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@xxxxxxxxxxxxxxx # v4.4+
> Suggested-by: Fangrui Song <maskray@xxxxxxxxxx>
> Signed-off-by: Danny Lin <danny@xxxxxxxxxxx>
> ---
> 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
