tools: add VMA merge tests

Add a variety of VMA merge unit tests to assert that the behaviour of VMA
merge is correct at an abstract level and VMAs are merged or not merged as
expected.

These are intentionally added _before_ we start refactoring vma_merge() in
order that we can continually assert correctness throughout the rest of
the series.

In order to reduce churn going forward, we backport the vma_merge_struct
data type to the test code which we introduce and use in a future commit,
and add wrappers around the merge new and existing VMA cases.

Link: https://lkml.kernel.org/r/1c7a0b43cfad2c511a6b1b52f3507696478ff51a.1725040657.git.lorenzo.stoakes@oracle.comSigned-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Bert Karwatzki <spasswolf@web.de>
Cc: Jeff Xu <jeffxu@chromium.org>
Cc: Jiri Olsa <olsajiri@gmail.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/vma/vma_internal.h

@@ -81,8 +81,6 @@
 
 #define AS_MM_ALL_LOCKS 2
 
-#define current NULL
-
 /* We hardcode this for now. */
 #define sysctl_max_map_count 0x1000000UL
 
@@ -92,6 +90,12 @@ typedef struct pgprot { pgprotval_t pgprot; } pgprot_t;
 typedef unsigned long vm_flags_t;
 typedef __bitwise unsigned int vm_fault_t;
 
+/*
+ * The shared stubs do not implement this, it amounts to an fprintf(STDERR,...)
+ * either way :)
+ */
+#define pr_warn_once pr_err
+
 typedef struct refcount_struct {
 	atomic_t refs;
 } refcount_t;
@@ -100,9 +104,30 @@ struct kref {
 	refcount_t refcount;
 };
 
+/*
+ * Define the task command name length as enum, then it can be visible to
+ * BPF programs.
+ */
+enum {
+	TASK_COMM_LEN = 16,
+};
+
+struct task_struct {
+	char comm[TASK_COMM_LEN];
+	pid_t pid;
+	struct mm_struct *mm;
+};
+
+struct task_struct *get_current(void);
+#define current get_current()
+
 struct anon_vma {
 	struct anon_vma *root;
 	struct rb_root_cached rb_root;
+
+	/* Test fields. */
+	bool was_cloned;
+	bool was_unlinked;
 };
 
 struct anon_vma_chain {
@@ -682,13 +707,21 @@ static inline int vma_dup_policy(struct vm_area_struct *, struct vm_area_struct
 	return 0;
 }
 
-static inline int anon_vma_clone(struct vm_area_struct *, struct vm_area_struct *)
+static inline int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
 {
+	/* For testing purposes. We indicate that an anon_vma has been cloned. */
+	if (src->anon_vma != NULL) {
+		dst->anon_vma = src->anon_vma;
+		dst->anon_vma->was_cloned = true;
+	}
+
 	return 0;
 }
 
-static inline void vma_start_write(struct vm_area_struct *)
+static inline void vma_start_write(struct vm_area_struct *vma)
 {
+	/* Used to indicate to tests that a write operation has begun. */
+	vma->vm_lock_seq++;
 }
 
 static inline void vma_adjust_trans_huge(struct vm_area_struct *vma,
@@ -759,8 +792,10 @@ static inline void vma_assert_write_locked(struct vm_area_struct *)
 {
 }
 
-static inline void unlink_anon_vmas(struct vm_area_struct *)
+static inline void unlink_anon_vmas(struct vm_area_struct *vma)
 {
+	/* For testing purposes, indicate that the anon_vma was unlinked. */
+	vma->anon_vma->was_unlinked = true;
 }
 
 static inline void anon_vma_unlock_write(struct anon_vma *)