linux-kernel - [PATCH] kbuild: Merge module sections if and only if CONFIG_LTO

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Message-Id: <20210322234438.502582-1-seanjc@google.com>
Date:   Mon, 22 Mar 2021 16:44:38 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Sean Christopherson <seanjc@...gle.com>
Subject: [PATCH] kbuild: Merge module sections if and only if CONFIG_LTO_CLANG
 is enabled

Merge module sections only when using Clang LTO.  With gcc-10, 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.

The stale symbol table breaks gdb's function disassambler, 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>
Cc: Sami Tolvanen <samitolvanen@...gle.com>
Cc: Kees Cook <keescook@...omium.org>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---

This is obviously the quick and dirty approach to fixing the problem,
presumably there is a way to actually update the symbols, but that is far,
far outside my area of expertise.

IIUC how the disassemblers work, the section headers are correctly updated,
e.g. objdump displays the correct info, and even gdb's disassembler shows
the "correct" offset, it's just the symbol tables that are out of whack.

An earlier version of the LTO series did have exactly this #ifdef, but
it was dropped when no one objected to Kees' suggestion to unconditionally
merge sections.  https://lkml.kernel.org/r/202010141548.47CB1BC@keescook

 scripts/module.lds.S | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/scripts/module.lds.S b/scripts/module.lds.S
index 168cd27e6122..3580c6d02957 100644
--- a/scripts/module.lds.S
+++ b/scripts/module.lds.S
@@ -25,6 +25,7 @@ SECTIONS {
 	 * -ffunction-sections, which increases the size of the final module.
 	 * Merge the split sections in the final binary.
 	 */
+#ifdef CONFIG_LTO_CLANG
 	.bss : {
 		*(.bss .bss.[0-9a-zA-Z_]*)
 		*(.bss..L*)
@@ -41,6 +42,7 @@ SECTIONS {
 	}

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

 /* bring in arch-specific sections */
-- 
2.31.0.rc2.261.g7f71774620-goog