x86/bug: Fix BUG_FORMAT vs KASLR

Encoding a relative NULL pointer doesn't work for KASLR, when the
whole kernel image gets shifted, the __bug_table and the target string
get shifted by the same amount and the relative offset is preserved.

However when the target is an absolute 0 value and the __bug_table
gets moved about, the end result in a pointer equivalent to
kaslr_offset(), not NULL.

Notably, this will generate SHN_UNDEF relocations, and Ard would
really like to not have those at all.

Use the empty string to denote no-string.

Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Author: Peter Zijlstra

arch/x86/include/asm/bug.h

@@ -86,7 +86,7 @@ extern void __WARN_trap(struct bug_entry *bug, ...);
 #ifdef CONFIG_DEBUG_BUGVERBOSE_DETAILED
 #define WARN_CONDITION_STR(cond_str) cond_str
 #else
-#define WARN_CONDITION_STR(cond_str) NULL
+#define WARN_CONDITION_STR(cond_str) ""
 #endif
 
 #define _BUG_FLAGS(cond_str, ins, flags, extra)				\
@@ -103,8 +103,12 @@ do {									\
 } while (0)
 
 #define ARCH_WARN_ASM(file, line, flags, size)				\
+	".pushsection .rodata.str1.1, \"aMS\", @progbits, 1\n"		\
+	"99:\n"								\
+	"\t.string \"\"\n"						\
+	".popsection\n"							\
 	"1:\t " ASM_UD2 "\n"						\
-	_BUG_FLAGS_ASM("0", file, line, flags, size, "")
+	_BUG_FLAGS_ASM("99b", file, line, flags, size, "")
 
 #else
 

lib/bug.c

@@ -144,7 +144,17 @@ static const char *bug_get_format(struct bug_entry *bug)
 	const char *format = NULL;
 #ifdef HAVE_ARCH_BUG_FORMAT
 #ifdef CONFIG_GENERIC_BUG_RELATIVE_POINTERS
-	format = (const char *)&bug->format_disp + bug->format_disp;
+	/*
+	 * Allow an architecture to:
+	 *  - relative encode NULL (difficult vs KASLR);
+	 *  - use a literal 0 (there are no valid objects inside
+	 *    the __bug_table itself to refer to after all);
+	 *  - use an empty string.
+	 */
+	if (bug->format_disp)
+		format = (const char *)&bug->format_disp + bug->format_disp;
+	if (format && format[0] == '\0')
+		format = NULL;
 #else
 	format = bug->format;
 #endif