[PATCH v2] device property: Get rid of union aliasing

From: Andy Shevchenko
Date: Tue May 08 2018 - 09:15:57 EST

The commit

318a19718261 ("device property: refactor built-in properties support")

went way too far and brought a union aliasing. Partially revert it here
to get rid of union aliasing.

Note, Apple properties support is still utilizing this trick.

What union aliasing is?
~~~~~~~~~~~~~~~~~~~~~~~
The C99 standard in section 6.2.5 paragraph 20 defines union type as
"an overlapping nonempty set of member objects". It also states in
section 6.7.2.1 paragraph 14 that "the value of at most one of the
members can be stored in a union object at any time'.

Union aliasing is a type punning mechanism using union members to store
as one type and read back as another.

Why it's not good?
~~~~~~~~~~~~~~~~~~
Section 6.2.6.1 paragraph 6 says that a union object may not be a trap
representation, although its member objects may be.

Meanwhile annex J.1 says that "the value of a union member other than
the last one stored into" is unspecified [removed in C11].

In TC3, a footnote is added which specifies that accessing a member of a
union other than the last one stored causes "the object representation"
to be re-interpreted in the new type and specifically refers to this as
"type punning". This conflicts to some degree with Annex J.1.

While it's working in Linux with GCC, the use of union members to do
type punning is not clear area in the C standard and might lead to
unspecified behaviour.

More information is available in this [1] blog post.

[1]: https://davmac.wordpress.com/2010/02/26/c99-revisited/

Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
---

- extend commit message to explain what union aliasing is and why it's not good

drivers/base/property.c | 99 ++++++++++++++++++++-----
drivers/firmware/efi/apple-properties.c | 8 +-
include/linux/property.h | 52 +++++++------
3 files changed, 112 insertions(+), 47 deletions(-)

diff --git a/drivers/base/property.c b/drivers/base/property.c
index 8f205f6461ed..de19d7cc073b 100644
--- a/drivers/base/property.c
+++ b/drivers/base/property.c
@@ -56,6 +56,67 @@ pset_prop_get(const struct property_set *pset, const char *name)
return NULL;
}

+static const void *property_get_pointer(const struct property_entry *prop)
+{
+ switch (prop->type) {
+ case DEV_PROP_U8:
+ if (prop->is_array)
+ return prop->pointer.u8_data;
+ return &prop->value.u8_data;
+ case DEV_PROP_U16:
+ if (prop->is_array)
+ return prop->pointer.u16_data;
+ return &prop->value.u16_data;
+ case DEV_PROP_U32:
+ if (prop->is_array)
+ return prop->pointer.u32_data;
+ return &prop->value.u32_data;
+ case DEV_PROP_U64:
+ if (prop->is_array)
+ return prop->pointer.u64_data;
+ return &prop->value.u64_data;
+ default:
+ if (prop->is_array)
+ return prop->pointer.str;
+ return &prop->value.str;
+ }
+}
+
+static void property_set_pointer(struct property_entry *prop, const void *pointer)
+{
+ switch (prop->type) {
+ case DEV_PROP_U8:
+ if (prop->is_array)
+ prop->pointer.u8_data = pointer;
+ else
+ prop->value.u8_data = *((u8 *)pointer);
+ break;
+ case DEV_PROP_U16:
+ if (prop->is_array)
+ prop->pointer.u16_data = pointer;
+ else
+ prop->value.u16_data = *((u16 *)pointer);
+ break;
+ case DEV_PROP_U32:
+ if (prop->is_array)
+ prop->pointer.u32_data = pointer;
+ else
+ prop->value.u32_data = *((u32 *)pointer);
+ break;
+ case DEV_PROP_U64:
+ if (prop->is_array)
+ prop->pointer.u64_data = pointer;
+ else
+ prop->value.u64_data = *((u64 *)pointer);
+ break;
+ default:
+ if (prop->is_array)
+ prop->pointer.str = pointer;
+ else
+ prop->value.str = pointer;
+ }
+}
+
static const void *pset_prop_find(const struct property_set *pset,
const char *propname, size_t length)
{
@@ -65,10 +126,7 @@ static const void *pset_prop_find(const struct property_set *pset,
prop = pset_prop_get(pset, propname);
if (!prop)
return ERR_PTR(-EINVAL);
- if (prop->is_array)
- pointer = prop->pointer.raw_data;
- else
- pointer = &prop->value.raw_data;
+ pointer = property_get_pointer(prop);
if (!pointer)
return ERR_PTR(-ENODATA);
if (length > prop->length)
@@ -698,16 +756,17 @@ EXPORT_SYMBOL_GPL(fwnode_property_get_reference_args);

static void property_entry_free_data(const struct property_entry *p)
{
+ const void *pointer = property_get_pointer(p);
size_t i, nval;

if (p->is_array) {
- if (p->is_string && p->pointer.str) {
+ if (p->type == DEV_PROP_STRING && p->pointer.str) {
nval = p->length / sizeof(const char *);
for (i = 0; i < nval; i++)
kfree(p->pointer.str[i]);
}
- kfree(p->pointer.raw_data);
- } else if (p->is_string) {
+ kfree(pointer);
+ } else if (p->type == DEV_PROP_STRING) {
kfree(p->value.str);
}
kfree(p->name);
@@ -716,7 +775,7 @@ static void property_entry_free_data(const struct property_entry *p)
static int property_copy_string_array(struct property_entry *dst,
const struct property_entry *src)
{
- char **d;
+ const char **d;
size_t nval = src->length / sizeof(*d);
int i;

@@ -734,40 +793,44 @@ static int property_copy_string_array(struct property_entry *dst,
}
}

- dst->pointer.raw_data = d;
+ dst->pointer.str = d;
return 0;
}

static int property_entry_copy_data(struct property_entry *dst,
const struct property_entry *src)
{
+ const void *pointer = property_get_pointer(src);
+ const void *new;
int error;

if (src->is_array) {
if (!src->length)
return -ENODATA;

- if (src->is_string) {
+ if (src->type == DEV_PROP_STRING) {
error = property_copy_string_array(dst, src);
if (error)
return error;
+ new = dst->pointer.str;
} else {
- dst->pointer.raw_data = kmemdup(src->pointer.raw_data,
- src->length, GFP_KERNEL);
- if (!dst->pointer.raw_data)
+ new = kmemdup(pointer, src->length, GFP_KERNEL);
+ if (!new)
return -ENOMEM;
}
- } else if (src->is_string) {
- dst->value.str = kstrdup(src->value.str, GFP_KERNEL);
- if (!dst->value.str && src->value.str)
+ } else if (src->type == DEV_PROP_STRING) {
+ new = kstrdup(src->value.str, GFP_KERNEL);
+ if (!new && src->value.str)
return -ENOMEM;
} else {
- dst->value.raw_data = src->value.raw_data;
+ new = pointer;
}

dst->length = src->length;
dst->is_array = src->is_array;
- dst->is_string = src->is_string;
+ dst->type = src->type;
+
+ property_set_pointer(dst, new);

dst->name = kstrdup(src->name, GFP_KERNEL);
if (!dst->name)
diff --git a/drivers/firmware/efi/apple-properties.c b/drivers/firmware/efi/apple-properties.c
index adaa9a3714b9..b2c2d5a45f91 100644
--- a/drivers/firmware/efi/apple-properties.c
+++ b/drivers/firmware/efi/apple-properties.c
@@ -13,6 +13,9 @@
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, see <http://www.gnu.org/licenses/>.
+ *
+ * FIXME: The approach is still based on union aliasing and should be
+ * replaced by a proper resource provider.
*/

#define pr_fmt(fmt) "apple-properties: " fmt
@@ -96,12 +99,13 @@ static void __init unmarshal_key_value_pairs(struct dev_header *dev_header,
entry[i].name = key;
entry[i].length = val_len - sizeof(val_len);
entry[i].is_array = !!entry[i].length;
- entry[i].pointer.raw_data = ptr + key_len + sizeof(val_len);
+ entry[i].type = DEV_PROP_U8;
+ entry[i].pointer.u8_data = ptr + key_len + sizeof(val_len);

if (dump_properties) {
dev_info(dev, "property: %s\n", entry[i].name);
print_hex_dump(KERN_INFO, pr_fmt(), DUMP_PREFIX_OFFSET,
- 16, 1, entry[i].pointer.raw_data,
+ 16, 1, entry[i].pointer.u8_data,
entry[i].length, true);
}

diff --git a/include/linux/property.h b/include/linux/property.h
index 2eea4b310fc2..ac8a1ebc4c1b 100644
--- a/include/linux/property.h
+++ b/include/linux/property.h
@@ -178,7 +178,7 @@ static inline int fwnode_property_read_u64(const struct fwnode_handle *fwnode,
* @name: Name of the property.
* @length: Length of data making up the value.
* @is_array: True when the property is an array.
- * @is_string: True when property is a string.
+ * @type: Type of the data in unions.
* @pointer: Pointer to the property (an array of items of the given type).
* @value: Value of the property (when it is a single item of the given type).
*/
@@ -186,10 +186,9 @@ struct property_entry {
const char *name;
size_t length;
bool is_array;
- bool is_string;
+ enum dev_prop_type type;
union {
union {
- const void *raw_data;
const u8 *u8_data;
const u16 *u16_data;
const u32 *u32_data;
@@ -197,7 +196,6 @@ struct property_entry {
const char * const *str;
} pointer;
union {
- unsigned long long raw_data;
u8 u8_data;
u16 u16_data;
u32 u32_data;
@@ -213,55 +211,55 @@ struct property_entry {
* and structs.
*/

-#define PROPERTY_ENTRY_INTEGER_ARRAY(_name_, _type_, _val_) \
-(struct property_entry) { \
- .name = _name_, \
- .length = ARRAY_SIZE(_val_) * sizeof(_type_), \
- .is_array = true, \
- .is_string = false, \
- { .pointer = { ._type_##_data = _val_ } }, \
+#define PROPERTY_ENTRY_INTEGER_ARRAY(_name_, _type_, _Type_, _val_) \
+(struct property_entry) { \
+ .name = _name_, \
+ .length = ARRAY_SIZE(_val_) * sizeof(_type_), \
+ .is_array = true, \
+ .type = DEV_PROP_##_Type_, \
+ { .pointer = { ._type_##_data = _val_ } }, \
}

#define PROPERTY_ENTRY_U8_ARRAY(_name_, _val_) \
- PROPERTY_ENTRY_INTEGER_ARRAY(_name_, u8, _val_)
+ PROPERTY_ENTRY_INTEGER_ARRAY(_name_, u8, U8, _val_)
#define PROPERTY_ENTRY_U16_ARRAY(_name_, _val_) \
- PROPERTY_ENTRY_INTEGER_ARRAY(_name_, u16, _val_)
+ PROPERTY_ENTRY_INTEGER_ARRAY(_name_, u16, U16, _val_)
#define PROPERTY_ENTRY_U32_ARRAY(_name_, _val_) \
- PROPERTY_ENTRY_INTEGER_ARRAY(_name_, u32, _val_)
+ PROPERTY_ENTRY_INTEGER_ARRAY(_name_, u32, U32, _val_)
#define PROPERTY_ENTRY_U64_ARRAY(_name_, _val_) \
- PROPERTY_ENTRY_INTEGER_ARRAY(_name_, u64, _val_)
+ PROPERTY_ENTRY_INTEGER_ARRAY(_name_, u64, U64, _val_)

#define PROPERTY_ENTRY_STRING_ARRAY(_name_, _val_) \
(struct property_entry) { \
.name = _name_, \
.length = ARRAY_SIZE(_val_) * sizeof(const char *), \
.is_array = true, \
- .is_string = true, \
+ .type = DEV_PROP_STRING, \
{ .pointer = { .str = _val_ } }, \
}

-#define PROPERTY_ENTRY_INTEGER(_name_, _type_, _val_) \
-(struct property_entry) { \
- .name = _name_, \
- .length = sizeof(_type_), \
- .is_string = false, \
- { .value = { ._type_##_data = _val_ } }, \
+#define PROPERTY_ENTRY_INTEGER(_name_, _type_, _Type_, _val_) \
+(struct property_entry) { \
+ .name = _name_, \
+ .length = sizeof(_type_), \
+ .type = DEV_PROP_##_Type_, \
+ { .value = { ._type_##_data = _val_ } }, \
}

#define PROPERTY_ENTRY_U8(_name_, _val_) \
- PROPERTY_ENTRY_INTEGER(_name_, u8, _val_)
+ PROPERTY_ENTRY_INTEGER(_name_, u8, U8, _val_)
#define PROPERTY_ENTRY_U16(_name_, _val_) \
- PROPERTY_ENTRY_INTEGER(_name_, u16, _val_)
+ PROPERTY_ENTRY_INTEGER(_name_, u16, U16, _val_)
#define PROPERTY_ENTRY_U32(_name_, _val_) \
- PROPERTY_ENTRY_INTEGER(_name_, u32, _val_)
+ PROPERTY_ENTRY_INTEGER(_name_, u32, U32, _val_)
#define PROPERTY_ENTRY_U64(_name_, _val_) \
- PROPERTY_ENTRY_INTEGER(_name_, u64, _val_)
+ PROPERTY_ENTRY_INTEGER(_name_, u64, U64, _val_)

#define PROPERTY_ENTRY_STRING(_name_, _val_) \
(struct property_entry) { \
.name = _name_, \
.length = sizeof(_val_), \
- .is_string = true, \
+ .type = DEV_PROP_STRING, \
{ .value = { .str = _val_ } }, \
}

--
2.17.0