FreeIPMI Coding

by 

Albert Chu
chu11@llnl.gov

These are some short descriptions on coding style, API style, design
decisions, and various other thoughts.

1) Code Style
-------------

The GNU coding style was selected for FreeIPMI.  Please try to follow
the coding style used in the rest of FreeIPMI.  Here's a short example
that covers the generics of our code style.

int
main(int argc, char **argv)
{
  int a = 0;
  int b = 1;

  if (a == 1)
    printf("yoda\n");

  if (a == 5
      || b == 1)
    {
      printf("foobar\n");
      printf("xyzzy\n");
    }

  while (a++ < 5)
    printf("lala\n");

  while (b++ < 7)
    {
      printf("blah\n");
      printf("garble\n");
    }
}

2) Parameter Checking
---------------------

Please carefully check the input parameters on the inputs your
program and/or functions take.  Minor parsing issues can lead to
catastrophic mistakes in IPMI.

For example, suppose you have a --power-control option that takes a
number to represent a type of operation (on, off, etc.).  Suppose a
user inputs "--power-control=foobar". The "foobar" will be read as a
'0' by strtoul().  If not properly checked, the '0' can be passed to
the IPMI Chassis Control command, which uses the '0' to power off a
node.

In programs, when appropriate, output error messages to the user
indicating that how and why the inputted parameters were incorrect.

3) Code Consistency
-------------------

Please keep your code as consistent as possible to other code in
FreeIPMI.  That includes code indenting style, brace style, API style,
and naming convention (which is discussed in more detail below).

Although there may be situations that a particular API style or naming
convention will make things easier for you and your code (such as
shortening the name of a function, decreasing the number of parameters
you need to pass to a function via a struct, etc.), we ask that your
code be consistent so that it does not confuse other developers.

If there is a distinct technical reason that you must use a different
API style, please bring it up with the FreeIPMI authors.

For example, pretty much all of the "fill" functions in libfreeipmi
take the exact parameters they need to fill the fiid object which is
passed along as a parameter.  All parameters are passed by value, not
by a pointer or other method (i.e. object, struct, etc.).

Exceptions do exist.  For example, fill_cmd_chassis_identify() takes
parameters passed by pointer instead of passed by value.  The reason
is that both fields are optional and need not be filled according to
the IPMI specification.  The pointer gives the caller the ability to
set values (by passing a valid pointer) or not (by passing NULL).

4) Libfreeipmi naming/function parameter conventions
----------------------------------------------------

The naming style in libfreeipmi was developed primarily for the
purpose of readability when code is being compared to the IPMI
specification.

Due to the size of the IPMI spec, there will be a lot of code.  In
earlier versions of FreeIPMI, there was confusion on where code was
located, what parameters were called, how parameters should be input,
etc. due to different people using different abbreviations styles,
putting functions out of order with the spec, in different files,
using/not-using different bitmasks, etc.  The code has been auditted
and cleaned up since then.

So when adding new functions/templates/parameters/files/etc. to
libfreeipmi, please name them consistently to the rest of the
libfreeipmi library and the IPMI specification.

This includes:
- naming functions/templates/parameters/files based on the spec
- in most cases, not abbreviating any words (or using consistent
  abbreviations in the rest of the library, check first!)
- matching parameter lists to the templates and in the same order
- ordering functions/templates/parameters/files/etc. consistently with the spec.

For example: 

ipmi-messaging-support-cmds.c

is the file for messaging support commands, chapter 22 of the IPMI 2.0 spec.

tmpl_cmd_get_channel_authentication_capabilities_rq
tmpl_cmd_get_channel_authentication_capabilities_rs

are the templates for the Get Channel Authentication Capapilities
command.  They are listed first in the file b/c they are the first
commands in the IPMI Messaging Support Commands chapter implemented in
that file.

fiid_template_t tmpl_cmd_get_channel_authentication_capabilities_rq =
  {
    {8, "cmd", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {4, "channel_number", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {4, "reserved1", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {4, "maximum_privilege_level", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {4, "reserved2", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {0, "", 0}
  };

fiid_template_t tmpl_cmd_get_channel_authentication_capabilities_rs =
  {
    {8,  "cmd", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {8,  "comp_code", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {8,  "channel_number", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {1,  "authentication_type.none", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {1,  "authentication_type.md2", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {1,  "authentication_type.md5", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {1,  "authentication_type.reserved1", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {1,  "authentication_type.straight_password_key", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {1,  "authentication_type.oem_prop", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {2,  "authentication_type.reserved2", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {1,  "authentication_status.anonymous_login", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {1,  "authentication_status.null_username", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {1,  "authentication_status.non_null_username", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {1,  "authentication_status.user_level_authentication", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {1,  "authentication_status.per_message_authentication", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {3,  "authentication_status.reserved", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {8,  "reserved1", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {24, "oem_id", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {8,  "oem_auxiliary_data", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED}
  };

The fields listed in the templates above directly match table 22-15,
the Get Channel Authentication Capabilities request and response
commands.

int8_t fill_cmd_get_channel_authentication_capabilities (uint8_t channel_number,
                                                         uint8_t maximum_privilege_level,
                                                         fiid_obj_t obj_cmd_rq);

The function above matches the naming and takes exactly the parameters
needed by the Get Channel Authentication Capabilities request
template.

The coding conditions specified above may lead to function names or
function parameters names that exceed the 80 column mark or having
very long parameter lists.  We accept this annoyance (or poor coding
style, we admit it), as we consider matching the specification as a
more important need in libfreeipmi.

For example: 

int8_t
fill_cmd_set_lan_configuration_parameters_authentication_type_enables (uint8_t channel_number,
                                                                       uint8_t callback_level_none,
                                                                       uint8_t callback_level_md2,
                                                                       uint8_t callback_level_md5,
                                                                       uint8_t callback_level_straight_password,
                                                                       uint8_t callback_level_oem_proprietary,
                                                                       uint8_t user_level_none,
                                                                       uint8_t user_level_md2,
                                                                       uint8_t user_level_md5,
                                                                       uint8_t user_level_straight_password,
                                                                       uint8_t user_level_oem_proprietary,
                                                                       uint8_t operator_level_none,
                                                                       uint8_t operator_level_md2,
                                                                       uint8_t operator_level_md5,
                                                                       uint8_t operator_level_straight_password,
                                                                       uint8_t operator_level_oem_proprietary,
                                                                       uint8_t admin_level_none,
                                                                       uint8_t admin_level_md2,
                                                                       uint8_t admin_level_md5,
                                                                       uint8_t admin_level_straight_password,
                                                                       uint8_t admin_level_oem_proprietary,
                                                                       uint8_t oem_level_none,
                                                                       uint8_t oem_level_md2,
                                                                       uint8_t oem_level_md5,
                                                                       uint8_t oem_level_straight_password,
                                                                       uint8_t oem_level_oem_proprietary,
                                                                       fiid_obj_t obj_cmd_rq);

The function name and parameters look pretty long and terrible.  But
the names and fields exactly match the get authentication type enables
fields listed in Table 23-4.  There should be very little difficulty
understanding what this funciton does, how it should be called, and
what the parameters are if you are reading along with the spec.

Because we want the code to match the IPMI spec as closely as
possible, we currently accept the code inefficiencies (due to large
stacks of parameters) that come with having long parameters lists and
the atrocities of having gigantic 25+ parameter function calls in
code.

5) Error tracing/syslogging macros
----------------------------------

Throughout libfreeipmi, you will see various macros such as:

ERR

or 

FIID_OBJ_CREATE_CLEANUP

These are macros that will expand to something along the lines of the
following:

#define ERR(expr)                                                       \
do {                                                                    \
  if (!(expr))                                                          \
    {                                                                   \
      __IPMI_SYSLOG;                                                    \
      __IPMI_TRACE;                                                     \
      return (-1);                                                      \
    }                                                                   \
} while (0)

The primary purpose of these macros is to log a trace of errors to
stderr/syslog when appropriate debugging is enabled.  It has allowed
the FreeIPMI developers to debug issues very quickly and figure out
vendor IPMI compliance very quickly.

6) Fiid vs. other Marshalling/Unmarshalling Styles
--------------------------------------------------

Several programmers have asked us why we have chosen a relatively
unpopular/different method to marshall/unmarshall IPMI packets and
build network packets.

First, there are several classic methods for marshalling/unmarshalling
data when using structs to represent a packet.

Method A: Marshall/Unmarshall "manually":
-----------------------------------------

struct packet
{
  int field_1; /* 1 bit */
  int field_2; /* 3 bits */
  int field_3; /* 4 bits */
  int field_4; /* 16 bits */
};

char packet_buffer[1024];
struct packet pkt;

/* receive data from the network into packet_buffer */

pkt.field_1 = packet_buffer[0] & 0x01;
pkt.field_2 = packet_buffer[0] & 0x0E >> 1;
pkt.field_3 = packet_buffer[0] & 0xF0 >> 4;
#if LITTLE_ENDIAN_HOST
pkt.field_4 = packet_buffer[2] | packet_buffer[1] << 8;;
#else
pkt.field_4 = packet_buffer[1] | packet_buffer[2] << 8;;
#endif

Pros:

A) No need to deal with struct packing issues in the compiler.
B) The struct definition describes packets exactly.
C) Relatively efficient.

Cons:

A) Have to deal with endian problems.
B) Lots of code.  Structs and marshalling/unmarshalling functions are
required for each packet type.
C) Relatively difficult to deal with optional fields. (You'll need
flags in the struct to indicate if a field was set/unset.)
D) Relatively difficult to deal with variable length fields. (You'll
need a length parameter in the struct to indicate the length of a
field.)
E) Difficult to do large packet dumps (you only get hex, or you have
to create a separate debug function for each struct or packet type if
you want nicer output).

Method B: Cast a buffer to a packed struct:
-------------------------------------------

For Example:

struct packet
{
  int field_1 : 1;
  int field_2 : 3;
  int field_3 : 4;
  int field_4 : 16; 
};

char my_packet_buffer[1024];

struct packet *pkt = (struct packet *)my_packet_buffer_ptr;

#if LITTLE_ENDIAN_HOST
pkt->field_4 = ntohs(pkt->field_4);
#endif

Pros:

A) Less code.  You may not need to create specific functions for
marshalling/unmarshalling each packet type.
B) The struct definition describes packets exactly.
C) Very efficient (little actual marshalling/unmarshalling needs to be done.)

Cons:

A) Have to deal with endian problems.
B) Have to deal with portability of struct packing techniques (there
are differences in compilers, but nowadays, this may be
easier/portable that I originally believed it to be).
C) No mechanism to deal with optional fields (how do you know if
something is set?)
D) No mechanism to deal with variable length fields (how do you know
how long the field is?)
E) Difficult to do large packet dumps (you only get hex, or you have
to create a separate debug function for each struct or packet type if
you want nicer output).

Our Method C: string_name -> bitmask mapping
--------------------------------------------

The "FreeIPMI Interface Definition" or 'fiid' API in libfreeipmi uses
a string_name/bit_count template and an API to get and set values in a
packet to handle marshalling/unmarshalling.  The following is an
example of this template:

fiid_template_t tmpl_cmd_set_channel_security_keys_rs =
  {
    {8,   "cmd", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {8,   "comp_code", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {2,   "lock_status", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {6,   "reserved", FIID_FIELD_REQUIRED | FIID_FIELD_LENGTH_FIXED},
    {160, "key_value", FIID_FIELD_OPTIONAL | FIID_FIELD_LENGTH_VARIABLE},
    {0, "", 0}
  };

The following are a few of the API functions used for FIID to give you
an idea for the fiid API:

fiid_obj_t fiid_obj_create (fiid_template_t tmpl);
int32_t fiid_obj_errnum(fiid_obj_t obj);
int8_t fiid_obj_clear (fiid_obj_t obj);
int8_t fiid_obj_set (fiid_obj_t obj, char *field, uint64_t val);
int8_t fiid_obj_get (fiid_obj_t obj, char *field, uint64_t *val);
int32_t fiid_obj_get_all (fiid_obj_t obj, uint8_t *data, uint32_t data_len);
int32_t fiid_obj_set_all (fiid_obj_t obj, uint8_t *data, uint32_t data_len);

The pros and cons of the fiid method are:

Pros:

A) No need to deal with endian problems (handled internally in the API).
B) No need to deal with struct packing issues (bit shifts are handled
internally in the API).
C) Easier to deal with optional fields (just don't set a field, on a
read the api can identify if a field is set or not).
D) Easier to deal with variable length fields (set whatever length you
want, on a read the api can identify the length of the field).
E) Templates describe the packets exactly.
F) Easy to do large packet dumps (fields and values easily output and identified).
G) Significantly reduce the amount of marshalling/unmarshalling/debug
code needed.

Cons:

A) Need to learn/use a reasonably sized API.
B) Pretty inefficient (lots of string comparisons).
C) Lots of duplicate templates (relative inability to "inherit"
structs without #define mangling/hackery).

The authors acknowledge that the fiid interface is far less efficient
than the traditional methods and may annoy some.  However, some of
the below are the big reasons why this was developed and chosen over
traditional methods.

A) The IPMI specification is very large, so reducing code size weighed
in as an important factor for the authors.  This allowed there to be
fewer marshalling/unmarshalling/debug functions.  By one authors
counting in the specification, there are 304 different payloads in the
IPMI specification.  (This doesn't count bitmask mapping, per packet
unique completion codes, header/trailer packet construction,
authentication, encryption, and all that other good stuff.)

B) Although IPMI has a specification, the lack of IPMI compliance from
vendors is a well known problem in the open-source community.  The
templates have saved developers countless hours of tcpdump debugging
time due to the easy method by which packets can be dumped with their
fields and values identified.  It is very easy to find vendor IPMI
compliance problems quickly.

Here's an example of a dump:

pwopr2: : RMCP Header:
pwopr2: : ------------
pwopr2: [               6h] = version[ 8b]
pwopr2: [               0h] = reserved[ 8b]
pwopr2: [              FFh] = sequence_number[ 8b]
pwopr2: [               7h] = message_class.class[ 5b]
pwopr2: [               0h] = message_class.reserved[ 2b]
pwopr2: [               0h] = message_class.ack[ 1b]
pwopr2: : IPMI Session Header:
pwopr2: : --------------------
pwopr2: [               0h] = authentication_type[ 8b]
pwopr2: [               0h] = session_sequence_number[32b]
pwopr2: [               0h] = session_id[32b]
pwopr2: [               9h] = ipmi_msg_len[ 8b]
pwopr2: : IPMI Message Header:
pwopr2: : --------------------
pwopr2: [              20h] = rs_addr[ 8b]
pwopr2: [               0h] = rs_lun[ 2b]
pwopr2: [               6h] = net_fn[ 6b]
pwopr2: [              C8h] = checksum1[ 8b]
pwopr2: [              81h] = rq_addr[ 8b]
pwopr2: [               0h] = rq_lun[ 2b]
pwopr2: [              26h] = rq_seq[ 6b]
pwopr2: : IPMI Command Data:
pwopr2: : ------------------
pwopr2: [              38h] = cmd[ 8b]
pwopr2: [               Eh] = channel_number[ 4b]
pwopr2: [               0h] = reserved1[ 3b]
pwopr2: [               1h] = get_ipmi_v2.0_extended_data[ 1b]
pwopr2: [               2h] = maximum_privilege_level[ 4b]
pwopr2: [               0h] = reserved2[ 4b]
pwopr2: : IPMI Trailer:
pwopr2: : --------------
pwopr2: [              1Fh] = checksum2[ 8b]

C) There are a relatively large number of optional fields and variable
length fields in the IPMI specification.  As stated above, the
traditional struct based marshalling/unmarshalling could have issues
with it.

7) Non-generic error messages
-----------------------------

Under some circumstances, it may be preferred to return generic error
messages to the user, so that a malicious user cannot infer remote
login information from different error messages returned.  For
example, returning a generic error message of "Permission Denied"
would not give a malicious user information on whether the username or
password was input incorrectly.

Although implemented earlier on, the authors have elected to not
implement this now.  There are many vendor implementations of IPMI and
many configuration options (authentication mechanism, cipher suite id,
username, password, K_g, privilege level) needed for proper IPMI
session establishment.  The number of error messages that could be
mapped into a generic "Permission Denied" would make it too difficult
for users to determine why they failed to connect properly.  The
overall worth of implementing a generic "Permission Denied" error
message just doesn't seem worth it now.

8) Get Channel Authentication Capabilities Command
--------------------------------------------------

The Get Channel Authentication Capabilities Command is typically the
first packet sent in the IPMI session.  It returns information 
on the remote machine's support of:

A) IPMI 1.5 authentication mechanisms (i.e. md2, md5, etc.)
B) IPMI 1.5 and/or IPMI 2.0
C) per msg authentication
D) K_g status
E) null username/non-null username/anonymous logins

Currently, in FreeIPMI, we check each of these values during the
session setup to determine if a person can connect to the remote
machine later in the protocol:

A) If the user input an unsupported authentication mechanism, we
return an error.

B) If the user requested IPMI 2.0, but the remote machine doesn't
support IPMI 2.0, we return an error.

C) We determine if per msg authentication should be considered later
in the protocol session.

D) If the user was required/not-required to input a K_g value, we
return an error appropriately.

E) If the user input an unsupported username/password combination, we
return an error appropriately.

There is a question as to what values above, if any, need to be
checked and appropriate errors returned to the user.  The Get Channel
Authentication Capabilities command is often implemented incorrectly
by a number of vendors, so that overall benefit of checks
has been put in question.  The authors have elected to keep 
all the checks for the following reasons.

* 'A' and 'B' should be checked to avoid potential timeouts:

  - Later in the protocol, the password could be sent/hashed
    incorrectly, leading to a timeout because packets are not accepted by
    the remote machine.

  - If the remote machine does not support IPMI 2.0, later packets
    could timeout because the remote machine does not recognize the packet
    format.

* 'C''s checks could be skipped as long as per msg authentication was not
  supported.

* 'D''s checks could be skipped, because an improper null vs non-null K_g
will be caught later during IPMI 2.0 authentication.

* 'E''s checks are the most complicated.  An improper null vs non-null
username will be caught later during IPMI 1.5 and IPMI 2.0
authentication.  An improper null vs non-null password can be caught
later during IPMI 2.0 authentication, but may result in a timeout
during IPMI 1.5 authentication.

An argument could also be made that the speed at which an invalid
username/password error is returned to a user could also give a
malicious user information on the username/password of the remote BMC.

In the end, the authors have felt the overall positive benefits
provided by the checking of these values provides more than the
negative implications.  Changes in the overall industry implementation
could change this viewpoint later.

9) Bmc-Config/Pef-config callback design
----------------------------------------

Bmc-config, Pef-config, and Ipmi-sensors-config (henceforce together
just called Bmc-config) are coded with a archicture that reads/writes
each configurable field in the BMC separately.

As an example, suppose we have the following BMC configuration file
we'd like to commit.

FieldA       Value1
FieldB.1     Value2
FieldB.2     Value3
FieldB.3     Value4
FieldB.4     Value5

Suppose FieldA is read/written using a single IPMI packet and fields
FieldB.1-FieldB.4 can be read/written using a single IPMI packet.

In the architecture that Bmc-config is currently based on, the above
would require 5 read requests to read all 5 values.  It would require
4 read requests (for FieldB.1-FieldB.4) and 5 write requests to write
the values.

Obviously, this sounds like (and is!) very inefficient. 

The authors acknowledge that the code is very inefficient b/c it will
cause an excess number of request/response packets to be generated.  With
a large number of inputs Bmc-config can be slow.

Here are some of the major reasons why this was done and is still
kept.

A) (This is the primary reason this architecture is still kept.)  Due
to widely varying hardware and implementations, this handles the write
configuration case best.  Suppose FieldB.2 is only configurable on
IPMI 2.0 systems but not IPMI 1.5 systems.  Suppose (perhaps b/c it is
optional in the IPMI specification) FieldB.3 is suppored by some
vendors but not other vendors.  Suppose FieldB.4 is simply not
implemented correctly by the vendor (which has been a common problem
that has to be dealt in the open-source community).

This architecture allows the majority of the configuration to succeed
on a specific platform, and allows the end user to know exactly what
may or may not be configurable.  If all 4 fields of FieldB.1-FieldB.4
were written at the same time, there is currently no method in the
IPMI protocol to know what field was configured incorrectly and why.

In the future, functionality could be added to retry each field
separately if there was such a failure, however that would add another
piece of complexity into the code we currently don't have time to add.

B) There are several (and possibly more future) vendor compliance
problems that can be (or will need to be) worked around.  By using
this architecture each specific field can be worked around
independently depending on the vendor.

C) In the beginning of Bmc-config's development, 'FieldA' type fields
were a bit more common than 'FieldB' type fields.  Currently, it's
about 50-50.

10) Dealing with workarounds
----------------------------

There is an admitted conflict in determining whether vendor compliance
issues should be handled automatically vs. a specified workaround.

On one hand, we would like for the tools to operate as simply for the
users as possible without the need to specify strange workarounds or
options on the command line.

On the other hand, a number of these workarounds are due to vendor
compliance problems.  In some cases, the workarounds are so intrusive
(i.e. using a different hashing algorithm for keys) they must require
a workaround on the command line b/c there is really no other way to
handle it.  However, some could be handled seemlessly, but would
require altered behavior to handle the "common case".

The general rule that I've come to is that if the workaround changes
some "normal" behavior, it must require a specific workaround on the
command line.  Although it may/will be annoying to a number of users,
I feel it is better for the long term.  It can hopefully pressure
vendors into their fixing the implementations down the road.
