linux-kernel - [PATCH 5.10 046/121] kbuild: lto: Merge module sections if and only if CONFIG_LTO

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Message-Id: <20220221084922.767725960@linuxfoundation.org>
Date:   Mon, 21 Feb 2022 09:48:58 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Nick Desaulniers <ndesaulniers@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Stephen Boyd <swboyd@...omium.org>
Subject: [PATCH 5.10 046/121] kbuild: lto: Merge module sections if and only if CONFIG_LTO_CLANG is enabled

From: Sean Christopherson <seanjc@...gle.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@...gle.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
Reviewed-by: Sami Tolvanen <samitolvanen@...gle.com>
Tested-by: Sami Tolvanen <samitolvanen@...gle.com>
Signed-off-by: Kees Cook <keescook@...omium.org>
Link: https://lore.kernel.org/r/20210322234438.502582-1-seanjc@google.com
Cc: Stephen Boyd <swboyd@...omium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.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 */