Re: [PATCH v9 06/16] kbuild: lto: add a default list of used symbols

[Kees Cook <keescook@chromium.org>]

On Fri, Dec 11, 2020 at 10:46:23AM -0800, Sami Tolvanen wrote:
> With CONFIG_LTO_CLANG, LLVM bitcode has not yet been compiled into a
> binary when the .mod files are generated, which means they don't yet
> contain references to certain symbols that will be present in the final
> binaries. This includes intrinsic functions, such as memcpy, memmove,
> and memset [1], and stack protector symbols [2]. This change adds a
> default symbol list to use with CONFIG_TRIM_UNUSED_KSYMS when Clang's
> LTO is used.
> 
> [1] https://llvm.org/docs/LangRef.html#standard-c-c-library-intrinsics
> [2] https://llvm.org/docs/LangRef.html#llvm-stackprotector-intrinsic
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

> ---
>  init/Kconfig                | 1 +
>  scripts/lto-used-symbollist | 5 +++++
>  2 files changed, 6 insertions(+)
>  create mode 100644 scripts/lto-used-symbollist
> 
> diff --git a/init/Kconfig b/init/Kconfig
> index 0872a5a2e759..e88c919c1bf1 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -2297,6 +2297,7 @@ config TRIM_UNUSED_KSYMS
>  config UNUSED_KSYMS_WHITELIST
>  	string "Whitelist of symbols to keep in ksymtab"
>  	depends on TRIM_UNUSED_KSYMS
> +	default "scripts/lto-used-symbollist" if LTO_CLANG
>  	help
>  	  By default, all unused exported symbols will be un-exported from the
>  	  build when TRIM_UNUSED_KSYMS is selected.
> diff --git a/scripts/lto-used-symbollist b/scripts/lto-used-symbollist
> new file mode 100644
> index 000000000000..38e7bb9ebaae
> --- /dev/null
> +++ b/scripts/lto-used-symbollist
> @@ -0,0 +1,5 @@
> +memcpy
> +memmove
> +memset
> +__stack_chk_fail
> +__stack_chk_guard
> -- 
> 2.29.2.576.ga3fc446d84-goog
> 

bikeshed: Should this filename use some kind of extension, like
lto-user-symbols.txt or .list, to make it more human-friendly?

-- 
Kees Cook