powerpc: Avoid dead code/data elimination when using recordmcount

Although powerpc now has objtool mcount support, it's not enabled in all
configurations due to dependencies.

On those configurations, with some linkers (binutils 2.37 at least),
it's still possible to hit the dreaded "recordmcount bug", eg. errors
such as:

    CC      kernel/kexec_file.o
  Cannot find symbol for section 10: .text.unlikely.
  kernel/kexec_file.o: failed
  make[1]: *** [scripts/Makefile.build:287 : kernel/kexec_file.o] Error 1

Those errors are much more prevalent when building with
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION, because it places every function
in a separate section.

CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is marked experimental and is not
enabled in any powerpc defconfigs or by major distros. Although it does
have at least some users on 32-bit where kernel size tends to be more
important.

Avoid the build errors by blocking CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
when the build is using recordmcount, rather than objtool. In practice
that means for 64-bit big endian builds, or 64-bit clang builds - both
because they lack CONFIG_MPROFILE_KERNEL.

On 32-bit objtool is always used, so
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is still available there.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230221130331.2714199-1-mpe@ellerman.id.au

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 2c9cdf1..a6c4407 100644 (file)
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -236,7 +236,7 @@ config PPC
        select HAVE_KPROBES
        select HAVE_KPROBES_ON_FTRACE
        select HAVE_KRETPROBES
-       select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
+       select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if HAVE_OBJTOOL_MCOUNT
        select HAVE_LIVEPATCH                   if HAVE_DYNAMIC_FTRACE_WITH_REGS
        select HAVE_MOD_ARCH_SPECIFIC
        select HAVE_NMI                         if PERF_EVENTS || (PPC64 && PPC_BOOK3S)