Chapter 5. Samba Internals

8 May 1996

Table of Contents

Character Handling
The new functions
Macros in byteorder.h
CVAL(buf,pos)
PVAL(buf,pos)
SCVAL(buf,pos,val)
SVAL(buf,pos)
IVAL(buf,pos)
SVALS(buf,pos)
IVALS(buf,pos)
SSVAL(buf,pos,val)
SIVAL(buf,pos,val)
SSVALS(buf,pos,val)
SIVALS(buf,pos,val)
RSVAL(buf,pos)
RIVAL(buf,pos)
RSSVAL(buf,pos,val)
RSIVAL(buf,pos,val)
LAN Manager Samba API
Parameters
Return value
Code character table

Character Handling

This section describes character set handling in Samba, as implemented in Samba 3.0 and above

In the past Samba had very ad-hoc character set handling. Scattered throughout the code were numerous calls which converted particular strings to/from DOS codepages. The problem is that there was no way of telling if a particular char* is in dos codepage or unix codepage. This led to a nightmare of code that tried to cope with particular cases without handlingt the general case.

The new functions

The new system works like this:

  1. all char* strings inside Samba are "unix" strings. These are multi-byte strings that are in the charset defined by the "unix charset" option in smb.conf.

  2. there is no single fixed character set for unix strings, but any character set that is used does need the following properties:

    1. must not contain NULLs except for termination

    2. must be 7-bit compatible with C strings, so that a constant string or character in C will be byte-for-byte identical to the equivalent string in the chosen character set.

    3. when you uppercase or lowercase a string it does not become longer than the original string

    4. must be able to correctly hold all characters that your client will throw at it

    For example, UTF-8 is fine, and most multi-byte asian character sets are fine, but UCS2 could not be used for unix strings as they contain nulls.

  3. when you need to put a string into a buffer that will be sent on the wire, or you need a string in a character set format that is compatible with the clients character set then you need to use a pull_ or push_ function. The pull_ functions pull a string from a wire buffer into a (multi-byte) unix string. The push_ functions push a string out to a wire buffer.

  4. the two main pull_ and push_ functions you need to understand are pull_string and push_string. These functions take a base pointer that should point at the start of the SMB packet that the string is in. The functions will check the flags field in this packet to automatically determine if the packet is marked as a unicode packet, and they will choose whether to use unicode for this string based on that flag. You may also force this decision using the STR_UNICODE or STR_ASCII flags. For use in smbd/ and libsmb/ there are wrapper functions clistr_ and srvstr_ that call the pull_/push_ functions with the appropriate first argument.

    You may also call the pull_ascii/pull_ucs2 or push_ascii/push_ucs2 functions if you know that a particular string is ascii or unicode. There are also a number of other convenience functions in charcnv.c that call the pull_/push_ functions with particularly common arguments, such as pull_ascii_pstring()

  5. The biggest thing to remember is that internal (unix) strings in Samba may now contain multi-byte characters. This means you cannot assume that characters are always 1 byte long. Often this means that you will have to convert strings to ucs2 and back again in order to do some (seemingly) simple task. For examples of how to do this see functions like strchr_m(). I know this is very slow, and we will eventually speed it up but right now we want this stuff correct not fast.

  6. all lp_ functions now return unix strings. The magic "DOS" flag on parameters is gone.

  7. all vfs functions take unix strings. Don't convert when passing to them

Macros in byteorder.h

This section describes the macros defined in byteorder.h. These macros are used extensively in the Samba code.

CVAL(buf,pos)

returns the byte at offset pos within buffer buf as an unsigned character.

PVAL(buf,pos)

returns the value of CVAL(buf,pos) cast to type unsigned integer.

SCVAL(buf,pos,val)

sets the byte at offset pos within buffer buf to value val.

SVAL(buf,pos)

returns the value of the unsigned short (16 bit) little-endian integer at offset pos within buffer buf. An integer of this type is sometimes refered to as "USHORT".

IVAL(buf,pos)

returns the value of the unsigned 32 bit little-endian integer at offset pos within buffer buf.

SVALS(buf,pos)

returns the value of the signed short (16 bit) little-endian integer at offset pos within buffer buf.

IVALS(buf,pos)

returns the value of the signed 32 bit little-endian integer at offset pos within buffer buf.

SSVAL(buf,pos,val)

sets the unsigned short (16 bit) little-endian integer at offset pos within buffer buf to value val.

SIVAL(buf,pos,val)

sets the unsigned 32 bit little-endian integer at offset pos within buffer buf to the value val.

SSVALS(buf,pos,val)

sets the short (16 bit) signed little-endian integer at offset pos within buffer buf to the value val.

SIVALS(buf,pos,val)

sets the signed 32 bit little-endian integer at offset pos within buffer buf to the value val.

RSVAL(buf,pos)

returns the value of the unsigned short (16 bit) big-endian integer at offset pos within buffer buf.

RIVAL(buf,pos)

returns the value of the unsigned 32 bit big-endian integer at offset pos within buffer buf.

RSSVAL(buf,pos,val)

sets the value of the unsigned short (16 bit) big-endian integer at offset pos within buffer buf to value val. refered to as "USHORT".

RSIVAL(buf,pos,val)

sets the value of the unsigned 32 bit big-endian integer at offset pos within buffer buf to value val.

LAN Manager Samba API

This section describes the functions need to make a LAN Manager RPC call. This information had been obtained by examining the Samba code and the LAN Manager 2.0 API documentation. It should not be considered entirely reliable.

call_api(int prcnt, int drcnt, int mprcnt, int mdrcnt,
	char *param, char *data, char **rparam, char **rdata);

This function is defined in client.c. It uses an SMB transaction to call a remote api.

Parameters

The parameters are as follows:

  1. prcnt: the number of bytes of parameters begin sent.

  2. drcnt: the number of bytes of data begin sent.

  3. mprcnt: the maximum number of bytes of parameters which should be returned

  4. mdrcnt: the maximum number of bytes of data which should be returned

  5. param: a pointer to the parameters to be sent.

  6. data: a pointer to the data to be sent.

  7. rparam: a pointer to a pointer which will be set to point to the returned parameters. The caller of call_api() must deallocate this memory.

  8. rdata: a pointer to a pointer which will be set to point to the returned data. The caller of call_api() must deallocate this memory.

These are the parameters which you ought to send, in the order of their appearance in the parameter block:

  1. An unsigned 16 bit integer API number. You should set this value with SSVAL(). I do not know where these numbers are described.

  2. An ASCIIZ string describing the parameters to the API function as defined in the LAN Manager documentation. The first parameter, which is the server name, is omitted. This string is based upon the API function as described in the manual, not the data which is actually passed.

  3. An ASCIIZ string describing the data structure which ought to be returned.

  4. Any parameters which appear in the function call, as defined in the LAN Manager API documentation, after the "Server" and up to and including the "uLevel" parameters.

  5. An unsigned 16 bit integer which gives the size in bytes of the buffer we will use to receive the returned array of data structures. Presumably this should be the same as mdrcnt. This value should be set with SSVAL().

  6. An ASCIIZ string describing substructures which should be returned. If no substructures apply, this string is of zero length.

The code in client.c always calls call_api() with no data. It is unclear when a non-zero length data buffer would be sent.

Return value

The returned parameters (pointed to by rparam), in their order of appearance are:

  1. An unsigned 16 bit integer which contains the API function's return code. This value should be read with SVAL().

  2. An adjustment which tells the amount by which pointers in the returned data should be adjusted. This value should be read with SVAL(). Basically, the address of the start of the returned data buffer should have the returned pointer value added to it and then have this value subtracted from it in order to obtain the currect offset into the returned data buffer.

  3. A count of the number of elements in the array of structures returned. It is also possible that this may sometimes be the number of bytes returned.

When call_api() returns, rparam points to the returned parameters. The first if these is the result code. It will be zero if the API call succeeded. This value by be read with "SVAL(rparam,0)".

The second parameter may be read as "SVAL(rparam,2)". It is a 16 bit offset which indicates what the base address of the returned data buffer was when it was built on the server. It should be used to correct pointer before use.

The returned data buffer contains the array of returned data structures. Note that all pointers must be adjusted before use. The function fix_char_ptr() in client.c can be used for this purpose.

The third parameter (which may be read as "SVAL(rparam,4)") has something to do with indicating the amount of data returned or possibly the amount of data which can be returned if enough buffer space is allowed.

Code character table

Certain data structures are described by means of ASCIIz strings containing code characters. These are the code characters:

  1. W a type byte little-endian unsigned integer

  2. N a count of substructures which follow

  3. D a four byte little-endian unsigned integer

  4. B a byte (with optional count expressed as trailing ASCII digits)

  5. z a four byte offset to a NULL terminated string

  6. l a four byte offset to non-string user data

  7. b an offset to data (with count expressed as trailing ASCII digits)

  8. r pointer to returned data buffer???

  9. L length in bytes of returned data buffer???

  10. h number of bytes of information available???