't all that informative.

Thank you for clearing this up, and sorry for not checking myself.

>> Out of curiosity, I applied the changes, allowed the selection for i386
>> (x86), and with Clang 1:11~++20200701093119+ffee8040534-1~exp1 from
>> Debian experimental, it failed with `Invalid absolute R_386_32
>> relocation: KERNEL_PAGES`:
> 
> I haven't looked at getting this to work on i386, which is why we only
> select ARCH_SUPPORTS_LTO for x86_64. I would expect there to be a few
> issues to address.
> 
>>>    arch/x86/tools/relocs vmlinux > arch/x86/boot/compressed/vmlinux.relocs;arch/x86/tools/relocs --abs-relocs vmlinux
>>> Invalid absolute R_386_32 relocation: KERNEL_PAGES
> 
> KERNEL_PAGES looks like a constant, so it's probably safe to ignore
> the absolute relocation in tools/relocs.c.

Thank you for pointing me to the right direction. I am happy to report, 
that with the diff below (no idea to what list to add the string), Linux 
5.8-rc5 with the LLVM/Clang/LTO patches on top, builds and boots on the 
ASRock E350M1.

```
diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
index 8f3bf34840cef..e91af127ed3c0 100644
--- a/arch/x86/tools/relocs.c
+++ b/arch/x86/tools/relocs.c
@@ -79,6 +79,7 @@ static const char * const 
sym_regex_kernel[S_NSYMTYPES] = {
         "__end_rodata_hpage_align|"
  #endif
         "__vvar_page|"
+       "KERNEL_PAGES|"
         "_end)$"
  };
```


Kind regards,

Paul