[PATCH bpf-next v3 1/7] bpf: Correct verifier diagnostic attribution for stack reads

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

Verifier memory diagnostics currently label every fixed-offset stack
read rejected by check_stack_read_fixed_off() as uninitialized. Dynptr,
iterator, and IRQ-flag slots instead contain initialized
verifier-managed state, so the report incorrectly suggests
initialization or CAP_PERFMON.

The variable-offset read without a destination register is currently
reached by atomic read-modify-write instructions, but that follows from
the call sites rather than the check_stack_read() interface. Its
diagnostic also attributes the access to a helper.

Classify rejected stack reads by slot type. Retain the existing
uninitialized report for STACK_INVALID, and describe verifier-managed
slots as opaque state. Use instruction-neutral wording for the
variable-offset read while leaving the existing verifier messages
unchanged.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Link: https://lore.kernel.org/bpf/20260815065956.49D2B1F000E9@smtp.kernel.org/
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/verifier.c | 63 ++++++++++++++++++++++++++++++-------------
 1 file changed, 45 insertions(+), 18 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 4c20e5bf8b69..7fbe32ce9449 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3978,19 +3978,46 @@ static int mark_reg_stack_read(struct bpf_verifier_env *env,
 	return 0;
 }
 
-static void bpf_diag_stack_read_uninit(struct bpf_verifier_env *env, int off, int i,
-				       int size)
+static void bpf_diag_stack_read_invalid(struct bpf_verifier_env *env, int off, int i, int size,
+					enum bpf_stack_slot_type type)
 {
-	const char *reason;
+	const char *problem, *reason, *suggestion, *kind;
+
+	if (type == STACK_INVALID) {
+		reason = bpf_diag_fmt(
+			env, "This rejected read uses %d bytes at stack offset %d, but byte %d in that range is uninitialized on this path. "
+			"Programs loaded with CAP_PERFMON can be allowed to read uninitialized stack bytes, but this program is being rejected without that allowance.",
+			size, off, i);
+		bpf_diag_memory(
+			env, env->insn_idx, "uninitialized stack read", reason,
+			"Initialize every byte in the stack range before reading it, adjust the offset and size so the read covers only initialized bytes, "
+			"or load with CAP_PERFMON if uninitialized stack reads are intended.");
+		return;
+	}
 
-	reason = bpf_diag_fmt(env,
-			      "This rejected read uses %d bytes at stack offset %d, but byte %d in that range is uninitialized on this path. "
-		"Programs loaded with CAP_PERFMON can be allowed to read uninitialized stack bytes, but this program is being rejected without that allowance.",
-		size, off, i);
-	bpf_diag_memory(
-		env, env->insn_idx, "uninitialized stack read", reason,
-		"Initialize every byte in the stack range before reading it, adjust the offset and size so the read covers only initialized bytes, "
-		"or load with CAP_PERFMON if uninitialized stack reads are intended.");
+	switch (type) {
+	case STACK_DYNPTR:
+		kind = "dynptr";
+		suggestion = "Use dynptr helpers or kfuncs to access the object represented by the dynptr instead of reading the dynptr state directly.";
+		break;
+	case STACK_ITER:
+		kind = "iterator";
+		suggestion = "Use iterator kfuncs to advance or destroy the iterator instead of reading its state directly.";
+		break;
+	case STACK_IRQ_FLAG:
+		kind = "IRQ flag";
+		suggestion = "Pass the saved IRQ flag to the matching restore kfunc instead of reading its state directly.";
+		break;
+	default:
+		return;
+	}
+
+	problem = bpf_diag_fmt(env, "direct read of %s stack state", kind);
+	reason = bpf_diag_fmt(
+		env, "This rejected read uses %d bytes at stack offset %d, but byte %d in that range belongs to verifier-managed %s state. "
+		"This state has an opaque representation that BPF programs cannot read directly.",
+		size, off, i, kind);
+	bpf_diag_memory(env, env->insn_idx, problem, reason, suggestion);
 }
 
 /* Read the stack at 'off' and put the results into the register indicated by
@@ -4081,7 +4108,7 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env,
 					} else {
 						verbose(env, "invalid read from stack off %d+%d size %d\n",
 							off, i, size);
-						bpf_diag_stack_read_uninit(env, off, i, size);
+						bpf_diag_stack_read_invalid(env, off, i, size, type);
 					}
 					return -EACCES;
 				}
@@ -4140,7 +4167,7 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env,
 			} else {
 				verbose(env, "invalid read from stack off %d+%d size %d\n",
 					off, i, size);
-				bpf_diag_stack_read_uninit(env, off, i, size);
+				bpf_diag_stack_read_invalid(env, off, i, size, type);
 			}
 			return -EACCES;
 		}
@@ -4238,13 +4265,13 @@ static int check_stack_read(struct bpf_verifier_env *env,
 		tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
 		verbose(env, "variable offset stack pointer cannot be passed into helper function; var_off=%s off=%d size=%d\n",
 			tn_buf, off, size);
-		reason = bpf_diag_fmt(env,
-				      "The helper would access the stack through variable offset %s plus fixed offset %d and size %d. "
-			"Helper stack memory arguments require a constant stack offset and a precise initialized range.",
+		reason = bpf_diag_fmt(
+			env, "The instruction would access the stack through variable offset %s plus fixed offset %d and size %d. "
+			"This stack access requires a constant stack offset and a precise initialized range.",
 			tn_buf, off, size);
 		bpf_diag_memory(
-			env, env->insn_idx, "variable stack access", reason,
-			"Use a fixed stack offset for helper memory arguments, or copy the needed bytes into a fixed stack slot first.");
+			env, env->insn_idx, "variable-offset stack access", reason,
+			"Use a fixed stack offset for the instruction, selecting the target stack slot on separate control-flow paths if necessary.");
 		return -EACCES;
 	}
 	/* Variable offset is prohibited for unprivileged mode for simplicity
-- 
2.53.0