diff options
Diffstat (limited to 'microkernel/mach/mig')
| -rw-r--r-- | microkernel/mach/mig/documentation.mdwn | 2 | ||||
| -rw-r--r-- | microkernel/mach/mig/documentation/mig-mutate.mdwn | 138 |
2 files changed, 139 insertions, 1 deletions
diff --git a/microkernel/mach/mig/documentation.mdwn b/microkernel/mach/mig/documentation.mdwn index 06536386..4c800e6b 100644 --- a/microkernel/mach/mig/documentation.mdwn +++ b/microkernel/mach/mig/documentation.mdwn @@ -85,7 +85,7 @@ pp. 67--77." * MIG *in action*: [[hurd/io_path]]. - * [*Introduction to a mig-mutate.h file*](https://mail.gnu.org/archive/html/bug-hurd/2024-11/msg00045.html), a text by Sergey Bugaev + * [[What is mig-mutate.h?|mig-mutate]] ## IRC, freenode, #hurd, 2013-09-04 diff --git a/microkernel/mach/mig/documentation/mig-mutate.mdwn b/microkernel/mach/mig/documentation/mig-mutate.mdwn new file mode 100644 index 00000000..dd027a04 --- /dev/null +++ b/microkernel/mach/mig/documentation/mig-mutate.mdwn @@ -0,0 +1,138 @@ +[[!meta copyright="Copyright © 2024 Free Software Foundation, +Inc."]] + +[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable +id="license" text="Permission is granted to copy, distribute and/or modify this +document under the terms of the GNU Free Documentation License, Version 1.2 or +any later version published by the Free Software Foundation; with no Invariant +Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license +is included in the section entitled [[GNU Free Documentation +License|/fdl]]."]]"""]] + +The text in this page is based on the following code (from `$(hurd)/rtc/mig-mutate.h`): + + #define IO_INTRAN trivfs_protid_t trivfs_begin_using_protid (io_t) + #define IO_INTRAN_PAYLOAD trivfs_protid_t trivfs_begin_using_protid_payload + #define IO_DESTRUCTOR trivfs_end_using_protid (trivfs_protid_t) + #define IO_IMPORTS import "libtrivfs/mig-decls.h"; + +First, a brief description of what a protid is. Hurd translators +typically represent "files" internally with three kinds of distinct +structures: + +1. **node** -- these are filesystem nodes, same concept as an "inode". +2. **peropen** -- this keeps the data "per open" of the file and +corresponds to an "open file description" in POSIX. Things like +current I/O offset and the open mode (`O_READ | O_WRITE` ...) live here. +3. **protid** (or "credential") -- describes a specific "user" (UIDs/GIDs) +on behalf of whom the file is being accessed. + +A protid has a pointer to the peropen, and the peropen has a pointer +to the node. A node can have multiple peropens referring to it (when +the file has been opened multiple times), and a peropen can have +multiple protids referring to it (when processes running as different +users share an open file description, e.g. your shell and a sudo +invocation share the pts). In trivfs, there's only a single node, so +the concept is deemphasized. + +The concept of protid doesn't exist in classic Unix, since a +monolithic kernel can just directly see which UID the current process +runs as. But Mach IPC is (intentionally) designed in a way that it's +inherently impossible to see "who's asking", so instead we represent +differently-privileged callers with different handles (protids) that +refer to the same peropen, and then we check which protid the request +was made on. + +It is a protid that corresponds to an Mach port (`io_t`, `file_t`, ...), +though the client side doesn't need to care. + +When an incoming request arrives, the thing you actually receive in a +message is the port name (ignoring protected payloads for now). What +you actually want is the protid that it corresponds to. + +trivfs has the API to look up the protid given the port, namely +`trivfs_begin_using_protid` (which wraps `ports_lookup_port` from +libports), and you could call that yourself: + + kern_return_t + rtc_S_foobar (io_t port, int foo, int *bar) + { + error_t err = 0; + struct trivfs_protid *cred = trivfs_begin_using_protid (port); + + if (!cred) + /* The request came in on a port that we listen for incoming + * messages on, but it doesn't correspond to a protid. Must + * be some other kind of port. */ + return EOPNOTSUPP; + + if (!(cred->po->openmodes & O_READ)) + { + err = EBADF; + goto out; + } + + do something with cred... + + out: + trivfs_end_using_protid (cred); + return err; + } + +But since we already have a code generator (MIG), why not make it +generate the conversion logic for us as well. And so, in MIG, when +defining a type, you can provide `intran` and `outtran` and +`destructor` function names, and MIG will generate the calls for you. + +So the proper MIG way to (but see below about the Hurd way) to do the +thing that you're trying to do would be to define your own flavor of +Mach ports, say `rtc_port_t`, like this: + + type rtc_port_t = mach_port_t + intran: trivfs_protid_t trivfs_begin_using_protid (io_t) + destructor: trivfs_end_using_protid (trivfs_protid_t); + +and then use that type in the routine definitions. MIG would then call +`trivfs_begin_using_protid` and `trivfs_end_using_protid` in the server-side +generated functions, only passing `trivfs_protid_t` (which is a typedef +for `struct trivfs_protid *`, since MIG can't deal with the full C type +notation) to your implementation. The downside of this is that it the +implementation details of the server leak into the API definition, and +for instance you'd have to edit the `.defs` if you switch the server +from trivfs to netfs. + +You can find some documentation about this MIG feature under "Type +Translation Information" on page 17 of the [Mach 3 Server Writer’s +Guide](http://shakthimaan.com/downloads/hurd/server_writer.pdf), +but of course keep in mind that the guide was written a long time +ago, about a much older version of MIG, without any of the Hurd +additions/specifics/best practices. + +Then, `hurd_types.defs` has this: + + type io_t = mach_port_copy_send_t + #ifdef IO_INTRAN + intran: IO_INTRAN + intranpayload: IO_INTRAN_PAYLOAD + #else + #ifdef HURD_DEFAULT_PAYLOAD_TO_PORT + intranpayload: io_t HURD_DEFAULT_PAYLOAD_TO_PORT + #endif + #endif + #ifdef IO_OUTTRAN + outtran: IO_OUTTRAN + #endif + #ifdef IO_DESTRUCTOR + destructor: IO_DESTRUCTOR + #endif + ; + +(and same for all the other types of ports, e.g. `FILE_INTRAN`, +`SHUTDOWN_DESTRUCTOR` etc) + +which lets you use the standard `io_t` type while plugging in your own +`intran/intranpayload/outtran/destructor` functions, in a way that +doesn't leak into the `defs`. You only have to define the macros in your +local `mig-mutate.h` header in your server. + +The content in this page is from [bug-hurd mail list](https://mail.gnu.org/archive/html/bug-hurd/2024-11/msg00045.html) with some modifications. |