objtool/powerpc: Add --mcount specific implementation

This patch enables objtool --mcount on powerpc, and adds implementation
specific to powerpc.

Tested-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Reviewed-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Sathvika Vasireddy <sv@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221114175754.1131267-17-sv@linux.ibm.com

Author: sathvika-v

arch/powerpc/Kconfig

@@ -239,6 +239,7 @@ config PPC
 	select HAVE_NMI				if PERF_EVENTS || (PPC64 && PPC_BOOK3S)
 	select HAVE_OPTPROBES
 	select HAVE_OBJTOOL			if PPC32 || MPROFILE_KERNEL
+	select HAVE_OBJTOOL_MCOUNT		if HAVE_OBJTOOL
 	select HAVE_PERF_EVENTS
 	select HAVE_PERF_EVENTS_NMI		if PPC64
 	select HAVE_PERF_REGS

tools/objtool/arch/powerpc/decode.c

@@ -9,6 +9,11 @@
 #include <objtool/builtin.h>
 #include <objtool/endianness.h>
 
+int arch_ftrace_match(char *name)
+{
+	return !strcmp(name, "_mcount");
+}
+
 unsigned long arch_dest_reloc_offset(int addend)
 {
 	return addend;
@@ -50,6 +55,17 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
 	typ = INSN_OTHER;
 	imm = 0;
 
+	switch (opcode) {
+	case 18: /* b[l][a] */
+		if ((insn & 3) == 1) /* bl */
+			typ = INSN_CALL;
+
+		imm = insn & 0x3fffffc;
+		if (imm & 0x2000000)
+			imm -= 0x4000000;
+		break;
+	}
+
 	if (opcode == 1)
 		*len = 8;
 	else

tools/objtool/arch/powerpc/include/arch/elf.h

@@ -4,5 +4,7 @@
 #define _OBJTOOL_ARCH_ELF
 
 #define R_NONE R_PPC_NONE
+#define R_ABS64 R_PPC64_ADDR64
+#define R_ABS32 R_PPC_ADDR32
 
 #endif /* _OBJTOOL_ARCH_ELF */