linux-kernel - Re: [PATCH] vmlinux.lds.h: Coalesce transient LLVM dead code elimination sections

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Date:   Thu, 2 Jul 2020 08:54:53 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Danny Lin <danny@...ag0n.dev>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Fangrui Song <maskray@...gle.com>, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        stable@...r.kernel.org
Subject: Re: [PATCH] vmlinux.lds.h: Coalesce transient LLVM dead code
 elimination sections

On Thu, Jul 02, 2020 at 01:54:00AM -0700, Danny Lin wrote:
> 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
>   [...]
> 
> 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, 1170972 KiB
>     After:    56 sections, 1171169 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>
> ---
>  include/asm-generic/vmlinux.lds.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index db600ef218d7..18968cba87c7 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -94,10 +94,10 @@
>   */
>  #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*
>  #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*

Are there .data.. and .bss.. sections we do NOT want to collect? i.e.
why not include .data..* and .bss..* ?

-- 
Kees Cook