[PATCH] powerpc: Fix build issue with LD_DEAD_CODE_DATA_ELIMINATION and FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY

We recently added support for -fpatchable-function-entry and it is
enabled by default on ppc32 (ppc64 needs gcc v13.1.0). When building the
kernel for ppc32 and also enabling CONFIG_LD_DEAD_CODE_DATA_ELIMINATION,
we see the below build error with older gcc versions:
  powerpc-linux-gnu-ld: init/main.o(__patchable_function_entries): error: need linked-to section for --gc-sections

This error is thrown since __patchable_function_entries section would be
garbage collected with --gc-sections since it does not reference any
other kept sections. This has subsequently been fixed with:
  https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=b7d072167715829eed0622616f6ae0182900de3e

Disable LD_DEAD_CODE_DATA_ELIMINATION for gcc versions before v11.1.0 if
using -fpatchable-function-entry to avoid this bug.

Fixes: 0f71dcfb4aef ("powerpc/ftrace: Add support for -fpatchable-function-entry")
Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Naveen N Rao <naveen@kernel.org>
---
 arch/powerpc/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 54b9387c3691..3aaadfd2c8eb 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig

@@ -255,7 +255,7 @@ config PPC
 	select HAVE_KPROBES
 	select HAVE_KPROBES_ON_FTRACE
 	select HAVE_KRETPROBES
-	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if HAVE_OBJTOOL_MCOUNT
+	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if HAVE_OBJTOOL_MCOUNT && (!ARCH_USING_PATCHABLE_FUNCTION_ENTRY || (!CC_IS_GCC || GCC_VERSION >= 110100))
 	select HAVE_LIVEPATCH			if HAVE_DYNAMIC_FTRACE_WITH_REGS
 	select HAVE_MOD_ARCH_SPECIFIC
 	select HAVE_NMI				if PERF_EVENTS || (PPC64 && PPC_BOOK3S)

base-commit: 0ad6bbfc7dab179bb3de79190140acec2862934d
-- 
2.41.0

On Wed, 13 Sep 2023 19:11:29 +0530, Naveen N Rao wrote:

We recently added support for -fpatchable-function-entry and it is
enabled by default on ppc32 (ppc64 needs gcc v13.1.0). When building the
kernel for ppc32 and also enabling CONFIG_LD_DEAD_CODE_DATA_ELIMINATION,
we see the below build error with older gcc versions:
  powerpc-linux-gnu-ld: init/main.o(__patchable_function_entries): error: need linked-to section for --gc-sections

This error is thrown since __patchable_function_entries section would be
garbage collected with --gc-sections since it does not reference any
other kept sections. This has subsequently been fixed with:
  https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=b7d072167715829eed0622616f6ae0182900de3e

[...]

Applied to powerpc/fixes.

[1/1] powerpc: Fix build issue with LD_DEAD_CODE_DATA_ELIMINATION and FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY
      https://git.kernel.org/powerpc/c/60d77ed24bb3068c0837fe45b8921b0a6598829d

cheers