Re: determine whether code is running in a signal handler context

[Carlos O'Donell <carlos at redhat dot com>]

On 10/18/2017 06:52 PM, Yubin Ruan wrote:
> 2017-10-19 2:34 GMT+08:00 Carlos O'Donell <carlos@redhat.com>:
>> On 10/18/2017 07:18 AM, Yubin Ruan wrote:
>>> Hi,
>>>
>>> I am writing to see if this is any util functions in libc that can
>>> help to determine it is currently running in a signal.
>>>
>>> I wrote some library and provide a function which will be used in many
>>> client code. However this function is not async-signal safe (it calls
>>> malloc(3)) so when it is called, I want to detect whether it is
>>> currently running in a signal handler. If it is, I can avoid calling
>>> those not-async-signal-safe functions which might cause deadlock.
>>>
>>> that is, I want a `in_signal_handler_context()' utility that can be
>>> used as this:
>>>
>>> int mylibfunc( void ) {
>>>     if( in_signal_handler_context() ) { return(-1) }
>>>     // rest of function goes here
>>>     return( 0 );
>>> }
>>
>> Only (b).
>>
>> (a) The kernel knows, but doesn't tell you.
>>
>> (b) Only via architecture-specific checks which involve inspecting the
>>     call frame to determine if there is a signal frame on the stack.
> 
> Is (b) deterministic? How can we know whether a call frame is a signal
> frame and how can we know at which point we should stop when
> inspecting the frame?

We could spend days talking about this. The answer should be deterministic,
and gdb solves it, but the solution involves some heuristic checks, and
unwinding (a complex topic).

> Somebody pointed out that on Linux all signal handlers are called from
> vdso, so I come up with the following code to "inspect" the call frame
> and see whether there is any address which points at some place inside
> the VDSO:

This is not true. It is architecture dependent. For examples on how complex
it is to get this 100% right you have to go read gdb sources to see how
they do this. It is non-trivial. It might be easier to use a library like
libunwind to determine this for you (if it even can).

-- 
Cheers,
Carlos.