kernel/api: add debugfs interface for kernel API specifications

Add a debugfs interface, enabled by CONFIG_KAPI_SPEC_DEBUGFS, to expose
kernel API specifications at runtime. This allows tools and users to
query the complete API specifications through the debugfs filesystem.

The interface provides:
- /sys/kernel/debug/kapi/list - lists all available API specifications
- /sys/kernel/debug/kapi/specs/<name> - detailed info for each API
- /sys/kernel/debug/kapi/specs-json/<name> - the same spec as JSON

Each specification file includes:
- Function name, version, and descriptions
- Execution context requirements and flags
- Parameter details with types, flags, and constraints
- Return value specifications and success conditions
- Error codes with descriptions and conditions
- Locking requirements and constraints
- Signal handling specifications
- Examples and notes

This enables runtime introspection of kernel APIs for documentation
tools, static analyzers, and debugging purposes.

Assisted-by: LLM
Signed-off-by: Sasha Levin <sashal@kernel.org>

1 files changed, 553 insertions, 0 deletions

diff --git a/kernel/api/kapi_debugfs.c b/kernel/api/kapi_debugfs.c
new file mode 100644
index 000000000000..5d9c2649a0dc
--- /dev/null
+++ b/kernel/api/kapi_debugfs.c
@@ -0,0 +1,553 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2026 Sasha Levin <sashal@kernel.org>
+ *
+ * Kernel API specification debugfs interface
+ *
+ * This provides a debugfs interface to expose kernel API specifications
+ * at runtime, allowing tools and users to query the complete API specs.
+ */
+
+#include <linux/debugfs.h>
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/seq_file.h>
+#include <linux/kernel_api_spec.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+
+#include "internal.h"
+
+/* Helper to print parameter flags */
+static void print_param_flags(struct seq_file *m, u32 flags)
+{
+	seq_puts(m, "    flags: ");
+	if ((flags & KAPI_PARAM_INOUT) == KAPI_PARAM_INOUT)
+		seq_puts(m, "INOUT ");
+	else if (flags & KAPI_PARAM_IN)
+		seq_puts(m, "IN ");
+	else if (flags & KAPI_PARAM_OUT)
+		seq_puts(m, "OUT ");
+	if (flags & KAPI_PARAM_OPTIONAL)
+		seq_puts(m, "OPTIONAL ");
+	if (flags & KAPI_PARAM_CONST)
+		seq_puts(m, "CONST ");
+	if (flags & KAPI_PARAM_USER)
+		seq_puts(m, "USER ");
+	if (flags & KAPI_PARAM_VOLATILE)
+		seq_puts(m, "VOLATILE ");
+	if (flags & KAPI_PARAM_DMA)
+		seq_puts(m, "DMA ");
+	if (flags & KAPI_PARAM_ALIGNED)
+		seq_puts(m, "ALIGNED ");
+	seq_puts(m, "\n");
+}
+
+/* Helper to print context flags */
+static void print_context_flags(struct seq_file *m, u32 flags)
+{
+	seq_puts(m, "Context flags: ");
+	if (flags & KAPI_CTX_PROCESS)
+		seq_puts(m, "PROCESS ");
+	if (flags & KAPI_CTX_HARDIRQ)
+		seq_puts(m, "HARDIRQ ");
+	if (flags & KAPI_CTX_SOFTIRQ)
+		seq_puts(m, "SOFTIRQ ");
+	if (flags & KAPI_CTX_NMI)
+		seq_puts(m, "NMI ");
+	if (flags & KAPI_CTX_SLEEPABLE)
+		seq_puts(m, "SLEEPABLE ");
+	if (flags & KAPI_CTX_ATOMIC)
+		seq_puts(m, "ATOMIC ");
+	if (flags & KAPI_CTX_PREEMPT_DISABLED)
+		seq_puts(m, "PREEMPT_DISABLED ");
+	if (flags & KAPI_CTX_IRQ_DISABLED)
+		seq_puts(m, "IRQ_DISABLED ");
+	seq_puts(m, "\n");
+}
+
+/*
+ * Print a multi-line value with every line indented, so that no line of it
+ * can be mistaken for a field of its own.
+ */
+static void print_block(struct seq_file *m, const char *text)
+{
+	while (*text) {
+		size_t len = strcspn(text, "\n");
+
+		if (len)
+			seq_printf(m, "  %.*s", (int)len, text);
+		seq_putc(m, '\n');
+		text += len;
+		if (*text == '\n')
+			text++;
+	}
+}
+
+/* Helper to print a list of signed values */
+static void print_s64_list(struct seq_file *m, const char *label,
+			   const s64 *vals, u32 count)
+{
+	u32 i;
+
+	if (!vals || !count)
+		return;
+
+	seq_printf(m, "%s:", label);
+	for (i = 0; i < count; i++)
+		seq_printf(m, " %lld", vals[i]);
+	seq_puts(m, "\n");
+}
+
+/* Helper to print which parameter carries a buffer's element count */
+static void print_size_param(struct seq_file *m,
+			     const struct kernel_api_spec *spec,
+			     const struct kapi_param_spec *param)
+{
+	int idx = param->size_param_idx - 1;
+
+	if (idx < 0)
+		return;
+
+	seq_printf(m, "      size_param: %s (index %d)\n",
+		   idx < spec->param_count && idx < KAPI_MAX_PARAMS ?
+		   spec->params[idx].name : "?", idx);
+	if (param->size_multiplier)
+		seq_printf(m, "      size_multiplier: %zu\n",
+			   param->size_multiplier);
+}
+
+/* Show function for individual API spec */
+static int kapi_spec_show(struct seq_file *m, void *v)
+{
+	struct kernel_api_spec *spec = m->private;
+	int i;
+
+	seq_puts(m, "Kernel API Specification\n");
+	seq_puts(m, "========================\n\n");
+
+	/* Basic info */
+	seq_printf(m, "Name: %s\n", spec->name);
+	seq_printf(m, "Version: %u\n", spec->version);
+	seq_printf(m, "Description: %s\n", spec->description);
+	if (spec->long_description && *spec->long_description) {
+		seq_puts(m, "Long description:\n");
+		print_block(m, spec->long_description);
+	}
+
+	/* Context */
+	print_context_flags(m, spec->context_flags);
+	seq_puts(m, "\n");
+
+	/* Parameters */
+	if (spec->param_count > 0) {
+		seq_printf(m, "Parameters (%u):\n", spec->param_count);
+		for (i = 0; i < spec->param_count && i < KAPI_MAX_PARAMS; i++) {
+			struct kapi_param_spec *param = &spec->params[i];
+
+			seq_printf(m, "  [%d] %s:\n", i, param->name);
+			seq_printf(m, "    type: %s (%s)\n",
+				   kapi_param_type_to_string(param->type), param->type_name);
+			print_param_flags(m, param->flags);
+			if (param->description && *param->description)
+				seq_printf(m, "    description: %s\n", param->description);
+			if (param->size > 0)
+				seq_printf(m, "    size: %zu\n", param->size);
+			if (param->alignment > 0)
+				seq_printf(m, "    alignment: %zu\n", param->alignment);
+
+			/* Print constraints if any */
+			if (param->constraint_type != KAPI_CONSTRAINT_NONE ||
+			    (param->constraints && *param->constraints)) {
+				seq_puts(m, "    constraints:\n");
+				switch (param->constraint_type) {
+				case KAPI_CONSTRAINT_NONE:
+					break;
+				case KAPI_CONSTRAINT_RANGE:
+					seq_puts(m, "      type: range\n");
+					seq_printf(m, "      min: %lld\n", param->min_value);
+					seq_printf(m, "      max: %lld\n", param->max_value);
+					break;
+				case KAPI_CONSTRAINT_MASK:
+					seq_puts(m, "      type: mask\n");
+					seq_printf(m, "      valid_bits: 0x%llx\n",
+						   param->valid_mask);
+					break;
+				case KAPI_CONSTRAINT_ENUM:
+					seq_puts(m, "      type: enum\n");
+					seq_printf(m, "      count: %u\n", param->enum_count);
+					print_s64_list(m, "      values",
+						       param->enum_values,
+						       param->enum_count);
+					break;
+				case KAPI_CONSTRAINT_USER_STRING:
+					seq_puts(m, "      type: user_string\n");
+					seq_printf(m, "      min_len: %lld\n", param->min_value);
+					seq_printf(m, "      max_len: %lld\n", param->max_value);
+					break;
+				case KAPI_CONSTRAINT_USER_PATH:
+					seq_puts(m, "      type: user_path\n");
+					seq_puts(m, "      max_len: PATH_MAX (4096)\n");
+					break;
+				case KAPI_CONSTRAINT_USER_PTR:
+					seq_puts(m, "      type: user_ptr\n");
+					seq_printf(m, "      size: %zu bytes\n", param->size);
+					print_size_param(m, spec, param);
+					break;
+				case KAPI_CONSTRAINT_BUFFER:
+					seq_puts(m, "      type: buffer\n");
+					print_size_param(m, spec, param);
+					break;
+				case KAPI_CONSTRAINT_ALIGNMENT:
+					seq_puts(m, "      type: alignment\n");
+					seq_printf(m, "      alignment: %zu\n", param->alignment);
+					break;
+				case KAPI_CONSTRAINT_POWER_OF_TWO:
+					seq_puts(m, "      type: power_of_two\n");
+					break;
+				case KAPI_CONSTRAINT_PAGE_ALIGNED:
+					seq_puts(m, "      type: page_aligned\n");
+					break;
+				case KAPI_CONSTRAINT_NONZERO:
+					seq_puts(m, "      type: nonzero\n");
+					break;
+				case KAPI_CONSTRAINT_CUSTOM:
+					seq_puts(m, "      type: custom\n");
+					break;
+				default:
+					seq_printf(m, "      type: unknown (%d)\n",
+						   param->constraint_type);
+					break;
+				}
+				if (param->constraints && *param->constraints)
+					seq_printf(m, "      description: %s\n",
+						   param->constraints);
+			}
+			seq_puts(m, "\n");
+		}
+	}
+
+	/* Return value */
+	seq_puts(m, "Return value:\n");
+	seq_printf(m, "  type: %s\n", spec->return_spec.type_name);
+	if (spec->return_spec.description && *spec->return_spec.description)
+		seq_printf(m, "  description: %s\n", spec->return_spec.description);
+
+	switch (spec->return_spec.check_type) {
+	case KAPI_RETURN_EXACT:
+		seq_printf(m, "  success: == %lld\n", spec->return_spec.success_value);
+		break;
+	case KAPI_RETURN_RANGE:
+		seq_printf(m, "  success: [%lld, %lld]\n",
+			   spec->return_spec.success_min,
+			   spec->return_spec.success_max);
+		break;
+	case KAPI_RETURN_FD:
+		seq_puts(m, "  success: valid file descriptor (>= 0)\n");
+		break;
+	case KAPI_RETURN_ERROR_CHECK:
+		seq_puts(m, "  success: error check\n");
+		print_s64_list(m, "  error values",
+			       spec->return_spec.error_values,
+			       spec->return_spec.error_count);
+		break;
+	case KAPI_RETURN_CUSTOM:
+		seq_puts(m, "  success: custom check\n");
+		break;
+	case KAPI_RETURN_NO_RETURN:
+		seq_puts(m, "  success: does not return\n");
+		break;
+	default:
+		break;
+	}
+	seq_puts(m, "\n");
+
+	/* Errors */
+	if (spec->error_count > 0) {
+		seq_printf(m, "Errors (%u):\n", spec->error_count);
+		for (i = 0; i < spec->error_count && i < KAPI_MAX_ERRORS; i++) {
+			struct kapi_error_spec *err = &spec->errors[i];
+
+			seq_printf(m, "  %s (%d): %s\n",
+				   err->name, err->error_code, err->description);
+			if (err->condition && *err->condition)
+				seq_printf(m, "    condition: %s\n", err->condition);
+		}
+		seq_puts(m, "\n");
+	}
+
+	/* Locks */
+	if (spec->lock_count > 0) {
+		seq_printf(m, "Locks (%u):\n", spec->lock_count);
+		for (i = 0; i < spec->lock_count && i < KAPI_MAX_LOCKS; i++) {
+			struct kapi_lock_spec *lock = &spec->locks[i];
+
+			seq_printf(m, "  %s (%s): %s\n", lock->lock_name,
+				   kapi_lock_type_to_string(lock->lock_type),
+				   lock->description);
+			seq_printf(m, "    scope: %s\n",
+				   kapi_lock_scope_to_string(lock->scope));
+		}
+		seq_puts(m, "\n");
+	}
+
+	/* Constraints */
+	if (spec->constraint_count > 0) {
+		seq_printf(m, "Additional constraints (%u):\n", spec->constraint_count);
+		for (i = 0; i < spec->constraint_count && i < KAPI_MAX_CONSTRAINTS; i++) {
+			struct kapi_constraint_spec *cons = &spec->constraints[i];
+
+			seq_printf(m, "  - %s", cons->name);
+			if (cons->description && *cons->description)
+				seq_printf(m, ": %s", cons->description);
+			seq_puts(m, "\n");
+			if (cons->expression && *cons->expression)
+				seq_printf(m, "    expression: %s\n", cons->expression);
+		}
+		seq_puts(m, "\n");
+	}
+
+	/* Signals */
+	if (spec->signal_count > 0) {
+		seq_printf(m, "Signal handling (%u):\n", spec->signal_count);
+		for (i = 0; i < spec->signal_count && i < KAPI_MAX_SIGNALS; i++) {
+			struct kapi_signal_spec *sig = &spec->signals[i];
+
+			seq_printf(m, "  %s (%d):\n", sig->signal_name, sig->signal_num);
+			seq_puts(m, "    direction: ");
+			if (sig->direction & KAPI_SIGNAL_SEND)
+				seq_puts(m, "send ");
+			if (sig->direction & KAPI_SIGNAL_RECEIVE)
+				seq_puts(m, "receive ");
+			if (sig->direction & KAPI_SIGNAL_HANDLE)
+				seq_puts(m, "handle ");
+			if (sig->direction & KAPI_SIGNAL_BLOCK)
+				seq_puts(m, "block ");
+			if (sig->direction & KAPI_SIGNAL_IGNORE)
+				seq_puts(m, "ignore ");
+			seq_puts(m, "\n");
+			seq_puts(m, "    action: ");
+			switch (sig->action) {
+			case KAPI_SIGNAL_ACTION_DEFAULT:
+				seq_puts(m, "default");
+				break;
+			case KAPI_SIGNAL_ACTION_TERMINATE:
+				seq_puts(m, "terminate");
+				break;
+			case KAPI_SIGNAL_ACTION_COREDUMP:
+				seq_puts(m, "coredump");
+				break;
+			case KAPI_SIGNAL_ACTION_STOP:
+				seq_puts(m, "stop");
+				break;
+			case KAPI_SIGNAL_ACTION_CONTINUE:
+				seq_puts(m, "continue");
+				break;
+			case KAPI_SIGNAL_ACTION_CUSTOM:
+				seq_puts(m, "custom");
+				break;
+			case KAPI_SIGNAL_ACTION_RETURN:
+				seq_puts(m, "return");
+				break;
+			case KAPI_SIGNAL_ACTION_RESTART:
+				seq_puts(m, "restart");
+				break;
+			case KAPI_SIGNAL_ACTION_QUEUE:
+				seq_puts(m, "queue");
+				break;
+			case KAPI_SIGNAL_ACTION_DISCARD:
+				seq_puts(m, "discard");
+				break;
+			case KAPI_SIGNAL_ACTION_TRANSFORM:
+				seq_puts(m, "transform");
+				break;
+			default:
+				seq_puts(m, "unknown");
+				break;
+			}
+			seq_puts(m, "\n");
+			if (sig->description && *sig->description)
+				seq_printf(m, "    description: %s\n", sig->description);
+		}
+		seq_puts(m, "\n");
+	}
+
+	/* Side effects */
+	if (spec->side_effect_count > 0) {
+		seq_printf(m, "Side effects (%u):\n", spec->side_effect_count);
+		for (i = 0; i < spec->side_effect_count && i < KAPI_MAX_SIDE_EFFECTS; i++) {
+			const struct kapi_side_effect *eff = &spec->side_effects[i];
+
+			seq_printf(m, "  - %s", eff->target);
+			if (eff->description && *eff->description)
+				seq_printf(m, ": %s", eff->description);
+			if (eff->reversible)
+				seq_puts(m, " (reversible)");
+			seq_puts(m, "\n");
+		}
+		seq_puts(m, "\n");
+	}
+
+	/* State transitions */
+	if (spec->state_trans_count > 0) {
+		seq_printf(m, "State transitions (%u):\n", spec->state_trans_count);
+		for (i = 0; i < spec->state_trans_count && i < KAPI_MAX_STATE_TRANS; i++) {
+			const struct kapi_state_transition *trans = &spec->state_transitions[i];
+
+			seq_printf(m, "  %s: %s -> %s\n", trans->object,
+				   trans->from_state, trans->to_state);
+			if (trans->description && *trans->description)
+				seq_printf(m, "    %s\n", trans->description);
+		}
+		seq_puts(m, "\n");
+	}
+
+	/* Capabilities */
+	if (spec->capability_count > 0) {
+		seq_printf(m, "Capabilities (%u):\n", spec->capability_count);
+		for (i = 0; i < spec->capability_count && i < KAPI_MAX_CAPABILITIES; i++) {
+			const struct kapi_capability_spec *cap = &spec->capabilities[i];
+
+			seq_printf(m, "  %s (%d):\n", cap->cap_name,
+				   cap->capability);
+			if (cap->allows && *cap->allows)
+				seq_printf(m, "    allows: %s\n", cap->allows);
+			if (cap->without_cap && *cap->without_cap)
+				seq_printf(m, "    without: %s\n", cap->without_cap);
+		}
+		seq_puts(m, "\n");
+	}
+
+	/* Additional info */
+	if (spec->examples && *spec->examples) {
+		seq_puts(m, "Examples:\n");
+		print_block(m, spec->examples);
+		seq_putc(m, '\n');
+	}
+	if (spec->notes && *spec->notes) {
+		seq_puts(m, "Notes:\n");
+		print_block(m, spec->notes);
+		seq_putc(m, '\n');
+	}
+
+	return 0;
+}
+
+static int kapi_spec_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, kapi_spec_show, inode->i_private);
+}
+
+static const struct file_operations kapi_spec_fops = {
+	.open = kapi_spec_open,
+	.read = seq_read,
+	.llseek = seq_lseek,
+	.release = single_release,
+};
+
+/*
+ * JSON view of a single spec.  kapi_export_json() writes straight into the
+ * seq_file buffer; when the output does not fit, flagging an overflow makes
+ * seq_file retry with a larger buffer, so no fixed size limit applies.
+ */
+static int kapi_spec_json_show(struct seq_file *m, void *v)
+{
+	const struct kernel_api_spec *spec = m->private;
+	size_t size;
+	char *buf;
+	int ret;
+
+	if (!spec)
+		return -EINVAL;
+
+	size = seq_get_buf(m, &buf);
+	ret = size ? kapi_export_json(spec, buf, size) : -E2BIG;
+	if (ret == -E2BIG) {
+		seq_commit(m, -1);
+		return 0;
+	}
+	if (ret < 0)
+		return ret;
+
+	seq_commit(m, ret);
+	return 0;
+}
+
+static int kapi_spec_json_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, kapi_spec_json_show, inode->i_private);
+}
+
+static const struct file_operations kapi_spec_json_fops = {
+	.open = kapi_spec_json_open,
+	.read = seq_read,
+	.llseek = seq_lseek,
+	.release = single_release,
+};
+
+/*
+ * Show all available API specs.
+ *
+ * Note: This only iterates the static .kapi_specs section. Specs registered
+ * dynamically via kapi_register_spec() are not included in this listing
+ * or in the per-spec debugfs files.
+ */
+static int kapi_list_show(struct seq_file *m, void *v)
+{
+	const struct kernel_api_spec * const *pp;
+	int count = 0;
+
+	seq_puts(m, "Available Kernel API Specifications\n");
+	seq_puts(m, "===================================\n\n");
+
+	for (pp = __start_kapi_specs; pp < __stop_kapi_specs; pp++) {
+		const struct kernel_api_spec *spec = *pp;
+
+		if (!spec)
+			continue;
+		seq_printf(m, "%s - %s\n", spec->name, spec->description);
+		count++;
+	}
+
+	seq_printf(m, "\nTotal: %d specifications\n", count);
+	return 0;
+}
+
+static int kapi_list_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, kapi_list_show, NULL);
+}
+
+static const struct file_operations kapi_list_fops = {
+	.open = kapi_list_open,
+	.read = seq_read,
+	.llseek = seq_lseek,
+	.release = single_release,
+};
+
+static int __init kapi_debugfs_init(void)
+{
+	const struct kernel_api_spec * const *pp;
+	struct dentry *root, *spec_dir, *json_dir;
+
+	root = debugfs_create_dir("kapi", NULL);
+	debugfs_create_file("list", 0444, root, NULL, &kapi_list_fops);
+	spec_dir = debugfs_create_dir("specs", root);
+	json_dir = debugfs_create_dir("specs-json", root);
+
+	for (pp = __start_kapi_specs; pp < __stop_kapi_specs; pp++) {
+		const struct kernel_api_spec *spec = *pp;
+
+		if (!spec || !spec->name)
+			continue;
+		debugfs_create_file(spec->name, 0444, spec_dir,
+				    (void *)spec, &kapi_spec_fops);
+		debugfs_create_file(spec->name, 0444, json_dir,
+				    (void *)spec, &kapi_spec_json_fops);
+	}
+
+	return 0;
+}
+
+/* Initialize as part of kernel, not as a module */
+fs_initcall(kapi_debugfs_init);