UPSTREAM: kbuild: lto: Merge module sections if and only if CONFIG_LTO_CLANG is enabled

Merge module sections only when using Clang LTO. With ld.bfd, merging
sections does not appear to update the symbol tables for the module,
e.g. 'readelf -s' shows the value that a symbol would have had, if
sections were not merged. ld.lld does not show this problem.

The stale symbol table breaks gdb's function disassembler, and presumably
other things, e.g.

  gdb -batch -ex "file arch/x86/kvm/kvm.ko" -ex "disassemble kvm_init"

reads the wrong bytes and dumps garbage.

Fixes: dd2776222abb ("kbuild: lto: merge module sections")
Cc: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210322234438.502582-1-seanjc@google.com
(cherry picked from commit 6a3193cdd5e5b96ac65f04ee42555c216da332af)
Bug: 187129171
Signed-off-by: Connor O'Brien <connoro@google.com>
Change-Id: I295d9a6b71a41fd1585059d0f48a2a2c13eb2361

scripts/module.lds.S

@@ -33,6 +33,7 @@ SECTIONS {
 
 	__patchable_function_entries : { *(__patchable_function_entries) }
 
+#ifdef CONFIG_LTO_CLANG
 	/*
 	 * With CONFIG_LTO_CLANG, LLD always enables -fdata-sections and
 	 * -ffunction-sections, which increases the size of the final module.
@@ -66,6 +67,7 @@ SECTIONS {
 		__cfi_jt_end = .;
 	}
 #endif
+#endif
 }
 
 /* bring in arch-specific sections */