bpf: Refactor active lock management

When bpf_spin_lock was introduced originally, there was deliberation on
whether to use an array of lock IDs, but since bpf_spin_lock is limited
to holding a single lock at any given time, we've been using a single ID
to identify the held lock.

In preparation for introducing spin locks that can be taken multiple
times, introduce support for acquiring multiple lock IDs. For this
purpose, reuse the acquired_refs array and store both lock and pointer
references. We tag the entry with REF_TYPE_PTR or REF_TYPE_LOCK to
disambiguate and find the relevant entry. The ptr field is used to track
the map_ptr or btf (for bpf_obj_new allocations) to ensure locks can be
matched with protected fields within the same "allocation", i.e.
bpf_obj_new object or map value.

The struct active_lock is changed to an int as the state is part of the
acquired_refs array, and we only need active_lock as a cheap way of
detecting lock presence.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20241109231430.2475236-2-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

1 files changed, 25 insertions, 28 deletions

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 4513372c5bc8..d84beed92ae4 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -48,22 +48,6 @@ enum bpf_reg_liveness {
 	REG_LIVE_DONE = 0x8, /* liveness won't be updating this register anymore */
 };
 
-/* For every reg representing a map value or allocated object pointer,
- * we consider the tuple of (ptr, id) for them to be unique in verifier
- * context and conside them to not alias each other for the purposes of
- * tracking lock state.
- */
-struct bpf_active_lock {
-	/* This can either be reg->map_ptr or reg->btf. If ptr is NULL,
-	 * there's no active lock held, and other fields have no
-	 * meaning. If non-NULL, it indicates that a lock is held and
-	 * id member has the reg->id of the register which can be >= 0.
-	 */
-	void *ptr;
-	/* This will be reg->id */
-	u32 id;
-};
-
 #define ITER_PREFIX "bpf_iter_"
 
 enum bpf_iter_state {
@@ -266,6 +250,13 @@ struct bpf_stack_state {
 };
 
 struct bpf_reference_state {
+	/* Each reference object has a type. Ensure REF_TYPE_PTR is zero to
+	 * default to pointer reference on zero initialization of a state.
+	 */
+	enum ref_state_type {
+		REF_TYPE_PTR = 0,
+		REF_TYPE_LOCK,
+	} type;
 	/* Track each reference created with a unique id, even if the same
 	 * instruction creates the reference multiple times (eg, via CALL).
 	 */
@@ -274,17 +265,23 @@ struct bpf_reference_state {
 	 * is used purely to inform the user of a reference leak.
 	 */
 	int insn_idx;
-	/* There can be a case like:
-	 * main (frame 0)
-	 *  cb (frame 1)
-	 *   func (frame 3)
-	 *    cb (frame 4)
-	 * Hence for frame 4, if callback_ref just stored boolean, it would be
-	 * impossible to distinguish nested callback refs. Hence store the
-	 * frameno and compare that to callback_ref in check_reference_leak when
-	 * exiting a callback function.
-	 */
-	int callback_ref;
+	union {
+		/* There can be a case like:
+		 * main (frame 0)
+		 *  cb (frame 1)
+		 *   func (frame 3)
+		 *    cb (frame 4)
+		 * Hence for frame 4, if callback_ref just stored boolean, it would be
+		 * impossible to distinguish nested callback refs. Hence store the
+		 * frameno and compare that to callback_ref in check_reference_leak when
+		 * exiting a callback function.
+		 */
+		int callback_ref;
+		/* Use to keep track of the source object of a lock, to ensure
+		 * it matches on unlock.
+		 */
+		void *ptr;
+	};
 };
 
 struct bpf_retval_range {
@@ -332,6 +329,7 @@ struct bpf_func_state {
 
 	/* The following fields should be last. See copy_func_state() */
 	int acquired_refs;
+	int active_locks;
 	struct bpf_reference_state *refs;
 	/* The state of the stack. Each element of the array describes BPF_REG_SIZE
 	 * (i.e. 8) bytes worth of stack memory.
@@ -434,7 +432,6 @@ struct bpf_verifier_state {
 	u32 insn_idx;
 	u32 curframe;
 
-	struct bpf_active_lock active_lock;
 	bool speculative;
 	bool active_rcu_lock;
 	u32 active_preempt_lock;