docs: add a script to check sysctl docs

This script allows sysctl documentation to be checked against the
kernel source code, to identify missing or obsolete entries. Running
it against 5.5 shows for example that sysctl/kernel.rst has two
obsolete entries and is missing 52 entries.
Signed-off-by: Stephen Kitt <steve@sk2.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/sysctl/kernel.rst

@@ -2,6 +2,9 @@
 Documentation for /proc/sys/kernel/
 ===================================
 
+.. See scripts/check-sysctl-docs to keep this up to date
+
+
 Copyright (c) 1998, 1999,  Rik van Riel <riel@nl.linux.org>
 
 Copyright (c) 2009,        Shen Feng<shen@cn.fujitsu.com>

scripts/check-sysctl-docs

+#!/usr/bin/gawk -f
+# SPDX-License-Identifier: GPL-2.0
+
+# Script to check sysctl documentation against source files
+#
+# Copyright (c) 2020 Stephen Kitt
+
+# Example invocation:
+#	scripts/check-sysctl-docs -vtable="kernel" \
+#		Documentation/admin-guide/sysctl/kernel.rst \
+#		$(git grep -l register_sysctl_)
+#
+# Specify -vdebug=1 to see debugging information
+
+BEGIN {
+    if (!table) {
+	print "Please specify the table to look for using the table variable" > "/dev/stderr"
+	exit 1
+    }
+}
+
+# The following globals are used:
+# children: maps ctl_table names and procnames to child ctl_table names
+# documented: maps documented entries (each key is an entry)
+# entries: maps ctl_table names and procnames to counts (so
+#          enumerating the subkeys for a given ctl_table lists its
+#          procnames)
+# files: maps procnames to source file names
+# paths: maps ctl_path names to paths
+# curpath: the name of the current ctl_path struct
+# curtable: the name of the current ctl_table struct
+# curentry: the name of the current proc entry (procname when parsing
+#           a ctl_table, constructed path when parsing a ctl_path)
+
+
+# Remove punctuation from the given value
+function trimpunct(value) {
+    while (value ~ /^["&]/) {
+	value = substr(value, 2)
+    }
+    while (value ~ /[]["&,}]$/) {
+	value = substr(value, 1, length(value) - 1)
+    }
+    return value
+}
+
+# Print the information for the given entry
+function printentry(entry) {
+    seen[entry]++
+    printf "* %s from %s", entry, file[entry]
+    if (documented[entry]) {
+	printf " (documented)"
+    }
+    print ""
+}
+
+
+# Stage 1: build the list of documented entries
+FNR == NR && /^=+$/ {
+    if (prevline ~ /Documentation for/) {
+	# This is the main title
+	next
+    }
+
+    # The previous line is a section title, parse it
+    $0 = prevline
+    if (debug) print "Parsing " $0
+    inbrackets = 0
+    for (i = 1; i <= NF; i++) {
+	if (length($i) == 0) {
+	    continue
+	}
+	if (!inbrackets && substr($i, 1, 1) == "(") {
+	    inbrackets = 1
+	}
+	if (!inbrackets) {
+	    token = trimpunct($i)
+	    if (length(token) > 0 && token != "and") {
+		if (debug) print trimpunct($i)
+		documented[trimpunct($i)]++
+	    }
+	}
+	if (inbrackets && substr($i, length($i), 1) == ")") {
+	    inbrackets = 0
+	}
+    }
+}
+
+FNR == NR {
+    prevline = $0
+    next
+}
+
+
+# Stage 2: process each file and find all sysctl tables
+BEGINFILE {
+    delete children
+    delete entries
+    delete paths
+    curpath = ""
+    curtable = ""
+    curentry = ""
+    if (debug) print "Processing file " FILENAME
+}
+
+/^static struct ctl_path/ {
+    match($0, /static struct ctl_path ([^][]+)/, tables)
+    curpath = tables[1]
+    if (debug) print "Processing path " curpath
+}
+
+/^static struct ctl_table/ {
+    match($0, /static struct ctl_table ([^][]+)/, tables)
+    curtable = tables[1]
+    if (debug) print "Processing table " curtable
+}
+
+/^};$/ {
+    curpath = ""
+    curtable = ""
+    curentry = ""
+}
+
+curpath && /\.procname[\t ]*=[\t ]*".+"/ {
+    match($0, /.procname[\t ]*=[\t ]*"([^"]+)"/, names)
+    if (curentry) {
+	curentry = curentry "/" names[1]
+    } else {
+	curentry = names[1]
+    }
+    if (debug) print "Setting path " curpath " to " curentry
+    paths[curpath] = curentry
+}
+
+curtable && /\.procname[\t ]*=[\t ]*".+"/ {
+    match($0, /.procname[\t ]*=[\t ]*"([^"]+)"/, names)
+    curentry = names[1]
+    if (debug) print "Adding entry " curentry " to table " curtable
+    entries[curtable][curentry]++
+    file[curentry] = FILENAME
+}
+
+/\.child[\t ]*=/ {
+    child = trimpunct($NF)
+    if (debug) print "Linking child " child " to table " curtable " entry " curentry
+    children[curtable][curentry] = child
+}
+
+/register_sysctl_table\(.*\)/ {
+    match($0, /register_sysctl_table\(([^)]+)\)/, tables)
+    if (debug) print "Registering table " tables[1]
+    if (children[tables[1]][table]) {
+	for (entry in entries[children[tables[1]][table]]) {
+	    printentry(entry)
+	}
+    }
+}
+
+/register_sysctl_paths\(.*\)/ {
+    match($0, /register_sysctl_paths\(([^)]+), ([^)]+)\)/, tables)
+    if (debug) print "Attaching table " tables[2] " to path " tables[1]
+    if (paths[tables[1]] == table) {
+	for (entry in entries[tables[2]]) {
+	    printentry(entry)
+	}
+    }
+    split(paths[tables[1]], components, "/")
+    if (length(components) > 1 && components[1] == table) {
+	# Count the first subdirectory as seen
+	seen[components[2]]++
+    }
+}
+
+
+END {
+    for (entry in documented) {
+	if (!seen[entry]) {
+	    print "No implementation for " entry
+	}
+    }
+}