[PATCH v4 12/15] objtool: Add support for more complex UACCESS control

[Christophe Leroy]

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 <christophe.leroy@xxxxxxxxxx>
---
 include/linux/objtool.h | 14 ++++++++++++++
 tools/objtool/check.c   | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 47 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__ */
 
 /*
@@ -156,6 +168,8 @@
 #define STACK_FRAME_NON_STANDARD_FP(func)
 #define ANNOTATE_NOENDBR
 #define ASM_REACHABLE
+#define ASM_UACCESS_BEGIN
+#define ASM_UACCESS_END
 #else
 #define ANNOTATE_INTRA_FUNCTION_CALL
 .macro UNWIND_HINT type:req sp_reg=0 sp_offset=0 signal=0
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index d2a0dfec5909..5af6c6c3fbed 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1052,6 +1052,38 @@ static void add_ignores(struct objtool_file *file)
 	}
 }
 
+static void __add_uaccess(struct objtool_file *file, const char *name,
+			  int type, const char *action)
+{
+	struct section *rsec;
+	struct reloc *reloc;
+	struct instruction *insn;
+
+	rsec = find_section_by_name(file->elf, name);
+	if (!rsec)
+		return;
+
+	for_each_reloc(rsec, reloc) {
+		if (reloc->sym->type != STT_SECTION) {
+			WARN("unexpected relocation symbol type in %s: ", rsec->name);
+			continue;
+		}
+		insn = find_insn(file, reloc->sym->sec, reloc_addend(reloc));
+		if (!insn) {
+			WARN("can't find UACCESS %s insn at %s+0x%" PRIx64,
+			     action, reloc->sym->sec->name, reloc_addend(reloc));
+			continue;
+		}
+		insn->type = type;
+	}
+}
+
+static void add_uaccess(struct objtool_file *file)
+{
+	__add_uaccess(file, ".rela.discard.uaccess_begin", INSN_STAC, "enable");
+	__add_uaccess(file, ".rela.discard.uaccess_end", INSN_CLAC, "disable");
+}
+
 /*
  * This is a whitelist of functions that is allowed to be called with AC set.
  * The list is meant to be minimal and only contains compiler instrumentation
@@ -2597,6 +2629,7 @@ static int decode_sections(struct objtool_file *file)
 		return ret;
 
 	add_ignores(file);
+	add_uaccess(file);
 	add_uaccess_safe(file);
 
 	ret = add_ignore_alternatives(file);
-- 
2.41.0