kbuild: lto: add a default list of used symbols

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>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20201211184633.3213045-7-samitolvanen@google.com

diff --git a/init/Kconfig b/init/Kconfig
index b77c60f8b963d4ae43140cce57b4c19af2e28732..5271138b87420b1d8b61410a2b2f22452cda79b8 100644 (file)
--- 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.txt" 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.txt b/scripts/lto-used-symbollist.txt
new file mode 100644 (file)
index 0000000..38e7bb9
--- /dev/null
+++ b/scripts/lto-used-symbollist.txt
@@ -0,0 +1,5 @@
+memcpy
+memmove
+memset
+__stack_chk_fail
+__stack_chk_guard