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x86: Add prefix symbols for function padding

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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: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Yujie Liu <yujie.liu@intel.com>
Link: https://lkml.kernel.org/r/20221028194453.592512209@infradead.org
This commit is contained in:
Peter Zijlstra 2022-10-28 21:08:19 +02:00
parent 9f2899fe36
commit b341b20d64
2 changed files with 5 additions and 0 deletions
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4
arch/x86/Kconfig
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@ -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

1
scripts/Makefile.lib
View File

@ -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) \