[PATCH v2] Makefile: fix GDB warning with CONFIG_RELR

[PATCH v2] Makefile: fix GDB warning with CONFIG_RELR

[Nick Desaulniers]

GDB produces the following warning when debugging kernels built with
CONFIG_RELR:

BFD: /android0/linux-next/vmlinux: unknown type [0x13] section `.relr.dyn'

when loading a kernel built with CONFIG_RELR into GDB. It can also
prevent debugging symbols using such relocations.

Peter sugguests:
  [That flag] means that lld will use dynamic tags and section type
  numbers in the OS-specific range rather than the generic range. The
  kernel itself doesn't care about these numbers; it determines the
  location of the RELR section using symbols defined by a linker script.

Link: https://github.com/ClangBuiltLinux/linux/issues/1057
Suggested-by: Peter Collingbourne <pcc@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Changes V1 -> V2:
* rebase
* pick up Nathan's reviewed by tag.

 Makefile                      | 2 +-
 scripts/tools-support-relr.sh | 3 ++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index 0ed7e061c8e9..2dbb56f831d4 100644
--- a/Makefile
+++ b/Makefile
@@ -1031,7 +1031,7 @@ LDFLAGS_vmlinux	+= $(call ld-option, -X,)
 endif
 
 ifeq ($(CONFIG_RELR),y)
-LDFLAGS_vmlinux	+= --pack-dyn-relocs=relr
+LDFLAGS_vmlinux	+= --pack-dyn-relocs=relr --use-android-relr-tags
 endif
 
 # We never want expected sections to be placed heuristically by the
diff --git a/scripts/tools-support-relr.sh b/scripts/tools-support-relr.sh
index 45e8aa360b45..cb55878bd5b8 100755
--- a/scripts/tools-support-relr.sh
+++ b/scripts/tools-support-relr.sh
@@ -7,7 +7,8 @@ trap "rm -f $tmp_file.o $tmp_file $tmp_file.bin" EXIT
 cat << "END" | $CC -c -x c - -o $tmp_file.o >/dev/null 2>&1
 void *p = &p;
 END
-$LD $tmp_file.o -shared -Bsymbolic --pack-dyn-relocs=relr -o $tmp_file
+$LD $tmp_file.o -shared -Bsymbolic --pack-dyn-relocs=relr \
+  --use-android-relr-tags -o $tmp_file
 
 # Despite printing an error message, GNU nm still exits with exit code 0 if it
 # sees a relr section. So we need to check that nothing is printed to stderr.

base-commit: 45af60e7ced07ae3def41368c3d260dbf496fbce
-- 
2.31.1.818.g46aad6cb9e-goog


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Re: [PATCH v2] Makefile: fix GDB warning with CONFIG_RELR

[Will Deacon]

On Fri, May 21, 2021 at 06:26:24PM -0700, Nick Desaulniers wrote:
> GDB produces the following warning when debugging kernels built with
> CONFIG_RELR:
> 
> BFD: /android0/linux-next/vmlinux: unknown type [0x13] section `.relr.dyn'
> 
> when loading a kernel built with CONFIG_RELR into GDB. It can also
> prevent debugging symbols using such relocations.
> 
> Peter sugguests:
>   [That flag] means that lld will use dynamic tags and section type
>   numbers in the OS-specific range rather than the generic range. The
>   kernel itself doesn't care about these numbers; it determines the
>   location of the RELR section using symbols defined by a linker script.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1057
> Suggested-by: Peter Collingbourne <pcc@google.com>
> Reviewed-by: Nathan Chancellor <nathan@kernel.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> Changes V1 -> V2:
> * rebase
> * pick up Nathan's reviewed by tag.
> 
>  Makefile                      | 2 +-
>  scripts/tools-support-relr.sh | 3 ++-
>  2 files changed, 3 insertions(+), 2 deletions(-)

Does lld support RELR relocations for any architectures other than arm64? If
so, is the "--use-android-relr-tags" option supported on all of those as
well?

Will

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Re: [PATCH v2] Makefile: fix GDB warning with CONFIG_RELR

[Nick Desaulniers]

On Wed, May 26, 2021 at 10:09 AM Will Deacon <will@kernel.org> wrote:
>
> On Fri, May 21, 2021 at 06:26:24PM -0700, Nick Desaulniers wrote:
> > GDB produces the following warning when debugging kernels built with
> > CONFIG_RELR:
> >
> > BFD: /android0/linux-next/vmlinux: unknown type [0x13] section `.relr.dyn'
> >
> > when loading a kernel built with CONFIG_RELR into GDB. It can also
> > prevent debugging symbols using such relocations.
> >
> > Peter sugguests:
> >   [That flag] means that lld will use dynamic tags and section type
> >   numbers in the OS-specific range rather than the generic range. The
> >   kernel itself doesn't care about these numbers; it determines the
> >   location of the RELR section using symbols defined by a linker script.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1057
> > Suggested-by: Peter Collingbourne <pcc@google.com>
> > Reviewed-by: Nathan Chancellor <nathan@kernel.org>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> > ---
> > Changes V1 -> V2:
> > * rebase
> > * pick up Nathan's reviewed by tag.
> >
> >  Makefile                      | 2 +-
> >  scripts/tools-support-relr.sh | 3 ++-
> >  2 files changed, 3 insertions(+), 2 deletions(-)
>
> Does lld support RELR relocations for any architectures other than arm64? If

Yes; from what I can tell it's not an architecture specific relocation
type. Combing through LLVM's sources, it seems Fuchsia sets it always
(at least when using lld) and I'm pretty sure they support x86.  At
least I don't get any errors out of LLD when building with
--pack-dyn-relocs=relr on x86.

I can force on RELR for x86 kernel builds with:
```
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 0045e1b44190..513272c77827 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2117,6 +2117,7 @@ config PHYSICAL_START

 config RELOCATABLE
        bool "Build a relocatable kernel"
+       select ARCH_HAS_RELR
        default y
        help
          This builds a kernel image that retains relocation information
```
That builds (it won't boot because we don't have the machinery in the
kernel to self relocate that type, yet).

> so, is the "--use-android-relr-tags" option supported on all of those as
> well?

I believe so; no issues building with this patch and with the above
diff applied on x86. All that flag does is change the elf section type
from SHT_RELR to SHT_ANDROID_RELR.

pcc@ can correct me if I'm wrong.
-- 
Thanks,
~Nick Desaulniers

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Re: [PATCH v2] Makefile: fix GDB warning with CONFIG_RELR

[Will Deacon]

On Fri, 21 May 2021 18:26:24 -0700, Nick Desaulniers wrote:
> GDB produces the following warning when debugging kernels built with
> CONFIG_RELR:
> 
> BFD: /android0/linux-next/vmlinux: unknown type [0x13] section `.relr.dyn'
> 
> when loading a kernel built with CONFIG_RELR into GDB. It can also
> prevent debugging symbols using such relocations.
> 
> [...]

Applied to arm64 (for-next/build), thanks!

[1/1] Makefile: fix GDB warning with CONFIG_RELR
      https://git.kernel.org/arm64/c/27f2a4db76e8

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev

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