bpf: add script and prepare bpf.h for new helpers documentation

Remove previous "overview" of eBPF helpers from user bpf.h header.
Replace it by a comment explaining how to process the new documentation
(to come in following patches) with a Python script to produce RST, then
man page documentation.

Also add the aforementioned Python script under scripts/. It is used to
process include/uapi/linux/bpf.h and to extract helper descriptions, to
turn it into a RST document that can further be processed with rst2man
to produce a man page. The script takes one "--filename <path/to/file>"
option. If the script is launched from scripts/ in the kernel root
directory, it should be able to find the location of the header to
parse, and "--filename <path/to/file>" is then optional. If it cannot
find the file, then the option becomes mandatory. RST-formatted
documentation is printed to standard output.

Typical workflow for producing the final man page would be:

    $ ./scripts/bpf_helpers_doc.py \
            --filename include/uapi/linux/bpf.h > /tmp/bpf-helpers.rst
    $ rst2man /tmp/bpf-helpers.rst > /tmp/bpf-helpers.7
    $ man /tmp/bpf-helpers.7

Note that the tool kernel-doc cannot be used to document eBPF helpers,
whose signatures are not available directly in the header files
(pre-processor directives are used to produce them at the beginning of
the compilation process).

v4:
- Also remove overviews for newly added bpf_xdp_adjust_tail() and
  bpf_skb_get_xfrm_state().
- Remove vague statement about what helpers are restricted to GPL
  programs in "LICENSE" section for man page footer.
- Replace license boilerplate with SPDX tag for Python script.

v3:
- Change license for man page.
- Remove "for safety reasons" from man page header text.
- Change "packets metadata" to "packets" in man page header text.
- Move and fix comment on helpers introducing no overhead.
- Remove "NOTES" section from man page footer.
- Add "LICENSE" section to man page footer.
- Edit description of file include/uapi/linux/bpf.h in man page footer.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

1 files changed, 421 insertions, 0 deletions

diff --git a/scripts/bpf_helpers_doc.py b/scripts/bpf_helpers_doc.py
new file mode 100755
index 000000000000..30ba0fee36e4
--- /dev/null
+++ b/scripts/bpf_helpers_doc.py
@@ -0,0 +1,421 @@
+#!/usr/bin/python3
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Copyright (C) 2018 Netronome Systems, Inc.
+
+# In case user attempts to run with Python 2.
+from __future__ import print_function
+
+import argparse
+import re
+import sys, os
+
+class NoHelperFound(BaseException):
+    pass
+
+class ParsingError(BaseException):
+    def __init__(self, line='<line not provided>', reader=None):
+        if reader:
+            BaseException.__init__(self,
+                                   'Error at file offset %d, parsing line: %s' %
+                                   (reader.tell(), line))
+        else:
+            BaseException.__init__(self, 'Error parsing line: %s' % line)
+
+class Helper(object):
+    """
+    An object representing the description of an eBPF helper function.
+    @proto: function prototype of the helper function
+    @desc: textual description of the helper function
+    @ret: description of the return value of the helper function
+    """
+    def __init__(self, proto='', desc='', ret=''):
+        self.proto = proto
+        self.desc = desc
+        self.ret = ret
+
+    def proto_break_down(self):
+        """
+        Break down helper function protocol into smaller chunks: return type,
+        name, distincts arguments.
+        """
+        arg_re = re.compile('^((const )?(struct )?(\w+|...))( (\**)(\w+))?$')
+        res = {}
+        proto_re = re.compile('^(.+) (\**)(\w+)\(((([^,]+)(, )?){1,5})\)$')
+
+        capture = proto_re.match(self.proto)
+        res['ret_type'] = capture.group(1)
+        res['ret_star'] = capture.group(2)
+        res['name']     = capture.group(3)
+        res['args'] = []
+
+        args    = capture.group(4).split(', ')
+        for a in args:
+            capture = arg_re.match(a)
+            res['args'].append({
+                'type' : capture.group(1),
+                'star' : capture.group(6),
+                'name' : capture.group(7)
+            })
+
+        return res
+
+class HeaderParser(object):
+    """
+    An object used to parse a file in order to extract the documentation of a
+    list of eBPF helper functions. All the helpers that can be retrieved are
+    stored as Helper object, in the self.helpers() array.
+    @filename: name of file to parse, usually include/uapi/linux/bpf.h in the
+               kernel tree
+    """
+    def __init__(self, filename):
+        self.reader = open(filename, 'r')
+        self.line = ''
+        self.helpers = []
+
+    def parse_helper(self):
+        proto    = self.parse_proto()
+        desc     = self.parse_desc()
+        ret      = self.parse_ret()
+        return Helper(proto=proto, desc=desc, ret=ret)
+
+    def parse_proto(self):
+        # Argument can be of shape:
+        #   - "void"
+        #   - "type  name"
+        #   - "type *name"
+        #   - Same as above, with "const" and/or "struct" in front of type
+        #   - "..." (undefined number of arguments, for bpf_trace_printk())
+        # There is at least one term ("void"), and at most five arguments.
+        p = re.compile('^ \* ((.+) \**\w+\((((const )?(struct )?(\w+|\.\.\.)( \**\w+)?)(, )?){1,5}\))$')
+        capture = p.match(self.line)
+        if not capture:
+            raise NoHelperFound
+        self.line = self.reader.readline()
+        return capture.group(1)
+
+    def parse_desc(self):
+        p = re.compile('^ \* \tDescription$')
+        capture = p.match(self.line)
+        if not capture:
+            # Helper can have empty description and we might be parsing another
+            # attribute: return but do not consume.
+            return ''
+        # Description can be several lines, some of them possibly empty, and it
+        # stops when another subsection title is met.
+        desc = ''
+        while True:
+            self.line = self.reader.readline()
+            if self.line == ' *\n':
+                desc += '\n'
+            else:
+                p = re.compile('^ \* \t\t(.*)')
+                capture = p.match(self.line)
+                if capture:
+                    desc += capture.group(1) + '\n'
+                else:
+                    break
+        return desc
+
+    def parse_ret(self):
+        p = re.compile('^ \* \tReturn$')
+        capture = p.match(self.line)
+        if not capture:
+            # Helper can have empty retval and we might be parsing another
+            # attribute: return but do not consume.
+            return ''
+        # Return value description can be several lines, some of them possibly
+        # empty, and it stops when another subsection title is met.
+        ret = ''
+        while True:
+            self.line = self.reader.readline()
+            if self.line == ' *\n':
+                ret += '\n'
+            else:
+                p = re.compile('^ \* \t\t(.*)')
+                capture = p.match(self.line)
+                if capture:
+                    ret += capture.group(1) + '\n'
+                else:
+                    break
+        return ret
+
+    def run(self):
+        # Advance to start of helper function descriptions.
+        offset = self.reader.read().find('* Start of BPF helper function descriptions:')
+        if offset == -1:
+            raise Exception('Could not find start of eBPF helper descriptions list')
+        self.reader.seek(offset)
+        self.reader.readline()
+        self.reader.readline()
+        self.line = self.reader.readline()
+
+        while True:
+            try:
+                helper = self.parse_helper()
+                self.helpers.append(helper)
+            except NoHelperFound:
+                break
+
+        self.reader.close()
+        print('Parsed description of %d helper function(s)' % len(self.helpers),
+              file=sys.stderr)
+
+###############################################################################
+
+class Printer(object):
+    """
+    A generic class for printers. Printers should be created with an array of
+    Helper objects, and implement a way to print them in the desired fashion.
+    @helpers: array of Helper objects to print to standard output
+    """
+    def __init__(self, helpers):
+        self.helpers = helpers
+
+    def print_header(self):
+        pass
+
+    def print_footer(self):
+        pass
+
+    def print_one(self, helper):
+        pass
+
+    def print_all(self):
+        self.print_header()
+        for helper in self.helpers:
+            self.print_one(helper)
+        self.print_footer()
+
+class PrinterRST(Printer):
+    """
+    A printer for dumping collected information about helpers as a ReStructured
+    Text page compatible with the rst2man program, which can be used to
+    generate a manual page for the helpers.
+    @helpers: array of Helper objects to print to standard output
+    """
+    def print_header(self):
+        header = '''\
+.. Copyright (C) All BPF authors and contributors from 2014 to present.
+.. See git log include/uapi/linux/bpf.h in kernel tree for details.
+.. 
+.. %%%LICENSE_START(VERBATIM)
+.. Permission is granted to make and distribute verbatim copies of this
+.. manual provided the copyright notice and this permission notice are
+.. preserved on all copies.
+.. 
+.. Permission is granted to copy and distribute modified versions of this
+.. manual under the conditions for verbatim copying, provided that the
+.. entire resulting derived work is distributed under the terms of a
+.. permission notice identical to this one.
+.. 
+.. Since the Linux kernel and libraries are constantly changing, this
+.. manual page may be incorrect or out-of-date.  The author(s) assume no
+.. responsibility for errors or omissions, or for damages resulting from
+.. the use of the information contained herein.  The author(s) may not
+.. have taken the same level of care in the production of this manual,
+.. which is licensed free of charge, as they might when working
+.. professionally.
+.. 
+.. Formatted or processed versions of this manual, if unaccompanied by
+.. the source, must acknowledge the copyright and authors of this work.
+.. %%%LICENSE_END
+.. 
+.. Please do not edit this file. It was generated from the documentation
+.. located in file include/uapi/linux/bpf.h of the Linux kernel sources
+.. (helpers description), and from scripts/bpf_helpers_doc.py in the same
+.. repository (header and footer).
+
+===========
+BPF-HELPERS
+===========
+-------------------------------------------------------------------------------
+list of eBPF helper functions
+-------------------------------------------------------------------------------
+
+:Manual section: 7
+
+DESCRIPTION
+===========
+
+The extended Berkeley Packet Filter (eBPF) subsystem consists in programs
+written in a pseudo-assembly language, then attached to one of the several
+kernel hooks and run in reaction of specific events. This framework differs
+from the older, "classic" BPF (or "cBPF") in several aspects, one of them being
+the ability to call special functions (or "helpers") from within a program.
+These functions are restricted to a white-list of helpers defined in the
+kernel.
+
+These helpers are used by eBPF programs to interact with the system, or with
+the context in which they work. For instance, they can be used to print
+debugging messages, to get the time since the system was booted, to interact
+with eBPF maps, or to manipulate network packets. Since there are several eBPF
+program types, and that they do not run in the same context, each program type
+can only call a subset of those helpers.
+
+Due to eBPF conventions, a helper can not have more than five arguments.
+
+Internally, eBPF programs call directly into the compiled helper functions
+without requiring any foreign-function interface. As a result, calling helpers
+introduces no overhead, thus offering excellent performance.
+
+This document is an attempt to list and document the helpers available to eBPF
+developers. They are sorted by chronological order (the oldest helpers in the
+kernel at the top).
+
+HELPERS
+=======
+'''
+        print(header)
+
+    def print_footer(self):
+        footer = '''
+EXAMPLES
+========
+
+Example usage for most of the eBPF helpers listed in this manual page are
+available within the Linux kernel sources, at the following locations:
+
+* *samples/bpf/*
+* *tools/testing/selftests/bpf/*
+
+LICENSE
+=======
+
+eBPF programs can have an associated license, passed along with the bytecode
+instructions to the kernel when the programs are loaded. The format for that
+string is identical to the one in use for kernel modules (Dual licenses, such
+as "Dual BSD/GPL", may be used). Some helper functions are only accessible to
+programs that are compatible with the GNU Privacy License (GPL).
+
+In order to use such helpers, the eBPF program must be loaded with the correct
+license string passed (via **attr**) to the **bpf**\ () system call, and this
+generally translates into the C source code of the program containing a line
+similar to the following:
+
+::
+
+        char ____license[] __attribute__((section("license"), used)) = "GPL";
+
+IMPLEMENTATION
+==============
+
+This manual page is an effort to document the existing eBPF helper functions.
+But as of this writing, the BPF sub-system is under heavy development. New eBPF
+program or map types are added, along with new helper functions. Some helpers
+are occasionally made available for additional program types. So in spite of
+the efforts of the community, this page might not be up-to-date. If you want to
+check by yourself what helper functions exist in your kernel, or what types of
+programs they can support, here are some files among the kernel tree that you
+may be interested in:
+
+* *include/uapi/linux/bpf.h* is the main BPF header. It contains the full list
+  of all helper functions, as well as many other BPF definitions including most
+  of the flags, structs or constants used by the helpers.
+* *net/core/filter.c* contains the definition of most network-related helper
+  functions, and the list of program types from which they can be used.
+* *kernel/trace/bpf_trace.c* is the equivalent for most tracing program-related
+  helpers.
+* *kernel/bpf/verifier.c* contains the functions used to check that valid types
+  of eBPF maps are used with a given helper function.
+* *kernel/bpf/* directory contains other files in which additional helpers are
+  defined (for cgroups, sockmaps, etc.).
+
+Compatibility between helper functions and program types can generally be found
+in the files where helper functions are defined. Look for the **struct
+bpf_func_proto** objects and for functions returning them: these functions
+contain a list of helpers that a given program type can call. Note that the
+**default:** label of the **switch ... case** used to filter helpers can call
+other functions, themselves allowing access to additional helpers. The
+requirement for GPL license is also in those **struct bpf_func_proto**.
+
+Compatibility between helper functions and map types can be found in the
+**check_map_func_compatibility**\ () function in file *kernel/bpf/verifier.c*.
+
+Helper functions that invalidate the checks on **data** and **data_end**
+pointers for network processing are listed in function
+**bpf_helper_changes_pkt_data**\ () in file *net/core/filter.c*.
+
+SEE ALSO
+========
+
+**bpf**\ (2),
+**cgroups**\ (7),
+**ip**\ (8),
+**perf_event_open**\ (2),
+**sendmsg**\ (2),
+**socket**\ (7),
+**tc-bpf**\ (8)'''
+        print(footer)
+
+    def print_proto(self, helper):
+        """
+        Format function protocol with bold and italics markers. This makes RST
+        file less readable, but gives nice results in the manual page.
+        """
+        proto = helper.proto_break_down()
+
+        print('**%s %s%s(' % (proto['ret_type'],
+                              proto['ret_star'].replace('*', '\\*'),
+                              proto['name']),
+              end='')
+
+        comma = ''
+        for a in proto['args']:
+            one_arg = '{}{}'.format(comma, a['type'])
+            if a['name']:
+                if a['star']:
+                    one_arg += ' {}**\ '.format(a['star'].replace('*', '\\*'))
+                else:
+                    one_arg += '** '
+                one_arg += '*{}*\\ **'.format(a['name'])
+            comma = ', '
+            print(one_arg, end='')
+
+        print(')**')
+
+    def print_one(self, helper):
+        self.print_proto(helper)
+
+        if (helper.desc):
+            print('\tDescription')
+            # Do not strip all newline characters: formatted code at the end of
+            # a section must be followed by a blank line.
+            for line in re.sub('\n$', '', helper.desc, count=1).split('\n'):
+                print('{}{}'.format('\t\t' if line else '', line))
+
+        if (helper.ret):
+            print('\tReturn')
+            for line in helper.ret.rstrip().split('\n'):
+                print('{}{}'.format('\t\t' if line else '', line))
+
+        print('')
+
+###############################################################################
+
+# If script is launched from scripts/ from kernel tree and can access
+# ../include/uapi/linux/bpf.h, use it as a default name for the file to parse,
+# otherwise the --filename argument will be required from the command line.
+script = os.path.abspath(sys.argv[0])
+linuxRoot = os.path.dirname(os.path.dirname(script))
+bpfh = os.path.join(linuxRoot, 'include/uapi/linux/bpf.h')
+
+argParser = argparse.ArgumentParser(description="""
+Parse eBPF header file and generate documentation for eBPF helper functions.
+The RST-formatted output produced can be turned into a manual page with the
+rst2man utility.
+""")
+if (os.path.isfile(bpfh)):
+    argParser.add_argument('--filename', help='path to include/uapi/linux/bpf.h',
+                           default=bpfh)
+else:
+    argParser.add_argument('--filename', help='path to include/uapi/linux/bpf.h')
+args = argParser.parse_args()
+
+# Parse file.
+headerParser = HeaderParser(args.filename)
+headerParser.run()
+
+# Print formatted output to standard output.
+printer = PrinterRST(headerParser.helpers)
+printer.print_all()