Re: raise() marked __leaf__ is not C-compliant?

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 28/10/2020 16:23, Tadeus Prastowo wrote:
> On Wed, Oct 28, 2020 at 6:34 PM Adhemerval Zanella
> <adhemerval.zanella@linaro.org> wrote:
>>
>> On 28/10/2020 10:19, Tadeus Prastowo wrote:
>>> On Wed, Oct 28, 2020 at 12:53 PM Adhemerval Zanella
>>> <adhemerval.zanella@linaro.org> wrote:
>>>>
>>>> The sentence "raise() definitely runs a signal handler" is not really
>>>> valid in a portable sense. Afaik neither C nor POSIX states which signals
>>>> should be delivered synchronously or asynchronously (although some do
>>>> only make sense to be delivered synchronously such as SIGSEGV).
>>>>
>>>> However, Linux does ran some signals synchronously and I agree that using
>>>> leaf attribute is incorrect and lead to this kind of problems. My point is
>>>> to be fully portable, you need to assume any signal might be delivered
>>>> asynchronously (and C standard specifies the volatile sig_atomic_t for
>>>> such cases).
>>>
>>> Thank you for your response.
>>>
>>> However, C, including C99, C11, and the latest C18 [1], says: "If a
>>> signal handler is called, the raise function shall not return until
>>> after the signal handler does."  And, POSIX [2] says: "If a signal
>>> handler is called, the raise() function shall not return until after
>>> the signal handler does."  So, the sentence "raise() definitely runs a
>>> signal handler" is valid in a portable sense as required by the
>>> standards, no?
>>
>> My understanding is it allows synchronous signals, not enforce it;
>> and if the signal is synchronous then it should complete prior hand.
> 
> I understand your point as the C standard says:
> 
> [...] distinct values that are the signal numbers, each corresponding
> to the specified condition:
> SIGABRT [...]
> SIGFPE [...]
> SIGILL [...]
> SIGINT [...]
> SIGSEGV [...]
> SIGTERM [...]
> An implementation need not generate any of these signals, except as a
> result of explicit calls to the raise function.  [...]  The complete
> set of signals, their semantics, and their default handling is
> implementation-defined; all signal numbers shall be positive.
> 
> [...]
> 
> void (*signal(int sig, void (*func)(int)))(int);
> Description
> [...]
> When a signal occurs and func points to a function, it is
> implementation-defined whether the equivalent of signal(sig, SIG_DFL);
> is executed [...]; in the case of SIGILL, the implementation may
> alternatively define that no action is taken.  Then the equivalent of
> (*func)(sig); is executed.
> 
> End quote.
> 
> So, yes, you are right that the sentence "raise() definitely runs a
> signal handler" is inaccurate because, as quoted above, C standard
> allows an implementation to not run the handler that the user has
> designated to handle the signal being generated by raise().  However,
> if the implementation decides that "the equivalent of (*func)(sig); is
> executed", then the C standard requires that "the raise function shall
> not return until after the signal handler does."

I agree this does make sense for *synchronous* signal triggering.  My
point ir neither C not POSIX specify which signal should be synchronous. 
In fact, afaik neither standard does make a clearly distinction nor 
specific the expected semantic for signal "synchronicity" (it is really 
implementation defined).

> 
> To conclude, in my earlier post, I meant to say: Glibc-2.30 raise()
> may run a signal handler that is defined in the current compilation
> unit to use static variables.  So, unless the C standard says that it
> is an undefined behavior to access a non-volatile object with static
> storage duration from within a signal handler that is called
> synchronously by raise() on a normal execution path, the marking of
> raise() with __leaf__ makes raise() non-compliant with the C standard.

I am not a language lawyer here, and I agree with you that adding leaf
attribute on 'raise' is wrong. But because glibc can't guarantee how
the underlying kernel will trigger the signal in a raise syscall.

I still think that once you can't assume the signal will be triggered
synchronously, you can't also assume that accessing non-volatile object
with static storage duration from within a signal handler might not
trigger undefined behavior.