ubsan: implement __ubsan_handle_alignment_assumption

When building ARCH=mips 32r2el_defconfig with CONFIG_UBSAN_ALIGNMENT:

  ld.lld: error: undefined symbol: __ubsan_handle_alignment_assumption
     referenced by slab.h:557 (include/linux/slab.h:557)
                   main.o:(do_initcalls) in archive init/built-in.a
     referenced by slab.h:448 (include/linux/slab.h:448)
                   do_mounts_rd.o:(rd_load_image) in archive init/built-in.a
     referenced by slab.h:448 (include/linux/slab.h:448)
                   do_mounts_rd.o:(identify_ramdisk_image) in archive init/built-in.a
     referenced 1579 more times

Implement this for the kernel based on LLVM's
handleAlignmentAssumptionImpl because the kernel is not linked against
the compiler runtime.

Link: https://github.com/ClangBuiltLinux/linux/issues/1245
Link: https://github.com/llvm/llvm-project/blob/llvmorg-11.0.1/compiler-rt/lib/ubsan/ubsan_handlers.cpp#L151-L190
Link: https://lkml.kernel.org/r/20210127224451.2587372-1-nathan@kernel.orgSigned-off-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/ubsan.c

@@ -427,3 +427,34 @@ void __ubsan_handle_load_invalid_value(void *_data, void *val)
 	ubsan_epilogue();
 }
 EXPORT_SYMBOL(__ubsan_handle_load_invalid_value);
+
+void __ubsan_handle_alignment_assumption(void *_data, unsigned long ptr,
+					 unsigned long align,
+					 unsigned long offset);
+void __ubsan_handle_alignment_assumption(void *_data, unsigned long ptr,
+					 unsigned long align,
+					 unsigned long offset)
+{
+	struct alignment_assumption_data *data = _data;
+	unsigned long real_ptr;
+
+	if (suppress_report(&data->location))
+		return;
+
+	ubsan_prologue(&data->location, "alignment-assumption");
+
+	if (offset)
+		pr_err("assumption of %lu byte alignment (with offset of %lu byte) for pointer of type %s failed",
+		       align, offset, data->type->type_name);
+	else
+		pr_err("assumption of %lu byte alignment for pointer of type %s failed",
+		       align, data->type->type_name);
+
+	real_ptr = ptr - offset;
+	pr_err("%saddress is %lu aligned, misalignment offset is %lu bytes",
+	       offset ? "offset " : "", BIT(real_ptr ? __ffs(real_ptr) : 0),
+	       real_ptr & (align - 1));
+
+	ubsan_epilogue();
+}
+EXPORT_SYMBOL(__ubsan_handle_alignment_assumption);

lib/ubsan.h

@@ -78,6 +78,12 @@ struct invalid_value_data {
 	struct type_descriptor *type;
 };
 
+struct alignment_assumption_data {
+	struct source_location location;
+	struct source_location assumption_location;
+	struct type_descriptor *type;
+};
+
 #if defined(CONFIG_ARCH_SUPPORTS_INT128)
 typedef __int128 s_max;
 typedef unsigned __int128 u_max;