powerpc: Allow relative pointers in bug table entries

This enables GENERIC_BUG_RELATIVE_POINTERS on Power so that 32-bit
offsets are stored in the bug entries rather than 64-bit pointers.
While this doesn't save space for 32-bit machines, use it anyway so
there is only one code path.
Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201201005203.15210-1-jniethe5@gmail.com

arch/powerpc/Kconfig

@@ -313,6 +313,10 @@ config GENERIC_BUG
 	default y
 	depends on BUG
 
+config GENERIC_BUG_RELATIVE_POINTERS
+	def_bool y
+	depends on GENERIC_BUG
+
 config SYS_SUPPORTS_APM_EMULATION
 	default y if PMAC_APM_EMU
 	bool

arch/powerpc/include/asm/bug.h

@@ -12,7 +12,7 @@
 #ifdef CONFIG_DEBUG_BUGVERBOSE
 .macro EMIT_BUG_ENTRY addr,file,line,flags
 	 .section __bug_table,"aw"
-5001:	 PPC_LONG \addr, 5002f
+5001:	 .4byte \addr - 5001b, 5002f - 5001b
 	 .short \line, \flags
 	 .org 5001b+BUG_ENTRY_SIZE
 	 .previous
@@ -23,7 +23,7 @@
 #else
 .macro EMIT_BUG_ENTRY addr,file,line,flags
 	 .section __bug_table,"aw"
-5001:	 PPC_LONG \addr
+5001:	 .4byte \addr - 5001b
 	 .short \flags
 	 .org 5001b+BUG_ENTRY_SIZE
 	 .previous
@@ -36,14 +36,14 @@
 #ifdef CONFIG_DEBUG_BUGVERBOSE
 #define _EMIT_BUG_ENTRY				\
 	".section __bug_table,\"aw\"\n"		\
-	"2:\t" PPC_LONG "1b, %0\n"		\
+	"2:\t.4byte 1b - 2b, %0 - 2b\n"		\
 	"\t.short %1, %2\n"			\
 	".org 2b+%3\n"				\
 	".previous\n"
 #else
 #define _EMIT_BUG_ENTRY				\
 	".section __bug_table,\"aw\"\n"		\
-	"2:\t" PPC_LONG "1b\n"			\
+	"2:\t.4byte 1b - 2b\n"			\
 	"\t.short %2\n"				\
 	".org 2b+%3\n"				\
 	".previous\n"

arch/powerpc/xmon/xmon.c

@@ -1747,9 +1747,9 @@ static void print_bug_trap(struct pt_regs *regs)
 
 #ifdef CONFIG_DEBUG_BUGVERBOSE
 	printf("kernel BUG at %s:%u!\n",
-	       bug->file, bug->line);
+	       (char *)bug + bug->file_disp, bug->line);
 #else
-	printf("kernel BUG at %px!\n", (void *)bug->bug_addr);
+	printf("kernel BUG at %px!\n", (void *)bug + bug->bug_addr_disp);
 #endif
 #endif /* CONFIG_BUG */
 }