Chapter 5. Samba Internals

Chapter 5. Samba Internals
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David Chappell

<David.Chappell@mail.trincoll.edu>

8 May 1996

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: +

+ 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. +
+ 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. +
+ 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. +
+ 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() +
+ 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. +
+ all lp_ functions now return unix strings. The magic "DOS" flag on + parameters is gone. +
+ 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 withing 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:

+ prcnt: the number of bytes of parameters begin sent. +
+ drcnt: the number of bytes of data begin sent. +
+ mprcnt: the maximum number of bytes of parameters which should be returned +
+ mdrcnt: the maximum number of bytes of data which should be returned +
+ param: a pointer to the parameters to be sent. +
+ data: a pointer to the data to be sent. +
+ rparam: a pointer to a pointer which will be set to point to the returned + paramters. The caller of call_api() must deallocate this memory. +
+ 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: +

+An unsigned 16 bit integer API number. You should set this value with +SSVAL(). I do not know where these numbers are described. +
+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 ommited. This string is based uppon the API function as described +in the manual, not the data which is actually passed. +
+An ASCIIZ string describing the data structure which ought to be returned. +
+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. +
+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(). +
+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:

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

+W a type byte little-endian unsigned integer +
+N a count of substructures which follow +
+D a four byte little-endian unsigned integer +
+B a byte (with optional count expressed as trailing ASCII digits) +
+z a four byte offset to a NULL terminated string +
+l a four byte offset to non-string user data +
+b an offset to data (with count expressed as trailing ASCII digits) +
+r pointer to returned data buffer??? +
+L length in bytes of returned data buffer??? +
+h number of bytes of information available??? +

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