Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[Sedat Dilek]

On Fri, Feb 26, 2021 at 11:02 AM Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote:
>
> On Fri, Feb 26, 2021 at 10:51 AM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
> >
> > On Fri, Feb 26, 2021 at 10:05 AM Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote:
> > > On Fri, Feb 26, 2021 at 9:14 AM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
> > >
> > > > > BTW, is CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y setable for x86 (64bit)?
> > > > > ( Did not look or check for it. )
> > > >
> > > > No, in mainline, HAVE_LD_DEAD_CODE_DATA_ELIMINATION is currently
> > > > only selected on MIPS and PowerPC. I only sent experimental patches to
> > > > enable it on arm64 and m68k, but have not tried booting them. If you
> > > > select the symbol on x86, you should see similar results.
> > > >
> > >
> > > OK, i see:
> > >
> > > $ git grep HAVE_LD_DEAD_CODE_DATA_ELIMINATION arch/mips/
> > > arch/mips/Kconfig:      select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
> > >
> > > $ git grep HAVE_LD_DEAD_CODE_DATA_ELIMINATION arch/powerpc/
> > > arch/powerpc/Kconfig:   select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
> > >
> > > So, I need to add this to arch/x86/Kconfig.
> > >
> > > You happen to know if changes to arch/x86/kernel/vmlinux.lds.S
> > > (sections) are needed?
> >
> > No idea. I'm still debugging a possible regression on arm64, but both
> > issues I found for arm64 are specific to that architecture and won't
> > happen on x86. It's likely that something else breaks though.
> >
>
> My first experiences with gc were with the freetz project (mips
> embedded router) and don't ask me what GCC version.
>
> I will try with gc + trim + cfi later after my current build has finished.
>
> > > Last question:
> > > The last days I see a lot of fixes touching inlining with LLVM/Clang v13-git.
> > > What git tag are you using?
> > > What are your experiences?
> > > Pending patches (kernel-side)?
> > >
> > > I use:
> > > $ /opt/llvm-toolchain/bin/clang --version
> > > dileks clang version 13.0.0 (https://github.com/llvm/llvm-project.git
> > > c465429f286f50e52a8d2b3b39f38344f3381cce)
> >
> > This is what I have on the build box:
> > Ubuntu clang version
> > 13.0.0-++20210223104451+ebca13c66504-1~exp1~20210223095200.234
> >
>
> Distro-clang takes much longer here.
> Selfmade stage1-only LLVM toolchain compiles here 10% faster.
> cfi takes approx. 20% longer.
> With trim + gc I suppose it will take much longer.
>
> Let me test.
> Will report later.
>

With the attached patch and diff (some modules need whitelisted
symbols) I was able to compile an x86-64 Linux-kernel.
It does not boot on bare metal.

If someone wants to play with it.

- Sedat -
From d5fce344393bb76b2370f09a0eb80a137c0047f7 Mon Sep 17 00:00:00 2001
From: Sedat Dilek <sedat.dilek@xxxxxxxxx>
Date: Fri, 26 Feb 2021 14:32:12 +0100
Subject: [PATCH] x86: kconfig: Enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

Signed-off-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
---
 arch/x86/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index cd4b9b1204a8..d5f5f296f92f 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -205,6 +205,7 @@ config X86
 	select HAVE_FUNCTION_ERROR_INJECTION
 	select HAVE_KRETPROBES
 	select HAVE_KVM
+	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
 	select HAVE_LIVEPATCH			if X86_64
 	select HAVE_MIXED_BREAKPOINTS_REGS
 	select HAVE_MOD_ARCH_SPECIFIC
-- 
2.30.1

diff --git a/scripts/gen_autoksyms.sh b/scripts/gen_autoksyms.sh
index da320151e7c3..4e3360c6e5ea 100755
--- a/scripts/gen_autoksyms.sh
+++ b/scripts/gen_autoksyms.sh
@@ -36,7 +36,28 @@ if [ -n "$CONFIG_LTO_CLANG" ]; then
 	needed_symbols="$needed_symbols _mcount"
 	# stack protector symbols
 	needed_symbols="$needed_symbols __stack_chk_fail __stack_chk_guard"
-fi
+    if [ -n "CONFIG_LD_DEAD_CODE_DATA_ELIMINATION" ] && [ -n "CONFIG_TRIM_UNUSED_KSYMS" ]; then
+        # ex_handler_XXX symbols
+        needed_symbols="$needed_symbols ex_handler_uaccess ex_handler_clear_fs ex_handler_fprestore ex_handler_rdmsr_unsafe ex_handler_wrmsr_unsafe"
+        # __fentry__ symbol
+        needed_symbols="$needed_symbols __fentry__"
+        # __get_user_XXX and __put_user_XXX symbols
+        needed_symbols="$needed_symbols __get_user_1 __get_user_2 __get_user_4 __get_user_8"
+        needed_symbols="$needed_symbols __put_user_1 __put_user_2 __put_user_4 __put_user_8"
+        needed_symbols="$needed_symbols __get_user_nocheck_1 __get_user_nocheck_2 __get_user_nocheck_4 __get_user_nocheck_8"
+        needed_symbols="$needed_symbols __put_user_nocheck_1 __put_user_nocheck_2 __put_user_nocheck_4 __put_user_nocheck_8"
+        # __sw_hweight32 and __sw_hweight64 symbols
+        needed_symbols="$needed_symbols __sw_hweight32 __sw_hweight64"
+        # __x86_indirect_thunk_r11 symbol
+        needed_symbols="$needed_symbols __x86_indirect_thunk_r11"
+        # __x86_retpoline_rax and __x86_retpoline_rsi symbols
+        needed_symbols="$needed_symbols __x86_retpoline_rax __x86_retpoline_rsi"
+    fi #  CONFIG_LD_DEAD_CODE_DATA_ELIMINATION && CONFIG_TRIM_UNUSED_KSYMS
+    if [ -n "$CONFIG_CFI_CLANG" ]; then
+        # __ubsan_handle_cfi_check_fail_abort symbol
+        needed_symbols="$needed_symbols __ubsan_handle_cfi_check_fail_abort"
+    fi # CONFIG_CFI_CLANG
+fi # CONFIG_LTO_CLANG
 
 ksym_wl=
 if [ -n "$CONFIG_UNUSED_KSYMS_WHITELIST" ]; then

Attachment: config-5.11.0-11603.3-amd64-clang13-cfi
Description: Binary data