futex: Fix inode life-time issue

As reported by Jann, ihold() does not in fact guarantee inode
persistence. And instead of making it so, replace the usage of inode
pointers with a per boot, machine wide, unique inode identifier.

This sequence number is global, but shared (file backed) futexes are
rare enough that this should not become a performance issue.

Reported-by: Jann Horn <jannh@google.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

2 files changed, 11 insertions, 7 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 3cd4fe6b845e..abedbffe2c9e 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -698,6 +698,7 @@ struct inode {
 		struct rcu_head		i_rcu;
 	};
 	atomic64_t		i_version;
+	atomic64_t		i_sequence; /* see futex */
 	atomic_t		i_count;
 	atomic_t		i_dio_count;
 	atomic_t		i_writecount;
diff --git a/include/linux/futex.h b/include/linux/futex.h
index 5cc3fed27d4c..b70df27d7e85 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -31,23 +31,26 @@ struct task_struct;
 
 union futex_key {
 	struct {
+		u64 i_seq;
 		unsigned long pgoff;
-		struct inode *inode;
-		int offset;
+		unsigned int offset;
 	} shared;
 	struct {
+		union {
+			struct mm_struct *mm;
+			u64 __tmp;
+		};
 		unsigned long address;
-		struct mm_struct *mm;
-		int offset;
+		unsigned int offset;
 	} private;
 	struct {
+		u64 ptr;
 		unsigned long word;
-		void *ptr;
-		int offset;
+		unsigned int offset;
 	} both;
 };
 
-#define FUTEX_KEY_INIT (union futex_key) { .both = { .ptr = NULL } }
+#define FUTEX_KEY_INIT (union futex_key) { .both = { .ptr = 0ULL } }
 
 #ifdef CONFIG_FUTEX
 enum {