Commit b341b20d authored by Peter Zijlstra's avatar Peter Zijlstra
Browse files

x86: Add prefix symbols for function padding 



When code is compiled with:

  -fpatchable-function-entry=${PADDING_BYTES},${PADDING_BYTES}

functions will have PADDING_BYTES of NOP in front of them. Unwinders
and other things that symbolize code locations will typically
attribute these bytes to the preceding function.

Given that these bytes nominally belong to the following symbol this
mis-attribution is confusing.

Inspired by the fact that CFI_CLANG emits __cfi_##name symbols to
claim these bytes, use objtool to emit __pfx_##name symbols to do
the same when CFI_CLANG is not used.

This then shows the callthunk for symbol 'name' as:

  __pfx_##name+0x6/0x10

Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: default avatarYujie Liu <yujie.liu@intel.com>
Link: https://lkml.kernel.org/r/20221028194453.592512209@infradead.org
parent 9f2899fe

arch/x86/Kconfig

+4 −0
Original line number Diff line number Diff line
@@ -2471,6 +2471,10 @@ config CALL_THUNKS 
	def_bool n

	select FUNCTION_ALIGNMENT_16B



config PREFIX_SYMBOLS

	def_bool y

	depends on CALL_THUNKS && !CFI_CLANG



menuconfig SPECULATION_MITIGATIONS

	bool "Mitigations for speculative execution vulnerabilities"

	default y

scripts/Makefile.lib

+1 −0
Original line number Diff line number Diff line
@@ -265,6 +265,7 @@ objtool-args-$(CONFIG_STACK_VALIDATION) += --stackval 
objtool-args-$(CONFIG_HAVE_STATIC_CALL_INLINE)		+= --static-call

objtool-args-$(CONFIG_HAVE_UACCESS_VALIDATION)		+= --uaccess

objtool-args-$(CONFIG_GCOV_KERNEL)			+= --no-unreachable

objtool-args-$(CONFIG_PREFIX_SYMBOLS)			+= --prefix=$(CONFIG_FUNCTION_PADDING_BYTES)



objtool-args = $(objtool-args-y)					\

	$(if $(delay-objtool), --link)					\