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ld.lld: warning: init/built-in.a(main.o):(.ARM.extab) is being placed in '.ARM.extab' #1152

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nathanchance opened this issue Sep 10, 2020 · 3 comments
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ld.lld: warning: init/built-in.a(main.o):(.ARM.extab) is being placed in '.ARM.extab' #1152

nathanchance opened this issue Sep 10, 2020 · 3 comments
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@nathanchance
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[ARCH] arm32 This bug impacts ARCH=arm [BUG] linux A bug that should be fixed in the mainline kernel. [FIXED][LINUX] 5.10 This bug was fixed in Linux 5.10 Orphan section An linker orphan section warning

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@nathanchance
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After the orphan section warnings series hit -next, 32-bit ARM kernels that use CONFIG_UNWINDER_FRAME_POINTER instead of CONFIG_UNWINDER_ARM (which distributions such as Debian and OpenSUSE do) show hundreds of warnings:

$ make -skj"$(nproc)" ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- LLVM=1 O=out/arm32 distclean defconfig

$ scripts/config --file out/arm32/.config -d CONFIG_UNWINDER_ARM -e CONFIG_UNWINDER_FRAME_POINTER

$ make -skj"$(nproc)" ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- LLVM=1 O=out/arm32 olddefconfig zImage
...
ld.lld: warning: init/built-in.a(main.o):(.ARM.extab) is being placed in '.ARM.extab'
ld.lld: warning: init/built-in.a(main.o):(.ARM.extab.init.text) is being placed in '.ARM.extab.init.text'
ld.lld: warning: init/built-in.a(main.o):(.ARM.extab.ref.text) is being placed in '.ARM.extab.ref.text'
ld.lld: warning: init/built-in.a(do_mounts.o):(.ARM.extab.init.text) is being placed in '.ARM.extab.init.text'
ld.lld: warning: init/built-in.a(do_mounts.o):(.ARM.extab) is being placed in '.ARM.extab'
ld.lld: warning: init/built-in.a(do_mounts_rd.o):(.ARM.extab.init.text) is being placed in '.ARM.extab.init.text'
ld.lld: warning: init/built-in.a(do_mounts_rd.o):(.ARM.extab) is being placed in '.ARM.extab'
ld.lld: warning: init/built-in.a(do_mounts_initrd.o):(.ARM.extab.init.text) is being placed in '.ARM.extab.init.text'
ld.lld: warning: init/built-in.a(initramfs.o):(.ARM.extab.init.text) is being placed in '.ARM.extab.init.text'
ld.lld: warning: init/built-in.a(initramfs.o):(.ARM.extab) is being placed in '.ARM.extab'
ld.lld: warning: init/built-in.a(calibrate.o):(.ARM.extab.init.text) is being placed in '.ARM.extab.init.text'
ld.lld: warning: init/built-in.a(calibrate.o):(.ARM.extab) is being placed in '.ARM.extab'
...

It looks like these sections are only dealt with when CONFIG_ARM_UNWIND is set, which is a hidden symbol selected by CONFIG_UNWINDER_ARM: https://elixir.bootlin.com/linux/v5.9-rc4/source/arch/arm/kernel/vmlinux.lds.S#L81

Maybe something like this?

diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
index 5f4922e858d0..a2c0d96b0580 100644
--- a/arch/arm/kernel/vmlinux.lds.S
+++ b/arch/arm/kernel/vmlinux.lds.S
@@ -40,6 +40,10 @@ SECTIONS
 		ARM_DISCARD
 #ifndef CONFIG_SMP_ON_UP
 		*(.alt.smp.init)
+#endif
+#ifndef CONFIG_ARM_UNWIND
+		*(.ARM.exidx*)
+		*(.ARM.extab*)
 #endif
 	}

cc @kees
@nathanchance nathanchance added [ARCH] arm32 This bug impacts ARCH=arm [BUG] linux-next This is an issue only seen in linux-next labels Sep 10, 2020
@nickdesaulniers
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cc @kees so I probably got that wrong in upstream review. Been spending too much time with CONFIG_UNWINDER_FRAME_POINTER on arm32...

@nathanchance
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Patch submitted for further review: https://lore.kernel.org/r/20200928224854.3224862-1-natechancellor@gmail.com

@nathanchance nathanchance self-assigned this Sep 28, 2020
@nathanchance nathanchance added the [PATCH] Submitted A patch has been submitted for review label Sep 28, 2020
torvalds pushed a commit to torvalds/linux that referenced this issue Oct 27, 2020
@nathanchance @kees
arm/build: Always handle .ARM.exidx and .ARM.extab sections
c39866f 
After turning on warnings for orphan section placement, enabling
CONFIG_UNWINDER_FRAME_POINTER instead of CONFIG_UNWINDER_ARM causes
thousands of warnings when clang + ld.lld are used:

$ scripts/config --file arch/arm/configs/multi_v7_defconfig \
                 -d CONFIG_UNWINDER_ARM \
                 -e CONFIG_UNWINDER_FRAME_POINTER
$ make -skj"$(nproc)" ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- LLVM=1 defconfig zImage
ld.lld: warning: init/built-in.a(main.o):(.ARM.extab) is being placed in '.ARM.extab'
ld.lld: warning: init/built-in.a(main.o):(.ARM.extab.init.text) is being placed in '.ARM.extab.init.text'
ld.lld: warning: init/built-in.a(main.o):(.ARM.extab.ref.text) is being placed in '.ARM.extab.ref.text'
ld.lld: warning: init/built-in.a(do_mounts.o):(.ARM.extab.init.text) is being placed in '.ARM.extab.init.text'
ld.lld: warning: init/built-in.a(do_mounts.o):(.ARM.extab) is being placed in '.ARM.extab'
ld.lld: warning: init/built-in.a(do_mounts_rd.o):(.ARM.extab.init.text) is being placed in '.ARM.extab.init.text'
ld.lld: warning: init/built-in.a(do_mounts_rd.o):(.ARM.extab) is being placed in '.ARM.extab'
ld.lld: warning: init/built-in.a(do_mounts_initrd.o):(.ARM.extab.init.text) is being placed in '.ARM.extab.init.text'
ld.lld: warning: init/built-in.a(initramfs.o):(.ARM.extab.init.text) is being placed in '.ARM.extab.init.text'
ld.lld: warning: init/built-in.a(initramfs.o):(.ARM.extab) is being placed in '.ARM.extab'
ld.lld: warning: init/built-in.a(calibrate.o):(.ARM.extab.init.text) is being placed in '.ARM.extab.init.text'
ld.lld: warning: init/built-in.a(calibrate.o):(.ARM.extab) is being placed in '.ARM.extab'

These sections are handled by the ARM_UNWIND_SECTIONS define, which is
only added to the list of sections when CONFIG_ARM_UNWIND is set.
CONFIG_ARM_UNWIND is a hidden symbol that is only selected when
CONFIG_UNWINDER_ARM is set so CONFIG_UNWINDER_FRAME_POINTER never
handles these sections. According to the help text of
CONFIG_UNWINDER_ARM, these sections should be discarded so that the
kernel image size is not affected.

Fixes: 5a17850 ("arm/build: Warn on orphan section placement")
Link: ClangBuiltLinux#1152
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Review-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
[kees: Made the discard slightly more specific]
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200928224854.3224862-1-natechancellor@gmail.com
@nathanchance
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Fixed, thanks @kees!

https://git.kernel.org/linus/c39866f268f89868df17724cd2262d121552d8c9

@nathanchance nathanchance added [BUG] linux A bug that should be fixed in the mainline kernel. [FIXED][LINUX] 5.10 This bug was fixed in Linux 5.10 and removed [BUG] linux-next This is an issue only seen in linux-next [PATCH] Submitted A patch has been submitted for review labels Oct 27, 2020
@nathanchance nathanchance added the Orphan section An linker orphan section warning label May 6, 2021
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[ARCH] arm32 This bug impacts ARCH=arm [BUG] linux A bug that should be fixed in the mainline kernel. [FIXED][LINUX] 5.10 This bug was fixed in Linux 5.10 Orphan section An linker orphan section warning
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