kernel/proc.c - sys_call() internals

I gave some idea about sys_call() in the preceding diagrams. sys_call() does a number of checks before it allows a message or notification to be passed from one process to another. These are all part of the security mechanisms in the kernel that keep processes from doing things they are not allowed to do and from corrupting each other's memory maps. Assuming that a process has just made a system call, here is the list of checks that happen:

1. Does the process have the privileges to make this particular system call? (the priv structure for this process in the process table is looked up to determine this)
2. In the case of a 'send' or 'notify', is the destination a valid process? In the case of a 'receive' is the source a valid process? Note that a caller can specify 'ANY' as its source in which case it is looking for a message or notification from any process, so this check is unnecessary for that case. 'ECHO' is also a special case where this check doesn't make sense.
3. Does the message pointer point to a valid region in memory i.e. is it really within the memory map limits of the calling process? Again, the memory map limits are available in the memmap struct for this process in the process table.
4. If the request is to 'send' to some process, is the caller allowed to send to the destination process?
5. Is the destination process running?
6. Has system shutdown started?
   

If all the checks above have passed then call one of the following functions: mini_notify(), mini_receive() or mini_send() depending upon the nature of the request.

Refer to the 'Message Primitives and Process States' diagrams (preceding posts) for details on what happens within the above functions.

Comments welcome.