samba-mirror/prog_guide.txt at 498d08518f4590edb3a092be68678a4c54b055f5

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Andrew Tridgell 498d08518f the beginnings of a samba4 programming guide

(This used to be commit f0b309cb30)

2003-10-27 11:23:35 +00:00

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 THIS IS INCOMPLETE! I'M ONLY COMMITING IT IN ORDER TO SOLICIT COMMENTS
 FROM A FEW PEOPLE. DON'T TAKE THIS AS THE FINAL VERSION YET.
 Samba4 Programming Guide
 ------------------------
 The internals of Samba4 are quite different from previous versions of
 Samba, so even if you are an experienced Samba developer please take
 the time to read through this document.
 This document will explain both the broad structure of Samba4, and
 some of the common coding elements such as memory management and
 dealing with macros.
 Coding Style
 ------------
 In past versions of Samba we have basically let each programmer choose
 their own programming style. Unfortunately the result has often been
 that code that other members of the team find difficult to read. For
 Samba version 4 I would like to standardise on a common coding style
 to make the whole tree more readable. For those of you who are
 horrified at the idea of having to learn a new style, I can assure you
 that it isn't as painful as you might think. I was forced to adopt a
 new style when I started working on the Linux kernel, and after some
 initial pain found it quite easy.
 That said, I don't want to invent a new style, instead I would like to
 adopt the style used by the Linux kernel. It is a widely used style
 with plenty of support tools available. See Documentation/CodingStyle
 in the Linux source tree. This is the style that I have used to write
 all of the core infrastructure for Samba4 and I think that we should
 continue with that style.
 I also think that we should most definately *not* adopt an automatic
 reformatting system in cvs (or whatever other source code system we
 end up using in the future). Such automatic formatters are, in my
 experience, incredibly error prone and don't understand the necessary
 exceptions. I don't mind if people use automated tools to reformat
 their own code before they commit it, but please do not run such
 automated tools on large slabs of existing code without being willing
 to spend a *lot* of time hand checking the results.
 Finally, I think that for code that is parsing or formatting protocol
 packets the code layout should strongly reflect the packet
 format. That means ordring the code so that it parses in the same
 order as the packet is stored on the while (where possible) and using
 white space to align packet offsets so that a reader can immediately
 map any line of the code to the corresponding place in the packet.
 Static and Global Data
 ----------------------
 The basic rule is "avoid static and global data like the plague". What
 do I mean by static data? The way to tell if you have static data in a
 file is to use the "size" utility in Linux. For example if we run:
   size libcli/raw/*.o
 in Samba4 then you get the following:
    text    data     bss     dec     hex filename
 0       0    2015     7df libcli/raw/clikrb5.o
 0       0     202      ca libcli/raw/clioplock.o
 0       0      35      23 libcli/raw/clirewrite.o
 0       0    3891     f33 libcli/raw/clisession.o
 0       0     869     365 libcli/raw/clisocket.o
 0       0    4962    1362 libcli/raw/clispnego.o
 0       0    1223     4c7 libcli/raw/clitransport.o
 0       0    2294     8f6 libcli/raw/clitree.o
 0       0    1081     439 libcli/raw/raweas.o
 0       0    6765    1a6d libcli/raw/rawfile.o
 0       0    6824    1aa8 libcli/raw/rawfileinfo.o
 0       0    2944     b80 libcli/raw/rawfsinfo.o
 0       0     541     21d libcli/raw/rawioctl.o
 0       0    1728     6c0 libcli/raw/rawnegotiate.o
 0       0     723     2d3 libcli/raw/rawnotify.o
 0       0    3779     ec3 libcli/raw/rawreadwrite.o
 0       0    6597    19c5 libcli/raw/rawrequest.o
 0       0    5580    15cc libcli/raw/rawsearch.o
 0       0    3034     bda libcli/raw/rawsetfileinfo.o
 0       0    5187    1443 libcli/raw/rawtrans.o
 0       0    2033     7f1 libcli/raw/smb_signing.o
 notice that the "data" and "bss" columns are all zero? That is
 good. If there are any non-zero values in data or bss then that
 indicates static data and is bad (as a rule of thumb).
 Lets compare that result to the equivalent in Samba3:
    text    data     bss     dec     hex filename
 0       0    3978     f8a libsmb/asn1.o
 0     288   19251    4b33 libsmb/cliconnect.o
 0    1024    3839     eff libsmb/clidgram.o
 0       0    4038     fc6 libsmb/clientgen.o
 664     256    4257    10a1 libsmb/clierror.o
 0       0   10043    273b libsmb/clifile.o
 0       0     332     14c libsmb/clifsinfo.o
 0       0     166      a6 libsmb/clikrb5.o
 0       0    5212    145c libsmb/clilist.o
 0       0    1367     557 libsmb/climessage.o
 0       0     259     103 libsmb/clioplock.o
 0       0    1584     630 libsmb/cliprint.o
 0     256    7821    1e8d libsmb/cliquota.o
 0       0    7694    1e0e libsmb/clirap.o
 0       0   27440    6b30 libsmb/clirap2.o
 0       0    2905     b59 libsmb/clireadwrite.o
 0       0    1698     6a2 libsmb/clisecdesc.o
 0       0    5517    158d libsmb/clispnego.o
 0       0     485     1e5 libsmb/clistr.o
 0       0    8449    2101 libsmb/clitrans.o
 0       4    2057     809 libsmb/conncache.o
 0     256    3297     ce1 libsmb/credentials.o
 0    1024    2285     8ed libsmb/doserr.o
 0       0   14560    38e0 libsmb/errormap.o
 0       0    3645     e3d libsmb/namecache.o
 0       8   16823    41b7 libsmb/namequery.o
 0       0    1626     65a libsmb/namequery_dc.o
 0    1076   15377    3c11 libsmb/nmblib.o
 0    2048   26564    67c4 libsmb/nterr.o
 0       8    8669    21dd libsmb/ntlmssp.o
 0       0    3188     c74 libsmb/ntlmssp_parse.o
 0       0    4945    1351 libsmb/ntlmssp_sign.o
 0       0    1303     517 libsmb/passchange.o
 0       0    1221     4c5 libsmb/pwd_cache.o
 0       4    2479     9af libsmb/samlogon_cache.o
 32       0   10800    2a30 libsmb/smb_signing.o
 0      16    4540    11bc libsmb/smbdes.o
 0       0    5708    164c libsmb/smbencrypt.o
 0    3072   10121    2789 libsmb/smberr.o
 0       0    2995     bb3 libsmb/spnego.o
 0       0    3186     c72 libsmb/trustdom_cache.o
 0       0    1742     6ce libsmb/trusts_util.o
 0      28     946     3b2 libsmb/unexpected.o
 notice all of the non-zero data and bss elements? Every bit of that
 data is a bug waiting to happen.
 Static data is evil as it has the following consequences:
  - it makes code much less likely to be thread-safe
  - it makes code much less likely to be recursion-safe
  - it leads to subtle side effects when the same code is called from
    multiple places
 Static data is particularly evil in library code (such as our internal
 smb and rpc libraries). If you can get rid of all static data in
 libraries then you can make some fairly strong guarantees about the
 behaviour of functions in that library, which really helps.
 Of course, it is possible to write code that uses static data and is
 safe, it's just much harder to do that than just avoid static data in
 the first place. We have been tripped up countless times by subtle
 bugs in Samba due to the use of static data, so I think it is time to
 start avoiding it in new code. Much of the core infrastructure of
 Samba4 was specifically written to avoid static data, so I'm going to
 be really annoyed if everyone starts adding lots of static data back
 in.
 So, how do we avoid static data? The basic method is to use context
 pointers. When reading the Samba4 code you will notice that just about
 every function takes a pointer to a context structure as its first
 argument. Any data that the function needs that isn't an explicit
 argument to the function can be found by traversing that context.
 Note that this includes all of the little caches that we have lying
 all over the code in Samba3. I'm referring to the ones that generally
 have a "static int initialised" and then some static string or integer
 that remembers the last return value of the function. Get rid of them!
 If you are *REALLY* absolutely completely certain that your personal
 favourite mini-cache is needed then you should do it properly by
 putting it into the appropriate context rather than doing it the lazy
 way by putting it inside the target function. I would suggest however
 that the vast majority of those little caches are useless - don't
 stick it in unless you have really firm benchmarking results that show
 that it is needed and helps by a significant amount.
 Note that Samba4 is not yet completely clean of static data like
 this. I've gotten the smbd/ directory down to 24 bytes of static data,
 and libcli/raw/ down to zero. I've also gotten the ntvfs layer and all
 backends down to just 8 bytes in ntvfs_base.c. The rest still needs
 some more work.
 Also note that truly constant data is OK, and will not in fact show up
 in the data and bss columns in "size" anyway (it will be included in
 "text"). So you can have constant tables of protocol data.
 Memory Contexts
 ---------------
 We introduced the talloc() system for memory contexts during the 2.2
 development cycle and it has been a great success. It has greatly
 simplified a lot of our code, particularly with regard to error
 handling.
 In Samba4 we use talloc even more extensively to give us much finer
 grained memory management. The really important thing to remember
 about talloc in Samba4 is:
   "don't just use the first talloc context that comes to hand - use
   the RIGHT talloc context"
 Just using the first talloc context that comes to hand is probably the
 most common systematic bug I have seen so far from programmers that
 have worked on the Samba4 code base. The reason this is vital is that
 different talloc contexts have vastly different lifetimes, so if you
 use a talloc context that has a long lifetime (such as one associated
 with a tree connection) for data that is very short lived (such as
 parsing an individual packet) then you have just introduced a huge
 memory leak.
 In fact, it is quite common that the correct thing to do is to create
 a new talloc context for some little function and then destroy it when
 you are done. That will give you a memory context that has exactly the
 right lifetime.
 You should also go and look at a new talloc function in Samba4 called
 talloc_steal(). By using talloc_steal() you can move a lump of memory
 from one memory context to another without copying the data. This
 should be used when a backend function (such as a packet parser)
 produces a result as a lump of talloc memory and you need to keep it
 around for a longer lifetime than the talloc context it is in. You
 just "steal" the memory from the short-lived context, putting it into
 your long lived context.
 Interface Structures
 --------------------
 One of the biggest changes in Samba4 is the universal use of interface
 structures. Go take a look through include/smb_interfaces.h now to get
 an idea of what I am talking about.
 In Samba3 many of the core wire structures in the SMB protocol were
 never explicitly defined in Samba. Instead, our parse and generation
 functions just worked directly with wire buffers. The biggest problem
 with this is that is tied our parse code with out "business logic"
 much too closely, which meant the code got extremely confusing to
 read.
 In Samba4 we have explicitly defined interface structures for
 everything in the protocol. When we receive a buffer we always parse
 it completely into one of these structures, then we pass a pointer to
 that structure to a backend handler. What we must *not* do is make any
 decisions about the data inside the parse functions. That is critical
 as different backends will need different portions of the data. This
 leads to a golden rule for Samba4:
   "don't design interfaces that lose information"
 In Samba3 our backends often received "condensed" versions of the
 information sent from clients, but this inevitably meant that some
 backends could not get at the data they needed to do what they wanted,
 so from now on we should expose the backends to all of the available
 information and let them choose which bits they want.
 Ok, so now some of you will be thinking "this sounds just like our
 msrpc code from Samba3", and while to some extent this is true there
 are extremely important differences in the approach that are worth
 pointing out.
 In the Samba3 msrpc code we used explicit parse strucrures for all
 msrpc functions. The problem is that we didn't just put all of the
 real variables in these structures, we also put in all the artifacts
 as well. A good example is the security descriptor strucrure that
 looks like this in Samba3:
 typedef struct security_descriptor_info
 {
 	uint16 revision;
 	uint16 type;
 	uint32 off_owner_sid;
 	uint32 off_grp_sid;
 	uint32 off_sacl;
 	uint32 off_dacl;
 	SEC_ACL *dacl;
 	SEC_ACL *sacl;
 	DOM_SID *owner_sid;
 	DOM_SID *grp_sid;
 } SEC_DESC;
 The problem with this structure is all the off_* variables. Those are
 not part of the interface, and do not appear in any real descriptions
 of Microsoft security descriptors. They are parsing artifacts
 generated by the IDL compiler that Microsoft use. That doesn't mean
 they aren't needed on the wire - indeed they are as they tell the
 parser where to find the following four variables, but they should
 *NOT* be in the interface structure.
 In Samba3 there were unwritten rules about which variables in a
 strucrure a high level caller has to fill in and which ones are filled
 in by the marshalling code. In Samba4 those rules are gone, because
 the redundent artifact variables are gone. The high level caller just
 sets up the real variables and the marshalling code worries about
 generating the right offsets.
 The same rule applies to strings. In many places in the SMB and MSRPC
 protocols complex strings are used on the wire, with complex rules
 about padding, format, alighment, termination etc. None of that
 information is useful to a high level calling routine or to a backend
 - its all just so much wire fluff. So, in Samba4 these strings are
 just "char *" and are always in our internal multi-byte format (which
 is usually UTF8). It is up to the parse functions to worry about
 translating the format and getting the padding right.
 The one exception to this is the use of the WIRE_STRING type, but that
 has a very good justification in terms of regression testing. Go and
 read the comment in smb_interfaces.h about that now.
 So, here is another rule to code by. When writing an interface
 structure think carefully about what variables in the structure can be
 left out as they are redundent. If some length is effectively defined
 twice on the wire then only put it once in the packet. If a length can
 be inferred from a null termination then do that and leave the length
 out of the structure completely. Don't put redundent stuff in
 structures!
 Async Design
 ------------
 Samba4 has an asynchronous design. That affects *lots* of the code,
 and the implications of the asynchronous design needs to be considered
 just about everywhere.
 The first aspect of the async design to look at is the SMB client
 library. Lets take a look at the following three functions in
 libcli/raw/rawfile.c:
 struct cli_request *smb_raw_seek_send(struct cli_tree *tree, struct smb_seek *parms);
 NTSTATUS smb_raw_seek_recv(struct cli_request *req, struct smb_seek *parms);
 NTSTATUS smb_raw_seek(struct cli_tree *tree, struct smb_seek *parms);
 Go and read them now then come back.
 Ok, first notice there there are 3 separate functions, whereas the
 equivalent code in Samba3 had just one. Also note that the 3rd
 function is extremely simple - its just a wrapper around calling the
 first two in order.
 The three separate functions are needed because we need to be able to
 generate SMB calls asynchronously. The first call, which for smb calls
 is always called smb_raw_XXXX_send(), constructs and sends a SMB
 request and returns a "struct cli_request" which acts as a handle for
 the request. The caller is then free to do lots of other calls if it
 wants to, then when it is ready it can call the smb_raw_XXX_recv()
 function to receive the reply.
 If all you want is a synchronous call then call the 3rd interface, the
 one called smb_raw_XXXX(). That just calls the first two in order, and
 blocks waiting for the reply.
 But what if you want to be called when the reply comes in? Yes, thats
 possible. You can do things like this:
     struct cli_request *req;
     req = smb_raw_XXX_send(tree, params);
     req->async.fn = my_callback;
     req->async.private = my_private_data;
 then in your callback function you can call the smb_raw_XXXX_recv()
 function to receive the reply. Your callback will receive the "req"
 pointer, which you can use to retrieve your private data.
 Then all you need to do is ensure that the main loop in the client
 library gets called. You can either do that by polling the connection
 using cli_transport_pending() and cli_request_receive_next() or you
 can use transport->idle.func to setup an idle function handler to call
 back to your main code. Either way, you can build a fully async
 application.
 In order to support all of this we have to make sure that when we
 write a piece of library code (SMB, MSRPC etc) that we build the
 separate _send() and _recv() functions. It really is worth the effort.
 Now about async in smbd, a much more complex topic.
 The SMB protocol is inherently async. Some functions (such as change
 notify) often don't return for hours, while hundreds of other
 functions pass through the socket. Take a look at the RAW-MUX test in
 the Samba4 smbtorture to see some really extreme examples of the sort
 of async operations that Windows supports.
 In Samba3 we handled this stuff very badly. We had awful "pending
 request" queues that allocated full 128k packet buffers, and even with
 all that crap we got the semantics wrong. In Samba4 I intend to make
 sure we get this stuff right.
 So, how do we do this? We now an async interface between smbd and the
 NTVFS backends. Whenever smbd calls into a backend the backend has an
 option of answer the request in a synchronous fashion if it wants to
 just like in Samba3, but it also has the option of answering the
 request asynchronously. The only backend that currently does this is
 the CIFS backend, but I hope the other backends will soon do this to.
 To make this work you need to do things like this in the backend:
   req->control_flags |= REQ_CONTROL_ASYNC;
 that tells smbd that the backend has elected to reply later rather
 than replying immediately. The backend must *only* do this if
 req->async.send_fn is not NULL. If send_fn is NULL then it means that
 smbd cannot handle this function being replied to in an async fashion.
 If the backend does this then it is up to the backend to call
 req->async.send_fn() when it is ready to reply. It the meantime smbd
 puts the call on hold and goes back to answering other requests on the
 socket.
 Inside smbd you will find that there is code to support this. The most
 obvious change is that smbd splits each SMB reply function into two
 parts - just like the client library has a _send() and _recv()
 function, so smbd has a _send() function and the parse function for
 each SMB.
 Go and have a look at reply_getatr_send() and reply_getatr() in
 smbd/reply.c. Read them? Good.
 Notice that reply_getatr() sets up the req->async structure to contain
 the send function. Thats how the backend gets to do an async
 reply. Also notice that reply_getatr() only does the parsing of the
 request, and does not to the reply generation. That is done by the
 _send() function. Nice and simple really.
 MSRPC
 -----
  - ntvfs
  - testing
  - command line handling
  - libcli structure
  - posix reliance
  - uid/gid handling
  - process models
  - static data
  - msrpc
 GMT vs TZ in printout of QFILEINFO timezones
 put in full UNC path in tconx
 test timezone handling by using a server in different zone from client
 don't just use any old TALLOC_CTX, use the right one!
 do {} while (0) system
 NT_STATUS_IS_OK() is NOT the opposite of NT_STATUS_IS_ERR()
 need to implement secondary parts of trans2 and nttrans in server and
 client
 add talloc_steal() to move a talloc ptr from one pool to another
 document access_mask in openx reply
 check all capabilities and flag1, flag2 fields (eg. EAs)
 large files -> pass thru levels
 setpathinfo is very fussy about null termination of the file name
 the overwrite flag doesn't seem to work on setpathinfo RENAME_INFORMATION
 END_OF_FILE_INFORMATION and ALLOCATION_INFORMATION don't seem to work
 via setpathinfo
 on w2k3 setpathinfo DISPOSITION_INFORMATION fails, but does have an
 effect. It leaves the file with SHARING_VIOLATION.
 on w2k3 trans2 setpathinfo with any invalid low numbered level causes
 the file to get into a state where DELETE_PENDING is reported, and the
 file cannot be deleted until you reboot
 trans2 qpathinfo doesn't see the delete_pending flag correctly, but
 qfileinfo does!
 get rid of pstring, fstring, strtok
 add programming documentation note about lp_set_cmdline()
 need to add a wct checking function in all client parsing code,
 similar to REQ_CHECK_WCT()
 need to make sure that NTTIME is a round number of seconds when
 converted from time_t
 not using a zero next offset in SMB_FILE_STREAM_INFORMATION for last
 entry causes explorer exception under win2000
 if the server sets the session key the same for a second SMB socket as
 an initial socket then the client will not re-authenticate, it will go
 straight to a tconx, skipping session setup and will use all the
 existing parameters! This allows two sockets with the same keys!?
 removed blocking lock code, we now queue the whole request the same as
 we queue any other pending request. This allows for things like a
 close() while a pending blocking lock is being processed to operate
 sanely.
 disabled change notify code
 disabled oplock code
 MILESTONES
 ==========
 client library and test code
 ----------------------------
   convert client library to new structure
   get smbtorture working
   get smbclient working
   expand client library for all requests
   write per-request test suite
   gentest randomised test suite
   separate client code as a library for non-Samba use
 server code
 -----------
   add remaining core SMB requests
   add IPC layer
   add nttrans layer
   add rpc layer
   fix auth models (share, server, rpc)
   get net command working
   connect CIFS backend to server level auth
   get nmbd working
   get winbindd working
   reconnect printing code
   restore removed smbd options
   add smb.conf macro substitution code
   add async backend notification
   add generic timer event mechanism
 clustering code
 ---------------
   write CIFS backend
   new server models (break 1-1)
   test clustered models
   add fulcrum statistics gathering
 docs
 ----
   conference paper
   developer docs

570 lines 23 KiB Plaintext Raw Blame History

570 lines

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Plaintext

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