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

include/uapi/linux/bpf.h

@@ -377,412 +377,22 @@ union bpf_attr {
 	};
 } __attribute__((aligned(8)));
 
-/* BPF helper function descriptions:
- *
- * void *bpf_map_lookup_elem(&map, &key)
- *     Return: Map value or NULL
- *
- * int bpf_map_update_elem(&map, &key, &value, flags)
- *     Return: 0 on success or negative error
- *
- * int bpf_map_delete_elem(&map, &key)
- *     Return: 0 on success or negative error
- *
- * int bpf_probe_read(void *dst, int size, void *src)
- *     Return: 0 on success or negative error
- *
- * u64 bpf_ktime_get_ns(void)
- *     Return: current ktime
- *
- * int bpf_trace_printk(const char *fmt, int fmt_size, ...)
- *     Return: length of buffer written or negative error
- *
- * u32 bpf_prandom_u32(void)
- *     Return: random value
- *
- * u32 bpf_raw_smp_processor_id(void)
- *     Return: SMP processor ID
- *
- * int bpf_skb_store_bytes(skb, offset, from, len, flags)
- *     store bytes into packet
- *     @skb: pointer to skb
- *     @offset: offset within packet from skb->mac_header
- *     @from: pointer where to copy bytes from
- *     @len: number of bytes to store into packet
- *     @flags: bit 0 - if true, recompute skb->csum
- *             other bits - reserved
- *     Return: 0 on success or negative error
- *
- * int bpf_l3_csum_replace(skb, offset, from, to, flags)
- *     recompute IP checksum
- *     @skb: pointer to skb
- *     @offset: offset within packet where IP checksum is located
- *     @from: old value of header field
- *     @to: new value of header field
- *     @flags: bits 0-3 - size of header field
- *             other bits - reserved
- *     Return: 0 on success or negative error
- *
- * int bpf_l4_csum_replace(skb, offset, from, to, flags)
- *     recompute TCP/UDP checksum
- *     @skb: pointer to skb
- *     @offset: offset within packet where TCP/UDP checksum is located
- *     @from: old value of header field
- *     @to: new value of header field
- *     @flags: bits 0-3 - size of header field
- *             bit 4 - is pseudo header
- *             other bits - reserved
- *     Return: 0 on success or negative error
- *
- * int bpf_tail_call(ctx, prog_array_map, index)
- *     jump into another BPF program
- *     @ctx: context pointer passed to next program
- *     @prog_array_map: pointer to map which type is BPF_MAP_TYPE_PROG_ARRAY
- *     @index: 32-bit index inside array that selects specific program to run
- *     Return: 0 on success or negative error
- *
- * int bpf_clone_redirect(skb, ifindex, flags)
- *     redirect to another netdev
- *     @skb: pointer to skb
- *     @ifindex: ifindex of the net device
- *     @flags: bit 0 - if set, redirect to ingress instead of egress
- *             other bits - reserved
- *     Return: 0 on success or negative error
- *
- * u64 bpf_get_current_pid_tgid(void)
- *     Return: current->tgid << 32 | current->pid
- *
- * u64 bpf_get_current_uid_gid(void)
- *     Return: current_gid << 32 | current_uid
- *
- * int bpf_get_current_comm(char *buf, int size_of_buf)
- *     stores current->comm into buf
- *     Return: 0 on success or negative error
- *
- * u32 bpf_get_cgroup_classid(skb)
- *     retrieve a proc's classid
- *     @skb: pointer to skb
- *     Return: classid if != 0
- *
- * int bpf_skb_vlan_push(skb, vlan_proto, vlan_tci)
- *     Return: 0 on success or negative error
- *
- * int bpf_skb_vlan_pop(skb)
- *     Return: 0 on success or negative error
- *
- * int bpf_skb_get_tunnel_key(skb, key, size, flags)
- * int bpf_skb_set_tunnel_key(skb, key, size, flags)
- *     retrieve or populate tunnel metadata
- *     @skb: pointer to skb
- *     @key: pointer to 'struct bpf_tunnel_key'
- *     @size: size of 'struct bpf_tunnel_key'
- *     @flags: room for future extensions
- *     Return: 0 on success or negative error
- *
- * u64 bpf_perf_event_read(map, flags)
- *     read perf event counter value
- *     @map: pointer to perf_event_array map
- *     @flags: index of event in the map or bitmask flags
- *     Return: value of perf event counter read or error code
- *
- * int bpf_redirect(ifindex, flags)
- *     redirect to another netdev
- *     @ifindex: ifindex of the net device
- *     @flags:
- *	  cls_bpf:
- *          bit 0 - if set, redirect to ingress instead of egress
- *          other bits - reserved
- *	  xdp_bpf:
- *	    all bits - reserved
- *     Return: cls_bpf: TC_ACT_REDIRECT on success or TC_ACT_SHOT on error
- *	       xdp_bfp: XDP_REDIRECT on success or XDP_ABORT on error
- * int bpf_redirect_map(map, key, flags)
- *     redirect to endpoint in map
- *     @map: pointer to dev map
- *     @key: index in map to lookup
- *     @flags: --
- *     Return: XDP_REDIRECT on success or XDP_ABORT on error
- *
- * u32 bpf_get_route_realm(skb)
- *     retrieve a dst's tclassid
- *     @skb: pointer to skb
- *     Return: realm if != 0
- *
- * int bpf_perf_event_output(ctx, map, flags, data, size)
- *     output perf raw sample
- *     @ctx: struct pt_regs*
- *     @map: pointer to perf_event_array map
- *     @flags: index of event in the map or bitmask flags
- *     @data: data on stack to be output as raw data
- *     @size: size of data
- *     Return: 0 on success or negative error
- *
- * int bpf_get_stackid(ctx, map, flags)
- *     walk user or kernel stack and return id
- *     @ctx: struct pt_regs*
- *     @map: pointer to stack_trace map
- *     @flags: bits 0-7 - numer of stack frames to skip
- *             bit 8 - collect user stack instead of kernel
- *             bit 9 - compare stacks by hash only
- *             bit 10 - if two different stacks hash into the same stackid
- *                      discard old
- *             other bits - reserved
- *     Return: >= 0 stackid on success or negative error
- *
- * s64 bpf_csum_diff(from, from_size, to, to_size, seed)
- *     calculate csum diff
- *     @from: raw from buffer
- *     @from_size: length of from buffer
- *     @to: raw to buffer
- *     @to_size: length of to buffer
- *     @seed: optional seed
- *     Return: csum result or negative error code
- *
- * int bpf_skb_get_tunnel_opt(skb, opt, size)
- *     retrieve tunnel options metadata
- *     @skb: pointer to skb
- *     @opt: pointer to raw tunnel option data
- *     @size: size of @opt
- *     Return: option size
- *
- * int bpf_skb_set_tunnel_opt(skb, opt, size)
- *     populate tunnel options metadata
- *     @skb: pointer to skb
- *     @opt: pointer to raw tunnel option data
- *     @size: size of @opt
- *     Return: 0 on success or negative error
- *
- * int bpf_skb_change_proto(skb, proto, flags)
- *     Change protocol of the skb. Currently supported is v4 -> v6,
- *     v6 -> v4 transitions. The helper will also resize the skb. eBPF
- *     program is expected to fill the new headers via skb_store_bytes
- *     and lX_csum_replace.
- *     @skb: pointer to skb
- *     @proto: new skb->protocol type
- *     @flags: reserved
- *     Return: 0 on success or negative error
- *
- * int bpf_skb_change_type(skb, type)
- *     Change packet type of skb.
- *     @skb: pointer to skb
- *     @type: new skb->pkt_type type
- *     Return: 0 on success or negative error
- *
- * int bpf_skb_under_cgroup(skb, map, index)
- *     Check cgroup2 membership of skb
- *     @skb: pointer to skb
- *     @map: pointer to bpf_map in BPF_MAP_TYPE_CGROUP_ARRAY type
- *     @index: index of the cgroup in the bpf_map
- *     Return:
- *       == 0 skb failed the cgroup2 descendant test
- *       == 1 skb succeeded the cgroup2 descendant test
- *        < 0 error
- *
- * u32 bpf_get_hash_recalc(skb)
- *     Retrieve and possibly recalculate skb->hash.
- *     @skb: pointer to skb
- *     Return: hash
- *
- * u64 bpf_get_current_task(void)
- *     Returns current task_struct
- *     Return: current
- *
- * int bpf_probe_write_user(void *dst, void *src, int len)
- *     safely attempt to write to a location
- *     @dst: destination address in userspace
- *     @src: source address on stack
- *     @len: number of bytes to copy
- *     Return: 0 on success or negative error
- *
- * int bpf_current_task_under_cgroup(map, index)
- *     Check cgroup2 membership of current task
- *     @map: pointer to bpf_map in BPF_MAP_TYPE_CGROUP_ARRAY type
- *     @index: index of the cgroup in the bpf_map
- *     Return:
- *       == 0 current failed the cgroup2 descendant test
- *       == 1 current succeeded the cgroup2 descendant test
- *        < 0 error
- *
- * int bpf_skb_change_tail(skb, len, flags)
- *     The helper will resize the skb to the given new size, to be used f.e.
- *     with control messages.
- *     @skb: pointer to skb
- *     @len: new skb length
- *     @flags: reserved
- *     Return: 0 on success or negative error
- *
- * int bpf_skb_pull_data(skb, len)
- *     The helper will pull in non-linear data in case the skb is non-linear
- *     and not all of len are part of the linear section. Only needed for
- *     read/write with direct packet access.
- *     @skb: pointer to skb
- *     @len: len to make read/writeable
- *     Return: 0 on success or negative error
- *
- * s64 bpf_csum_update(skb, csum)
- *     Adds csum into skb->csum in case of CHECKSUM_COMPLETE.
- *     @skb: pointer to skb
- *     @csum: csum to add
- *     Return: csum on success or negative error
- *
- * void bpf_set_hash_invalid(skb)
- *     Invalidate current skb->hash.
- *     @skb: pointer to skb
- *
- * int bpf_get_numa_node_id()
- *     Return: Id of current NUMA node.
- *
- * int bpf_skb_change_head()
- *     Grows headroom of skb and adjusts MAC header offset accordingly.
- *     Will extends/reallocae as required automatically.
- *     May change skb data pointer and will thus invalidate any check
- *     performed for direct packet access.
- *     @skb: pointer to skb
- *     @len: length of header to be pushed in front
- *     @flags: Flags (unused for now)
- *     Return: 0 on success or negative error
- *
- * int bpf_xdp_adjust_head(xdp_md, delta)
- *     Adjust the xdp_md.data by delta
- *     @xdp_md: pointer to xdp_md
- *     @delta: An positive/negative integer to be added to xdp_md.data
- *     Return: 0 on success or negative on error
- *
- * int bpf_probe_read_str(void *dst, int size, const void *unsafe_ptr)
- *     Copy a NUL terminated string from unsafe address. In case the string
- *     length is smaller than size, the target is not padded with further NUL
- *     bytes. In case the string length is larger than size, just count-1
- *     bytes are copied and the last byte is set to NUL.
- *     @dst: destination address
- *     @size: maximum number of bytes to copy, including the trailing NUL
- *     @unsafe_ptr: unsafe address
- *     Return:
- *       > 0 length of the string including the trailing NUL on success
- *       < 0 error
- *
- * u64 bpf_get_socket_cookie(skb)
- *     Get the cookie for the socket stored inside sk_buff.
- *     @skb: pointer to skb
- *     Return: 8 Bytes non-decreasing number on success or 0 if the socket
- *     field is missing inside sk_buff
- *
- * u32 bpf_get_socket_uid(skb)
- *     Get the owner uid of the socket stored inside sk_buff.
- *     @skb: pointer to skb
- *     Return: uid of the socket owner on success or overflowuid if failed.
- *
- * u32 bpf_set_hash(skb, hash)
- *     Set full skb->hash.
- *     @skb: pointer to skb
- *     @hash: hash to set
- *
- * int bpf_setsockopt(bpf_socket, level, optname, optval, optlen)
- *     Calls setsockopt. Not all opts are available, only those with
- *     integer optvals plus TCP_CONGESTION.
- *     Supported levels: SOL_SOCKET and IPPROTO_TCP
- *     @bpf_socket: pointer to bpf_socket
- *     @level: SOL_SOCKET or IPPROTO_TCP
- *     @optname: option name
- *     @optval: pointer to option value
- *     @optlen: length of optval in bytes
- *     Return: 0 or negative error
- *
- * int bpf_getsockopt(bpf_socket, level, optname, optval, optlen)
- *     Calls getsockopt. Not all opts are available.
- *     Supported levels: IPPROTO_TCP
- *     @bpf_socket: pointer to bpf_socket
- *     @level: IPPROTO_TCP
- *     @optname: option name
- *     @optval: pointer to option value
- *     @optlen: length of optval in bytes
- *     Return: 0 or negative error
- *
- * int bpf_sock_ops_cb_flags_set(bpf_sock_ops, flags)
- *     Set callback flags for sock_ops
- *     @bpf_sock_ops: pointer to bpf_sock_ops_kern struct
- *     @flags: flags value
- *     Return: 0 for no error
- *             -EINVAL if there is no full tcp socket
- *             bits in flags that are not supported by current kernel
- *
- * int bpf_skb_adjust_room(skb, len_diff, mode, flags)
- *     Grow or shrink room in sk_buff.
- *     @skb: pointer to skb
- *     @len_diff: (signed) amount of room to grow/shrink
- *     @mode: operation mode (enum bpf_adj_room_mode)
- *     @flags: reserved for future use
- *     Return: 0 on success or negative error code
- *
- * int bpf_sk_redirect_map(map, key, flags)
- *     Redirect skb to a sock in map using key as a lookup key for the
- *     sock in map.
- *     @map: pointer to sockmap
- *     @key: key to lookup sock in map
- *     @flags: reserved for future use
- *     Return: SK_PASS
- *
- * int bpf_sock_map_update(skops, map, key, flags)
- *	@skops: pointer to bpf_sock_ops
- *	@map: pointer to sockmap to update
- *	@key: key to insert/update sock in map
- *	@flags: same flags as map update elem
- *
- * int bpf_xdp_adjust_meta(xdp_md, delta)
- *     Adjust the xdp_md.data_meta by delta
- *     @xdp_md: pointer to xdp_md
- *     @delta: An positive/negative integer to be added to xdp_md.data_meta
- *     Return: 0 on success or negative on error
- *
- * int bpf_perf_event_read_value(map, flags, buf, buf_size)
- *     read perf event counter value and perf event enabled/running time
- *     @map: pointer to perf_event_array map
- *     @flags: index of event in the map or bitmask flags
- *     @buf: buf to fill
- *     @buf_size: size of the buf
- *     Return: 0 on success or negative error code
- *
- * int bpf_perf_prog_read_value(ctx, buf, buf_size)
- *     read perf prog attached perf event counter and enabled/running time
- *     @ctx: pointer to ctx
- *     @buf: buf to fill
- *     @buf_size: size of the buf
- *     Return : 0 on success or negative error code
- *
- * int bpf_override_return(pt_regs, rc)
- *	@pt_regs: pointer to struct pt_regs
- *	@rc: the return value to set
- *
- * int bpf_msg_redirect_map(map, key, flags)
- *     Redirect msg to a sock in map using key as a lookup key for the
- *     sock in map.
- *     @map: pointer to sockmap
- *     @key: key to lookup sock in map
- *     @flags: reserved for future use
- *     Return: SK_PASS
- *
- * int bpf_bind(ctx, addr, addr_len)
- *     Bind socket to address. Only binding to IP is supported, no port can be
- *     set in addr.
- *     @ctx: pointer to context of type bpf_sock_addr
- *     @addr: pointer to struct sockaddr to bind socket to
- *     @addr_len: length of sockaddr structure
- *     Return: 0 on success or negative error code
- *
- * int bpf_xdp_adjust_tail(xdp_md, delta)
- *     Adjust the xdp_md.data_end by delta. Only shrinking of packet's
- *     size is supported.
- *     @xdp_md: pointer to xdp_md
- *     @delta: A negative integer to be added to xdp_md.data_end
- *     Return: 0 on success or negative on error
- *
- * int bpf_skb_get_xfrm_state(skb, index, xfrm_state, size, flags)
- *     retrieve XFRM state
- *     @skb: pointer to skb
- *     @index: index of the xfrm state in the secpath
- *     @key: pointer to 'struct bpf_xfrm_state'
- *     @size: size of 'struct bpf_xfrm_state'
- *     @flags: room for future extensions
- *     Return: 0 on success or negative error
+/* The description below is an attempt at providing documentation to eBPF
+ * developers about the multiple available eBPF helper functions. It can be
+ * parsed and used to produce a manual page. The workflow is the following,
+ * and requires the rst2man utility:
+ *
+ *     $ ./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 in order to produce this external documentation, some RST
+ * formatting is used in the descriptions to get "bold" and "italics" in
+ * manual pages. Also note that the few trailing white spaces are
+ * intentional, removing them would break paragraphs for rst2man.
+ *
+ * Start of BPF helper function descriptions:
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\

scripts/bpf_helpers_doc.py

+#!/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()