158 lines
8.3 KiB
Text
158 lines
8.3 KiB
Text
Contributed by Christopher Faylor
|
|
|
|
[note that the following discussion is still incomplete]
|
|
|
|
How do signals work?
|
|
|
|
On process startup, cygwin starts a secondary thread which deals with
|
|
signals. This thread contains a loop which blocks waiting for
|
|
information to arrive on a pipe whose handle (sendsig) is currently
|
|
stored in _pinfo (this may change).
|
|
|
|
Communication on the sendsig pipe is via the 'sigpacket' structure.
|
|
This structure is filled out by the sig_send function with information
|
|
about the signal being sent, such as (as of this writing) the signal
|
|
number, the originating pid, the originating thread, and the address of
|
|
the mask to use (this may change).
|
|
|
|
Any cygwin function which calls a win32 api function is wrapped by the
|
|
assembly functions "_sigfe" and "_sigbe". These functions maintain a
|
|
cygwin "signal stack" which is used by the signal thread to control
|
|
handling of signal interrupts. Cygwin functions which need to be
|
|
wrapped by these functions (the majority) are labelled by the SIGFE
|
|
option in the file cygwin.din.
|
|
|
|
The cygwin.din function is translated into a standard cygwin.def file by
|
|
the perl script "gendef". This function notices exported cygwin
|
|
functions which are labelled as SIGFE and generates a front end assembly
|
|
file "sigfe.s" which contains the wrapper glue necessary for every
|
|
function to call sigfe prior to actually dispatching to the real cygwin
|
|
function. This generated file contains low-level signal related
|
|
functions: _sigfe, _sigbe, sigdelayed, sigreturn, longjmp, and setjmp.
|
|
|
|
The signal stack maintained by sigfe/sigbe and friends is a secondary
|
|
shadow stack. Addresses from this stack are swapped into the "real"
|
|
stack as needed to control program flow. The intent is that executing
|
|
cygwin functions will still see the same stack layout as if they had
|
|
been called directly and will be able to retrieve arguments from the
|
|
stack but will always return to the _sigbe routine so that any signal
|
|
handlers will be properly called.
|
|
|
|
Upon receipt of a "non-special" (see below) signal, the function
|
|
sigpacket::process is called. This function determines what action, if
|
|
any, to take on the signal. Possible actions are: Ignore the signal
|
|
(e.g., SIGUSR1), terminate the program (SIGKILL, SIGTERM), stop the
|
|
program (SIGSTOP, SIGTSTP, etc.), wake up a sigwait or sigwaitinfo in a
|
|
targetted thread, or call a signal handler (possibly in a thread). If
|
|
no thread information has been sent to sigpacket::process, it determines
|
|
the correct thread to use based on various heuristics, as per UNIX. As
|
|
per linux, the only time a handler is called in a thread is when there
|
|
is some kind of fault like SIGSEGV, SIGILL, etc. Signals sent via the
|
|
UNIX kill() function are normally sent to the main thread. Ditto
|
|
signals sent as the result of pressing tty keys, like CTRL-C.
|
|
|
|
Signals which stop a process are handled by a special internal handler:
|
|
sig_handle_tty_stop. Some signals (e.g., SIGKILL, SIGSTOP) are
|
|
uncatchable, as on UNIX.
|
|
|
|
If the signal has an associated signal handler, then the setup_handler
|
|
function is eventually called. It is passed the signal, the address of
|
|
the handler, a standard UNIX sigaction structure, and a pointer to the
|
|
thread's "_cygtls" information. The meat of signal processing is in
|
|
setup_handler.
|
|
|
|
setup_handler has a "simple" task. It tries to stop the appropriate
|
|
thread and either redirect its execution to the signal handler function,
|
|
flag that a signal has been received (sigwait) or both (sigpause).
|
|
|
|
To accomplish its task, setup_handler first inspects the target thread's
|
|
local storage (_cygtls) structure. This structure contains information
|
|
on any not-yet-handled signals that may have been set up by a previous
|
|
call to setup_handler but not yet dispatched in the target thread. If this
|
|
structure seems to be "active", then setup_handler returns, notifying it's
|
|
parent via a false value. Otherwise processing continues.
|
|
|
|
(For pending signals, the theory is that the signal handler thread will
|
|
be forced to be rerun by having some strategic cygwin function call
|
|
sig_send with a __SIGFLUSH argument. This causes the signal handler to
|
|
rescan the signal array looking for pending signals.)
|
|
|
|
After determining that it's ok to send a signal, setup_handler will lock
|
|
the cygtls stack to ensure that it has complete access. It will then
|
|
inspect the thread's 'incyg' boolean. If this is true, the thread is
|
|
currently executing a cygwin function. If it is false, the thread is
|
|
unlocked and it is assumed that the thread is executing "user" code.
|
|
The actions taken by setup_handler differ based on whether the program
|
|
is executing a cygwin routine or not.
|
|
|
|
If the program is executing a cygwin routine, then the
|
|
interrupt_on_return function is called which causes the address of the
|
|
'sigdelayed' function to be pushed onto the thread's signal stack, and
|
|
the signal's mask and handler to be saved in the tls structure. After
|
|
performing these operations, the 'signal_arrived' event is signalled, as
|
|
well as any thread-specific wait event.
|
|
|
|
Since the sigdelayed function was saved on the thread's signal stack,
|
|
when the cygwin function returns, it will eventually return to the
|
|
sigdelayed "front end". The sigdelayed function will save a lot of
|
|
state on the stack and set the signal mask as appropriate for POSIX.
|
|
It uses information from the _cygtls structure which has been filled in
|
|
by interrupt_setup, as called by setup_handler. sigdelayed pushes a
|
|
"call" to the function "sigreturn" on the thread's signal stack. This
|
|
will be the return address eventually seen by the signal handler. After
|
|
setting up the return value, modifying the signal mask, and saving other
|
|
information on the stack, sigreturn clears the signal number in the
|
|
_cygtls structure so that setup_handler can use it and jumps to the
|
|
signal handler function. And, so a UNIX signal handler function is
|
|
emulated.
|
|
|
|
The signal handler function operates as normal for UNIX but, upon
|
|
return, it does not go directly back to the return address of the
|
|
original cygwin function. Instead it returns to the previously
|
|
mentioned 'sigreturn' assembly language function.
|
|
|
|
sigreturn resets the process mask to its state prior to calling the
|
|
signal handler. It checks to see if a cygwin routine has set a special
|
|
"restore this errno on returning from a signal" value and sets errno to
|
|
this, if so. It pops the signal stack, places the new return address on
|
|
the real stack, restores all of the register values that were in effect
|
|
when sigdelayed was called, and then returns.
|
|
|
|
Ok. That is more or less how cygwin interrupts a process which is
|
|
executing a cygwin function. We are almost ready to talk about how
|
|
cygwin interrupts user code but there is one more thing to talk about:
|
|
SA_RESTART.
|
|
|
|
UNIX allows some blocking functions to be interrupted by a signal
|
|
handler and then return to blocking. In cygwin, so far, only
|
|
read/readv() and the wait* functions operate in this fashion. To
|
|
accommodate this behavior, a function notices when a signal comes in and
|
|
then calls the _cygtls function 'call_signal_handler_now'.
|
|
'call_signal_handler_now' emulates the behavior of both sigdelayed and
|
|
sigreturn. It sets the appropriate masks and calls the handler,
|
|
returning true to the caller if SA_RESTART is active. If SA_RESTART is
|
|
active, the function will loop. Otherwise it will typically return -1
|
|
and set the errno to EINTR.
|
|
|
|
Phew. So, now we turn to the case where cygwin needs to interrupt the
|
|
program when it is not executing a cygwin function. In this scenario,
|
|
we rely on the win32 "SuspendThread" function. Cygwin will suspend the
|
|
thread using this function and then inspect the location at which the
|
|
thread is executing using the win32 "GetThreadContext" call. In theory,
|
|
the program should not be executing in a win32 api since attempts to
|
|
suspend a process executing a win32 call can cause disastrous results,
|
|
especially on Win9x.
|
|
|
|
If the process is executing in an unsafe location then setup_handler
|
|
will (quickly!) return false as in the case above. Otherwise, the
|
|
current location of the thread is pushed on the thread's signal stack
|
|
and the thread is redirected to the sigdelayed function via the win32
|
|
"SetThreadContext" call. Then the thread is restarted using the win32
|
|
"ResumeThread" call and things proceed as per the sigdelayed discussion
|
|
above.
|
|
|
|
This leads us to the sig_send function. This is the "client side" part
|
|
of the signal manipulation process. sig_send is the low-level function
|
|
called by a high level process like kill() or pthread_kill().
|
|
|
|
** More to come **
|