On x86, UACCESS is controlled by two instructions: STAC and CLAC.
STAC instruction enables UACCESS while CLAC disables UACCESS.
This is simple enough for objtool to locate UACCESS enable and
disable.

But on powerpc it is a bit more complex, the same instruction is
used for enabling and disabling UACCESS, and the same instruction
can be used for many other things. It would be too complex to use
exclusively instruction decoding.

To help objtool, mark such instruction into .discard.uaccess_begin
and .discard.uaccess_end sections, on the same principle as for
reachable/unreachable instructions. And add ASM_UACCESS_BEGIN
and ASM_UACCESS_END macros to be used in inline assembly code to
annotate UACCESS enable and UACCESS disable instructions.

Signed-off-by: Christophe Leroy <redacted>
---
 include/linux/objtool.h | 14 +++++++++++++
 tools/objtool/check.c   | 44 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 58 insertions(+)

diff --git a/include/linux/objtool.h b/include/linux/objtool.h
index 03f82c2c2ebf..d8fde4158a40 100644
--- a/include/linux/objtool.h
+++ b/include/linux/objtool.h

@@ -57,6 +57,18 @@
 	".long 998b - .\n\t"						\
 	".popsection\n\t"
 
+#define ASM_UACCESS_BEGIN						\
+	"998:\n\t"							\
+	".pushsection .discard.uaccess_begin\n\t"			\
+	".long 998b - .\n\t"						\
+	".popsection\n\t"
+
+#define ASM_UACCESS_END							\
+	"998:\n\t"							\
+	".pushsection .discard.uaccess_end\n\t"				\
+	".long 998b - .\n\t"						\
+	".popsection\n\t"
+
 #else /* __ASSEMBLY__ */