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Date: Thu, 2 Jul 2020 16:27:13 -0700
From: Danny Lin <danny@...ag0n.dev>
To: Arnd Bergmann <arnd@...db.de>
Cc: Nathan Chancellor <natechancellor@...il.com>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Sami Tolvanen <samitolvanen@...gle.com>,
Kees Cook <keescook@...omium.org>,
Fangrui Song <maskray@...gle.com>, linux-arch@...r.kernel.org,
linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
Danny Lin <danny@...ag0n.dev>, stable@...r.kernel.org
Subject: [PATCH v2] vmlinux.lds.h: Coalesce transient LLVM dead code elimination sections
A recent LLVM 11 commit [1] made LLD stop implicitly coalescing some
temporary LLVM sections, namely .{data,bss}..compoundliteral.XXX:
[30] .data..compoundli PROGBITS ffffffff9ac9a000 19e9a000
000000000000cea0 0000000000000000 WA 0 0 32
[31] .rela.data..compo RELA 0000000000000000 40965440
0000000000001d88 0000000000000018 I 2238 30 8
[32] .data..compoundli PROGBITS ffffffff9aca6ea0 19ea6ea0
00000000000033c0 0000000000000000 WA 0 0 32
[33] .rela.data..compo RELA 0000000000000000 409671c8
0000000000000948 0000000000000018 I 2238 32 8
[...]
[2213] .bss..compoundlit NOBITS ffffffffa3000000 1d85c000
00000000000000a0 0000000000000000 WA 0 0 32
[2214] .bss..compoundlit NOBITS ffffffffa30000a0 1d85c000
0000000000000040 0000000000000000 WA 0 0 32
[...]
.{data,bss}..L<symbol name> sections are also created in some cases.
While there aren't any in this example, they should also be coalesced to
be safe in case some config or future LLVM change makes it start
creating more of those sections in the future. For example, enabling
global merging causes ..L_MergedGlobals sections to be created, but it's
likely that other changes will result in such sections as well.
While these extra sections don't typically cause any breakage, they do
inflate the vmlinux size due to the overhead of storing metadata for
thousands of extra sections.
It's also worth noting that for some reason, some downstream Android
kernels can't boot at all if these sections aren't coalesced.
This issue isn't limited to any specific architecture; it affects arm64
and x86 if CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is forced on.
Example on x86 allyesconfig:
Before: 2241 sections, 1171169 KiB
After: 56 sections, 1170972 KiB
[1] https://github.com/llvm/llvm-project/commit/9e33c096476ab5e02ab1c8442cc3cb4e32e29f17
Link: https://github.com/ClangBuiltLinux/linux/issues/958
Cc: stable@...r.kernel.org # v4.4+
Suggested-by: Fangrui Song <maskray@...gle.com>
Signed-off-by: Danny Lin <danny@...ag0n.dev>
Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
---
v2:
- Fixed swapped example sizes
- Added .{data,bss}..L* sections, since it looks like they're emitted
in some cases even when LTO is disabled
include/asm-generic/vmlinux.lds.h | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index db600ef218d7..737ecf782229 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -94,10 +94,11 @@
*/
#ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
#define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
-#define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..LPBX*
+#define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..LPBX* \
+ .data..compoundliteral* .data..L*
#define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]*
#define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]*
-#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]*
+#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]* .bss..compoundliteral* .bss..L*
#define SBSS_MAIN .sbss .sbss.[0-9a-zA-Z_]*
#else
#define TEXT_MAIN .text
--
2.27.0
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