[PATCH 5.10 046/121] kbuild: lto: Merge module sections if and only if CONFIG_LTO_CLANG is enabled

Greg Kroah-Hartman
2022-02-21T09:26:59Z

From: Sean Christopherson <seanjc@google.com>

commit 6a3193cdd5e5b96ac65f04ee42555c216da332af upstream.

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
Cc: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
scripts/module.lds.S | 2 ++
1 file changed, 2 insertions(+)

--- a/scripts/module.lds.S
+++ b/scripts/module.lds.S
@@ -26,6 +26,7 @@ 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.
@@ -47,6 +48,7 @@ SECTIONS {
}

.text : { *(.text .text.[0-9a-zA-Z_]*) }
+#endif
}

/* bring in arch-specific sections */