Merge tag 'overflow-v5.16-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux

Pull overflow updates from Kees Cook:
 "The end goal of the current buffer overflow detection work[0] is to
  gain full compile-time and run-time coverage of all detectable buffer
  overflows seen via array indexing or memcpy(), memmove(), and
  memset(). The str*() family of functions already have full coverage.

  While much of the work for these changes have been on-going for many
  releases (i.e. 0-element and 1-element array replacements, as well as
  avoiding false positives and fixing discovered overflows[1]), this
  series contains the foundational elements of several related buffer
  overflow detection improvements by providing new common helpers and
  FORTIFY_SOURCE changes needed to gain the introspection required for
  compiler visibility into array sizes. Also included are a handful of
  already Acked instances using the helpers (or related clean-ups), with
  many more waiting at the ready to be taken via subsystem-specific
  trees[2].

  The new helpers are:

   - struct_group() for gaining struct member range introspection

   - memset_after() and memset_startat() for clearing to the end of
     structures

   - DECLARE_FLEX_ARRAY() for using flex arrays in unions or alone in
     structs

  Also included is the beginning of the refactoring of FORTIFY_SOURCE to
  support memcpy() introspection, fix missing and regressed coverage
  under GCC, and to prepare to fix the currently broken Clang support.
  Finishing this work is part of the larger series[0], but depends on
  all the false positives and buffer overflow bug fixes to have landed
  already and those that depend on this series to land.

  As part of the FORTIFY_SOURCE refactoring, a set of both a
  compile-time and run-time tests are added for FORTIFY_SOURCE and the
  mem*()-family functions respectively. The compile time tests have
  found a legitimate (though corner-case) bug[6] already.

  Please note that the appearance of "panic" and "BUG" in the
  FORTIFY_SOURCE refactoring are the result of relocating existing code,
  and no new use of those code-paths are expected nor desired.

  Finally, there are two tree-wide conversions for 0-element arrays and
  flexible array unions to gain sane compiler introspection coverage
  that result in no known object code differences.

  After this series (and the changes that have now landed via netdev and
  usb), we are very close to finally being able to build with
  -Warray-bounds and -Wzero-length-bounds.

  However, due corner cases in GCC[3] and Clang[4], I have not included
  the last two patches that turn on these options, as I don't want to
  introduce any known warnings to the build. Hopefully these can be
  solved soon"

Link: https://lore.kernel.org/lkml/20210818060533.3569517-1-keescook@chromium.org/ [0]
Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/log/?qt=grep&q=FORTIFY_SOURCE [1]
Link: https://lore.kernel.org/lkml/202108220107.3E26FE6C9C@keescook/ [2]
Link: https://lore.kernel.org/lkml/3ab153ec-2798-da4c-f7b1-81b0ac8b0c5b@roeck-us.net/ [3]
Link: https://bugs.llvm.org/show_bug.cgi?id=51682 [4]
Link: https://lore.kernel.org/lkml/202109051257.29B29745C0@keescook/ [5]
Link: https://lore.kernel.org/lkml/20211020200039.170424-1-keescook@chromium.org/ [6]

* tag 'overflow-v5.16-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux: (30 commits)
  fortify: strlen: Avoid shadowing previous locals
  compiler-gcc.h: Define __SANITIZE_ADDRESS__ under hwaddress sanitizer
  treewide: Replace 0-element memcpy() destinations with flexible arrays
  treewide: Replace open-coded flex arrays in unions
  stddef: Introduce DECLARE_FLEX_ARRAY() helper
  btrfs: Use memset_startat() to clear end of struct
  string.h: Introduce memset_startat() for wiping trailing members and padding
  xfrm: Use memset_after() to clear padding
  string.h: Introduce memset_after() for wiping trailing members/padding
  lib: Introduce CONFIG_MEMCPY_KUNIT_TEST
  fortify: Add compile-time FORTIFY_SOURCE tests
  fortify: Allow strlen() and strnlen() to pass compile-time known lengths
  fortify: Prepare to improve strnlen() and strlen() warnings
  fortify: Fix dropped strcpy() compile-time write overflow check
  fortify: Explicitly disable Clang support
  fortify: Move remaining fortify helpers into fortify-string.h
  lib/string: Move helper functions out of string.c
  compiler_types.h: Remove __compiletime_object_size()
  cm4000_cs: Use struct_group() to zero struct cm4000_dev region
  can: flexcan: Use struct_group() to zero struct flexcan_regs regions
  ...

14 files changed, 246 insertions, 76 deletions

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index bd2b881c6b63..7bbd8df02532 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -41,8 +41,6 @@
 
 #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)
 
-#define __compiletime_object_size(obj) __builtin_object_size(obj, 0)
-
 #if defined(LATENT_ENTROPY_PLUGIN) && !defined(__CHECKER__)
 #define __latent_entropy __attribute__((latent_entropy))
 #endif
@@ -124,6 +122,14 @@
 #endif
 
 /*
+ * Treat __SANITIZE_HWADDRESS__ the same as __SANITIZE_ADDRESS__ in the kernel,
+ * matching the defines used by Clang.
+ */
+#ifdef __SANITIZE_HWADDRESS__
+#define __SANITIZE_ADDRESS__
+#endif
+
+/*
  * Turn individual warnings and errors on and off locally, depending
  * on version.
  */
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index b6ff83a714ca..05ceb2e92b0e 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -290,11 +290,6 @@ struct ftrace_likely_data {
 	(sizeof(t) == sizeof(char) || sizeof(t) == sizeof(short) || \
 	 sizeof(t) == sizeof(int) || sizeof(t) == sizeof(long))
 
-/* Compile time object size, -1 for unknown */
-#ifndef __compiletime_object_size
-# define __compiletime_object_size(obj) -1
-#endif
-
 #ifdef __OPTIMIZE__
 # define __compiletime_assert(condition, msg, prefix, suffix)		\
 	do {								\
diff --git a/include/linux/filter.h b/include/linux/filter.h
index ef03ff34234d..b956eeb0f9a8 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -586,8 +586,10 @@ struct bpf_prog {
 	struct bpf_prog_aux	*aux;		/* Auxiliary fields */
 	struct sock_fprog_kern	*orig_prog;	/* Original BPF program */
 	/* Instructions for interpreter */
-	struct sock_filter	insns[0];
-	struct bpf_insn		insnsi[];
+	union {
+		DECLARE_FLEX_ARRAY(struct sock_filter, insns);
+		DECLARE_FLEX_ARRAY(struct bpf_insn, insnsi);
+	};
 };
 
 struct sk_filter {
diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
index c1be37437e77..a6cd6815f249 100644
--- a/include/linux/fortify-string.h
+++ b/include/linux/fortify-string.h
@@ -2,6 +2,27 @@
 #ifndef _LINUX_FORTIFY_STRING_H_
 #define _LINUX_FORTIFY_STRING_H_
 
+#define __FORTIFY_INLINE extern __always_inline __attribute__((gnu_inline))
+#define __RENAME(x) __asm__(#x)
+
+void fortify_panic(const char *name) __noreturn __cold;
+void __read_overflow(void) __compiletime_error("detected read beyond size of object (1st parameter)");
+void __read_overflow2(void) __compiletime_error("detected read beyond size of object (2nd parameter)");
+void __write_overflow(void) __compiletime_error("detected write beyond size of object (1st parameter)");
+
+#define __compiletime_strlen(p)					\
+({								\
+	unsigned char *__p = (unsigned char *)(p);		\
+	size_t __ret = (size_t)-1;				\
+	size_t __p_size = __builtin_object_size(p, 1);		\
+	if (__p_size != (size_t)-1) {				\
+		size_t __p_len = __p_size - 1;			\
+		if (__builtin_constant_p(__p[__p_len]) &&	\
+		    __p[__p_len] == '\0')			\
+			__ret = __builtin_strlen(__p);		\
+	}							\
+	__ret;							\
+})
 
 #if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
 extern void *__underlying_memchr(const void *p, int c, __kernel_size_t size) __RENAME(memchr);
@@ -49,28 +70,38 @@ __FORTIFY_INLINE char *strcat(char *p, const char *q)
 	return p;
 }
 
-__FORTIFY_INLINE __kernel_size_t strlen(const char *p)
+extern __kernel_size_t __real_strnlen(const char *, __kernel_size_t) __RENAME(strnlen);
+__FORTIFY_INLINE __kernel_size_t strnlen(const char *p, __kernel_size_t maxlen)
 {
-	__kernel_size_t ret;
 	size_t p_size = __builtin_object_size(p, 1);
+	size_t p_len = __compiletime_strlen(p);
+	size_t ret;
 
-	/* Work around gcc excess stack consumption issue */
-	if (p_size == (size_t)-1 ||
-		(__builtin_constant_p(p[p_size - 1]) && p[p_size - 1] == '\0'))
-		return __underlying_strlen(p);
-	ret = strnlen(p, p_size);
-	if (p_size <= ret)
+	/* We can take compile-time actions when maxlen is const. */
+	if (__builtin_constant_p(maxlen) && p_len != (size_t)-1) {
+		/* If p is const, we can use its compile-time-known len. */
+		if (maxlen >= p_size)
+			return p_len;
+	}
+
+	/* Do not check characters beyond the end of p. */
+	ret = __real_strnlen(p, maxlen < p_size ? maxlen : p_size);
+	if (p_size <= ret && maxlen != ret)
 		fortify_panic(__func__);
 	return ret;
 }
 
-extern __kernel_size_t __real_strnlen(const char *, __kernel_size_t) __RENAME(strnlen);
-__FORTIFY_INLINE __kernel_size_t strnlen(const char *p, __kernel_size_t maxlen)
+/* defined after fortified strnlen to reuse it. */
+__FORTIFY_INLINE __kernel_size_t strlen(const char *p)
 {
+	__kernel_size_t ret;
 	size_t p_size = __builtin_object_size(p, 1);
-	__kernel_size_t ret = __real_strnlen(p, maxlen < p_size ? maxlen : p_size);
 
-	if (p_size <= ret && maxlen != ret)
+	/* Give up if we don't know how large p is. */
+	if (p_size == (size_t)-1)
+		return __underlying_strlen(p);
+	ret = strnlen(p, p_size);
+	if (p_size <= ret)
 		fortify_panic(__func__);
 	return ret;
 }
@@ -79,24 +110,27 @@ __FORTIFY_INLINE __kernel_size_t strnlen(const char *p, __kernel_size_t maxlen)
 extern size_t __real_strlcpy(char *, const char *, size_t) __RENAME(strlcpy);
 __FORTIFY_INLINE size_t strlcpy(char *p, const char *q, size_t size)
 {
-	size_t ret;
 	size_t p_size = __builtin_object_size(p, 1);
 	size_t q_size = __builtin_object_size(q, 1);
+	size_t q_len;	/* Full count of source string length. */
+	size_t len;	/* Count of characters going into destination. */
 
 	if (p_size == (size_t)-1 && q_size == (size_t)-1)
 		return __real_strlcpy(p, q, size);
-	ret = strlen(q);
-	if (size) {
-		size_t len = (ret >= size) ? size - 1 : ret;
-
-		if (__builtin_constant_p(len) && len >= p_size)
+	q_len = strlen(q);
+	len = (q_len >= size) ? size - 1 : q_len;
+	if (__builtin_constant_p(size) && __builtin_constant_p(q_len) && size) {
+		/* Write size is always larger than destination. */
+		if (len >= p_size)
 			__write_overflow();
+	}
+	if (size) {
 		if (len >= p_size)
 			fortify_panic(__func__);
 		__underlying_memcpy(p, q, len);
 		p[len] = '\0';
 	}
-	return ret;
+	return q_len;
 }
 
 /* defined after fortified strnlen to reuse it */
@@ -280,7 +314,10 @@ __FORTIFY_INLINE char *strcpy(char *p, const char *q)
 	if (p_size == (size_t)-1 && q_size == (size_t)-1)
 		return __underlying_strcpy(p, q);
 	size = strlen(q) + 1;
-	/* test here to use the more stringent object size */
+	/* Compile-time check for const size overflow. */
+	if (__builtin_constant_p(size) && p_size < size)
+		__write_overflow();
+	/* Run-time check for dynamic size overflow. */
 	if (p_size < size)
 		fortify_panic(__func__);
 	memcpy(p, q, size);
diff --git a/include/linux/ieee80211.h b/include/linux/ieee80211.h
index 694264503119..ada3dd79cd08 100644
--- a/include/linux/ieee80211.h
+++ b/include/linux/ieee80211.h
@@ -1143,7 +1143,7 @@ struct ieee80211_mgmt {
 			__le16 auth_transaction;
 			__le16 status_code;
 			/* possibly followed by Challenge text */
-			u8 variable[0];
+			u8 variable[];
 		} __packed auth;
 		struct {
 			__le16 reason_code;
@@ -1152,26 +1152,26 @@ struct ieee80211_mgmt {
 			__le16 capab_info;
 			__le16 listen_interval;
 			/* followed by SSID and Supported rates */
-			u8 variable[0];
+			u8 variable[];
 		} __packed assoc_req;
 		struct {
 			__le16 capab_info;
 			__le16 status_code;
 			__le16 aid;
 			/* followed by Supported rates */
-			u8 variable[0];
+			u8 variable[];
 		} __packed assoc_resp, reassoc_resp;
 		struct {
 			__le16 capab_info;
 			__le16 status_code;
-			u8 variable[0];
+			u8 variable[];
 		} __packed s1g_assoc_resp, s1g_reassoc_resp;
 		struct {
 			__le16 capab_info;
 			__le16 listen_interval;
 			u8 current_ap[ETH_ALEN];
 			/* followed by SSID and Supported rates */
-			u8 variable[0];
+			u8 variable[];
 		} __packed reassoc_req;
 		struct {
 			__le16 reason_code;
@@ -1182,11 +1182,11 @@ struct ieee80211_mgmt {
 			__le16 capab_info;
 			/* followed by some of SSID, Supported rates,
 			 * FH Params, DS Params, CF Params, IBSS Params, TIM */
-			u8 variable[0];
+			u8 variable[];
 		} __packed beacon;
 		struct {
 			/* only variable items: SSID, Supported rates */
-			u8 variable[0];
+			DECLARE_FLEX_ARRAY(u8, variable);
 		} __packed probe_req;
 		struct {
 			__le64 timestamp;
@@ -1194,7 +1194,7 @@ struct ieee80211_mgmt {
 			__le16 capab_info;
 			/* followed by some of SSID, Supported rates,
 			 * FH Params, DS Params, CF Params, IBSS Params */
-			u8 variable[0];
+			u8 variable[];
 		} __packed probe_resp;
 		struct {
 			u8 category;
@@ -1203,16 +1203,16 @@ struct ieee80211_mgmt {
 					u8 action_code;
 					u8 dialog_token;
 					u8 status_code;
-					u8 variable[0];
+					u8 variable[];
 				} __packed wme_action;
 				struct{
 					u8 action_code;
-					u8 variable[0];
+					u8 variable[];
 				} __packed chan_switch;
 				struct{
 					u8 action_code;
 					struct ieee80211_ext_chansw_ie data;
-					u8 variable[0];
+					u8 variable[];
 				} __packed ext_chan_switch;
 				struct{
 					u8 action_code;
@@ -1228,7 +1228,7 @@ struct ieee80211_mgmt {
 					__le16 timeout;
 					__le16 start_seq_num;
 					/* followed by BA Extension */
-					u8 variable[0];
+					u8 variable[];
 				} __packed addba_req;
 				struct{
 					u8 action_code;
@@ -1244,11 +1244,11 @@ struct ieee80211_mgmt {
 				} __packed delba;
 				struct {
 					u8 action_code;
-					u8 variable[0];
+					u8 variable[];
 				} __packed self_prot;
 				struct{
 					u8 action_code;
-					u8 variable[0];
+					u8 variable[];
 				} __packed mesh_action;
 				struct {
 					u8 action;
@@ -1292,7 +1292,7 @@ struct ieee80211_mgmt {
 					u8 toa[6];
 					__le16 tod_error;
 					__le16 toa_error;
-					u8 variable[0];
+					u8 variable[];
 				} __packed ftm;
 				struct {
 					u8 action_code;
diff --git a/include/linux/stddef.h b/include/linux/stddef.h
index 998a4ba28eba..ca507bd5f808 100644
--- a/include/linux/stddef.h
+++ b/include/linux/stddef.h
@@ -20,7 +20,7 @@ enum {
 #endif
 
 /**
- * sizeof_field(TYPE, MEMBER)
+ * sizeof_field() - Report the size of a struct field in bytes
  *
  * @TYPE: The structure containing the field of interest
  * @MEMBER: The field to return the size of
@@ -28,7 +28,7 @@ enum {
 #define sizeof_field(TYPE, MEMBER) sizeof((((TYPE *)0)->MEMBER))
 
 /**
- * offsetofend(TYPE, MEMBER)
+ * offsetofend() - Report the offset of a struct field within the struct
  *
  * @TYPE: The type of the structure
  * @MEMBER: The member within the structure to get the end offset of
@@ -36,4 +36,65 @@ enum {
 #define offsetofend(TYPE, MEMBER) \
 	(offsetof(TYPE, MEMBER)	+ sizeof_field(TYPE, MEMBER))
 
+/**
+ * struct_group() - Wrap a set of declarations in a mirrored struct
+ *
+ * @NAME: The identifier name of the mirrored sub-struct
+ * @MEMBERS: The member declarations for the mirrored structs
+ *
+ * Used to create an anonymous union of two structs with identical
+ * layout and size: one anonymous and one named. The former can be
+ * used normally without sub-struct naming, and the latter can be
+ * used to reason about the start, end, and size of the group of
+ * struct members.
+ */
+#define struct_group(NAME, MEMBERS...)	\
+	__struct_group(/* no tag */, NAME, /* no attrs */, MEMBERS)
+
+/**
+ * struct_group_attr() - Create a struct_group() with trailing attributes
+ *
+ * @NAME: The identifier name of the mirrored sub-struct
+ * @ATTRS: Any struct attributes to apply
+ * @MEMBERS: The member declarations for the mirrored structs
+ *
+ * Used to create an anonymous union of two structs with identical
+ * layout and size: one anonymous and one named. The former can be
+ * used normally without sub-struct naming, and the latter can be
+ * used to reason about the start, end, and size of the group of
+ * struct members. Includes structure attributes argument.
+ */
+#define struct_group_attr(NAME, ATTRS, MEMBERS...) \
+	__struct_group(/* no tag */, NAME, ATTRS, MEMBERS)
+
+/**
+ * struct_group_tagged() - Create a struct_group with a reusable tag
+ *
+ * @TAG: The tag name for the named sub-struct
+ * @NAME: The identifier name of the mirrored sub-struct
+ * @MEMBERS: The member declarations for the mirrored structs
+ *
+ * Used to create an anonymous union of two structs with identical
+ * layout and size: one anonymous and one named. The former can be
+ * used normally without sub-struct naming, and the latter can be
+ * used to reason about the start, end, and size of the group of
+ * struct members. Includes struct tag argument for the named copy,
+ * so the specified layout can be reused later.
+ */
+#define struct_group_tagged(TAG, NAME, MEMBERS...) \
+	__struct_group(TAG, NAME, /* no attrs */, MEMBERS)
+
+/**
+ * DECLARE_FLEX_ARRAY() - Declare a flexible array usable in a union
+ *
+ * @TYPE: The type of each flexible array element
+ * @NAME: The name of the flexible array member
+ *
+ * In order to have a flexible array member in a union or alone in a
+ * struct, it needs to be wrapped in an anonymous struct with at least 1
+ * named member, but that member can be empty.
+ */
+#define DECLARE_FLEX_ARRAY(TYPE, NAME) \
+	__DECLARE_FLEX_ARRAY(TYPE, NAME)
+
 #endif
diff --git a/include/linux/string.h b/include/linux/string.h
index 5e96d656be7a..5a36608144a9 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -249,15 +249,6 @@ static inline const char *kbasename(const char *path)
 	return tail ? tail + 1 : path;
 }
 
-#define __FORTIFY_INLINE extern __always_inline __attribute__((gnu_inline))
-#define __RENAME(x) __asm__(#x)
-
-void fortify_panic(const char *name) __noreturn __cold;
-void __read_overflow(void) __compiletime_error("detected read beyond size of object passed as 1st parameter");
-void __read_overflow2(void) __compiletime_error("detected read beyond size of object passed as 2nd parameter");
-void __read_overflow3(void) __compiletime_error("detected read beyond size of object passed as 3rd parameter");
-void __write_overflow(void) __compiletime_error("detected write beyond size of object passed as 1st parameter");
-
 #if !defined(__NO_FORTIFY) && defined(__OPTIMIZE__) && defined(CONFIG_FORTIFY_SOURCE)
 #include <linux/fortify-string.h>
 #endif
@@ -281,6 +272,41 @@ static inline void memcpy_and_pad(void *dest, size_t dest_len,
 }
 
 /**
+ * memset_after - Set a value after a struct member to the end of a struct
+ *
+ * @obj: Address of target struct instance
+ * @v: Byte value to repeatedly write
+ * @member: after which struct member to start writing bytes
+ *
+ * This is good for clearing padding following the given member.
+ */
+#define memset_after(obj, v, member)					\
+({									\
+	u8 *__ptr = (u8 *)(obj);					\
+	typeof(v) __val = (v);						\
+	memset(__ptr + offsetofend(typeof(*(obj)), member), __val,	\
+	       sizeof(*(obj)) - offsetofend(typeof(*(obj)), member));	\
+})
+
+/**
+ * memset_startat - Set a value starting at a member to the end of a struct
+ *
+ * @obj: Address of target struct instance
+ * @v: Byte value to repeatedly write
+ * @member: struct member to start writing at
+ *
+ * Note that if there is padding between the prior member and the target
+ * member, memset_after() should be used to clear the prior padding.
+ */
+#define memset_startat(obj, v, member)					\
+({									\
+	u8 *__ptr = (u8 *)(obj);					\
+	typeof(v) __val = (v);						\
+	memset(__ptr + offsetof(typeof(*(obj)), member), __val,		\
+	       sizeof(*(obj)) - offsetof(typeof(*(obj)), member));	\
+})
+
+/**
  * str_has_prefix - Test if a string has a given prefix
  * @str: The string to test
  * @prefix: The string to see if @str starts with
diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index 0999f6317978..ad0c4e041030 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -203,7 +203,7 @@ static inline void copy_overflow(int size, unsigned long count)
 static __always_inline __must_check bool
 check_copy_size(const void *addr, size_t bytes, bool is_source)
 {
-	int sz = __compiletime_object_size(addr);
+	int sz = __builtin_object_size(addr, 0);
 	if (unlikely(sz >= 0 && sz < bytes)) {
 		if (!__builtin_constant_p(bytes))
 			copy_overflow(sz, bytes);
diff --git a/include/scsi/sas.h b/include/scsi/sas.h
index 4726c1bbec65..64154c1fed02 100644
--- a/include/scsi/sas.h
+++ b/include/scsi/sas.h
@@ -323,8 +323,10 @@ struct ssp_response_iu {
 	__be32 sense_data_len;
 	__be32 response_data_len;
 
-	u8     resp_data[0];
-	u8     sense_data[];
+	union {
+		DECLARE_FLEX_ARRAY(u8, resp_data);
+		DECLARE_FLEX_ARRAY(u8, sense_data);
+	};
 } __attribute__ ((packed));
 
 struct ssp_command_iu {
@@ -554,8 +556,10 @@ struct ssp_response_iu {
 	__be32 sense_data_len;
 	__be32 response_data_len;
 
-	u8     resp_data[0];
-	u8     sense_data[];
+	union {
+		DECLARE_FLEX_ARRAY(u8, resp_data);
+		DECLARE_FLEX_ARRAY(u8, sense_data);
+	};
 } __attribute__ ((packed));
 
 struct ssp_command_iu {
diff --git a/include/uapi/drm/mga_drm.h b/include/uapi/drm/mga_drm.h
index 8c4337548ab5..bb31567e66c0 100644
--- a/include/uapi/drm/mga_drm.h
+++ b/include/uapi/drm/mga_drm.h
@@ -279,20 +279,22 @@ typedef struct drm_mga_init {
 
 	unsigned long sarea_priv_offset;
 
-	int chipset;
-	int sgram;
+	__struct_group(/* no tag */, always32bit, /* no attrs */,
+		int chipset;
+		int sgram;
 
-	unsigned int maccess;
+		unsigned int maccess;
 
-	unsigned int fb_cpp;
-	unsigned int front_offset, front_pitch;
-	unsigned int back_offset, back_pitch;
+		unsigned int fb_cpp;
+		unsigned int front_offset, front_pitch;
+		unsigned int back_offset, back_pitch;
 
-	unsigned int depth_cpp;
-	unsigned int depth_offset, depth_pitch;
+		unsigned int depth_cpp;
+		unsigned int depth_offset, depth_pitch;
 
-	unsigned int texture_offset[MGA_NR_TEX_HEAPS];
-	unsigned int texture_size[MGA_NR_TEX_HEAPS];
+		unsigned int texture_offset[MGA_NR_TEX_HEAPS];
+		unsigned int texture_size[MGA_NR_TEX_HEAPS];
+	);
 
 	unsigned long fb_offset;
 	unsigned long mmio_offset;
diff --git a/include/uapi/linux/dlm_device.h b/include/uapi/linux/dlm_device.h
index f880d2831160..e83954c69fff 100644
--- a/include/uapi/linux/dlm_device.h
+++ b/include/uapi/linux/dlm_device.h
@@ -45,13 +45,13 @@ struct dlm_lock_params {
 	void __user *bastaddr;
 	struct dlm_lksb __user *lksb;
 	char lvb[DLM_USER_LVB_LEN];
-	char name[0];
+	char name[];
 };
 
 struct dlm_lspace_params {
 	__u32 flags;
 	__u32 minor;
-	char name[0];
+	char name[];
 };
 
 struct dlm_purge_params {
diff --git a/include/uapi/linux/stddef.h b/include/uapi/linux/stddef.h
index ee8220f8dcf5..3021ea25a284 100644
--- a/include/uapi/linux/stddef.h
+++ b/include/uapi/linux/stddef.h
@@ -4,3 +4,40 @@
 #ifndef __always_inline
 #define __always_inline inline
 #endif
+
+/**
+ * __struct_group() - Create a mirrored named and anonyomous struct
+ *
+ * @TAG: The tag name for the named sub-struct (usually empty)
+ * @NAME: The identifier name of the mirrored sub-struct
+ * @ATTRS: Any struct attributes (usually empty)
+ * @MEMBERS: The member declarations for the mirrored structs
+ *
+ * Used to create an anonymous union of two structs with identical layout
+ * and size: one anonymous and one named. The former's members can be used
+ * normally without sub-struct naming, and the latter can be used to
+ * reason about the start, end, and size of the group of struct members.
+ * The named struct can also be explicitly tagged for layer reuse, as well
+ * as both having struct attributes appended.
+ */
+#define __struct_group(TAG, NAME, ATTRS, MEMBERS...) \
+	union { \
+		struct { MEMBERS } ATTRS; \
+		struct TAG { MEMBERS } ATTRS NAME; \
+	}
+
+/**
+ * __DECLARE_FLEX_ARRAY() - Declare a flexible array usable in a union
+ *
+ * @TYPE: The type of each flexible array element
+ * @NAME: The name of the flexible array member
+ *
+ * In order to have a flexible array member in a union or alone in a
+ * struct, it needs to be wrapped in an anonymous struct with at least 1
+ * named member, but that member can be empty.
+ */
+#define __DECLARE_FLEX_ARRAY(TYPE, NAME)	\
+	struct { \
+		struct { } __empty_ ## NAME; \
+		TYPE NAME[]; \
+	}
diff --git a/include/uapi/rdma/rdma_user_rxe.h b/include/uapi/rdma/rdma_user_rxe.h
index e283c2220aba..7f44d54bb0ab 100644
--- a/include/uapi/rdma/rdma_user_rxe.h
+++ b/include/uapi/rdma/rdma_user_rxe.h
@@ -141,8 +141,8 @@ struct rxe_dma_info {
 	__u32			sge_offset;
 	__u32			reserved;
 	union {
-		__u8		inline_data[0];
-		struct rxe_sge	sge[0];
+		__DECLARE_FLEX_ARRAY(__u8, inline_data);
+		__DECLARE_FLEX_ARRAY(struct rxe_sge, sge);
 	};
 };
 
diff --git a/include/uapi/sound/asoc.h b/include/uapi/sound/asoc.h
index da61398b1f8f..053949287ce8 100644
--- a/include/uapi/sound/asoc.h
+++ b/include/uapi/sound/asoc.h
@@ -240,8 +240,8 @@ struct snd_soc_tplg_vendor_array {
 struct snd_soc_tplg_private {
 	__le32 size;	/* in bytes of private data */
 	union {
-		char data[0];
-		struct snd_soc_tplg_vendor_array array[0];
+		__DECLARE_FLEX_ARRAY(char, data);
+		__DECLARE_FLEX_ARRAY(struct snd_soc_tplg_vendor_array, array);
 	};
 } __attribute__((packed));