linux-kernel - [PATCH RFC 03/17] bpf: Add UAPI fields for BCF proof interaction

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Message-Id: <20251106125255.1969938-4-hao.sun@inf.ethz.ch>
Date: Thu,  6 Nov 2025 13:52:41 +0100
From: Hao Sun <sunhao.th@...il.com>
To: bpf@...r.kernel.org
Cc: ast@...nel.org,
	daniel@...earbox.net,
	andrii@...nel.org,
	eddyz87@...il.com,
	john.fastabend@...il.com,
	martin.lau@...ux.dev,
	song@...nel.org,
	yonghong.song@...ux.dev,
	linux-kernel@...r.kernel.org,
	sunhao.th@...il.com,
	Hao Sun <hao.sun@....ethz.ch>
Subject: [PATCH RFC 03/17] bpf: Add UAPI fields for BCF proof interaction

Extend union bpf_attr for BPF_PROG_LOAD to support proof-based verifier
refinement. The new fields let the verifier export the refinement
condition to userspace, preserve its state behind an anon-fd, and later
accept a proof and resume analysis.

- bcf_buf, bcf_buf_size, bcf_buf_true_size: shared buffer used first to
  export expressions+condition to userspace and later to receive a proof.
  true_size refers to the condition size and proof size accordingly.

- bcf_fd: anon-fd that owns the preserved verifier_env. Set by the kernel on
  request; userspace passes it back when submitting a proof.

Tools uapi is also updated.

Signed-off-by: Hao Sun <hao.sun@....ethz.ch>
---
 include/uapi/linux/bpf.h       | 21 +++++++++++++++++++++
 kernel/bpf/syscall.c           | 23 ++++++++++++++++++++++-
 tools/include/uapi/linux/bpf.h | 21 +++++++++++++++++++++
 3 files changed, 64 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 1d73f165394d..fbe99fbc39ab 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -1484,6 +1484,15 @@ enum {
 	BPF_STREAM_STDERR = 2,
 };
 
+/* bcf_flags used in BPF_PROG_LOAD command to indicate if the verifier
+ * requests or the user provides proofs.
+ */
+enum {
+	BCF_F_PROOF_REQUESTED		= (1U << 0),
+	BCF_F_PROOF_PROVIDED		= (1U << 1),
+	BCF_F_PROOF_PATH_UNREACHABLE	= (1U << 2),
+};
+
 union bpf_attr {
 	struct { /* anonymous struct used by BPF_MAP_CREATE command */
 		__u32	map_type;	/* one of enum bpf_map_type */
@@ -1624,6 +1633,18 @@ union bpf_attr {
 		 * verification.
 		 */
 		__s32		keyring_id;
+		/* BCF buffer for both the condition to be proved required by
+		 * the verifier and the proof provided from user space.
+		 */
+		__aligned_u64	bcf_buf;
+		__u32		bcf_buf_size;
+		__u32		bcf_buf_true_size;
+		/* output: BCF fd for loading proof, set by the verifier when
+		 * proof is requested.
+		 */
+		__u32		bcf_fd;
+		/* input/output: proof requested or provided. */
+		__u32		bcf_flags;
 	};
 
 	struct { /* anonymous struct used by BPF_OBJ_* commands */
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 8a129746bd6c..5968b74ed7b2 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2854,7 +2854,7 @@ static int bpf_prog_verify_signature(struct bpf_prog *prog, union bpf_attr *attr
 }
 
 /* last field in 'union bpf_attr' used by this command */
-#define BPF_PROG_LOAD_LAST_FIELD keyring_id
+#define BPF_PROG_LOAD_LAST_FIELD bcf_flags
 
 static int bpf_prog_load(union bpf_attr *attr, bpfptr_t uattr, u32 uattr_size)
 {
@@ -2869,6 +2869,24 @@ static int bpf_prog_load(union bpf_attr *attr, bpfptr_t uattr, u32 uattr_size)
 	if (CHECK_ATTR(BPF_PROG_LOAD))
 		return -EINVAL;
 
+	if (!!attr->bcf_buf != !!attr->bcf_buf_size ||
+	    (attr->bcf_flags & ~(BCF_F_PROOF_PROVIDED |
+				 BCF_F_PROOF_PATH_UNREACHABLE)))
+		return -EINVAL;
+
+	/* Check proof and resume the last analysis. */
+	if (attr->bcf_flags & BCF_F_PROOF_PROVIDED) {
+		if (attr->bcf_buf_true_size > attr->bcf_buf_size ||
+		    !attr->bcf_buf_size)
+			return -EINVAL;
+		/* The resumed analysis must only uses the old, first attr. */
+		memset(attr, 0, offsetof(union bpf_attr, bcf_buf));
+		return -ENOTSUPP;
+	}
+
+	if (attr->bcf_fd || attr->bcf_buf_true_size || attr->bcf_flags)
+		return -EINVAL;
+
 	if (attr->prog_flags & ~(BPF_F_STRICT_ALIGNMENT |
 				 BPF_F_ANY_ALIGNMENT |
 				 BPF_F_TEST_STATE_FREQ |
@@ -2901,6 +2919,9 @@ static int bpf_prog_load(union bpf_attr *attr, bpfptr_t uattr, u32 uattr_size)
 	bpf_cap = bpf_token_capable(token, CAP_BPF);
 	err = -EPERM;
 
+	if (attr->bcf_buf && !bpf_cap)
+		goto put_token;
+
 	if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) &&
 	    (attr->prog_flags & BPF_F_ANY_ALIGNMENT) &&
 	    !bpf_cap)
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 1d73f165394d..fbe99fbc39ab 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -1484,6 +1484,15 @@ enum {
 	BPF_STREAM_STDERR = 2,
 };
 
+/* bcf_flags used in BPF_PROG_LOAD command to indicate if the verifier
+ * requests or the user provides proofs.
+ */
+enum {
+	BCF_F_PROOF_REQUESTED		= (1U << 0),
+	BCF_F_PROOF_PROVIDED		= (1U << 1),
+	BCF_F_PROOF_PATH_UNREACHABLE	= (1U << 2),
+};
+
 union bpf_attr {
 	struct { /* anonymous struct used by BPF_MAP_CREATE command */
 		__u32	map_type;	/* one of enum bpf_map_type */
@@ -1624,6 +1633,18 @@ union bpf_attr {
 		 * verification.
 		 */
 		__s32		keyring_id;
+		/* BCF buffer for both the condition to be proved required by
+		 * the verifier and the proof provided from user space.
+		 */
+		__aligned_u64	bcf_buf;
+		__u32		bcf_buf_size;
+		__u32		bcf_buf_true_size;
+		/* output: BCF fd for loading proof, set by the verifier when
+		 * proof is requested.
+		 */
+		__u32		bcf_fd;
+		/* input/output: proof requested or provided. */
+		__u32		bcf_flags;
 	};
 
 	struct { /* anonymous struct used by BPF_OBJ_* commands */
-- 
2.34.1