[PATCH v3] ubsan: Implement __ubsan_handle_alignment_assumption

[Nathan Chancellor <nathan@kernel.org>]

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
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---

v2 -> v3:

* Add prototype right above definition to avoid introducing a warning
  with W=1.

v1 -> v2:

* Use __ffs instead of ffs because due to size of input (unsigned long
  vs int) and we want a zero based index (Nick Desaulniers).

* Pick up Kees's ack.

 lib/ubsan.c | 31 +++++++++++++++++++++++++++++++
 lib/ubsan.h |  6 ++++++
 2 files changed, 37 insertions(+)

diff --git a/lib/ubsan.c b/lib/ubsan.c
index 3e3352f3d0da..bec38c64d6a6 100644
--- a/lib/ubsan.c
+++ b/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);
diff --git a/lib/ubsan.h b/lib/ubsan.h
index 7b56c09473a9..9a0b71c5ff9f 100644
--- a/lib/ubsan.h
+++ b/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;

base-commit: 6ee1d745b7c9fd573fba142a2efdad76a9f1cb04
-- 
2.30.0