arm64: Document why we enable PAC support for leaf functions

Document the fact that we enable pointer authentication protection for
leaf functions since there is some narrow potential for ROP protection
benefits and little overhead has been observed.
Signed-off-by: NMark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20200506195138.22086-2-broonie@kernel.orgSigned-off-by: NWill Deacon <will@kernel.org>

arch/arm64/Makefile

@@ -71,6 +71,9 @@ branch-prot-flags-y += $(call cc-option,-mbranch-protection=none)
 
 ifeq ($(CONFIG_ARM64_PTR_AUTH),y)
 branch-prot-flags-$(CONFIG_CC_HAS_SIGN_RETURN_ADDRESS) := -msign-return-address=all
+# We enable additional protection for leaf functions as there is some
+# narrow potential for ROP protection benefits and no substantial
+# performance impact has been observed.
 branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET) := -mbranch-protection=pac-ret+leaf
 # -march=armv8.3-a enables the non-nops instructions for PAC, to avoid the
 # compiler to generate them and consequently to break the single image contract