[PATCH 07/11] signal/arm64: Document conflicts with SI_USER and SIGFPE, SIGTRAP, SIGBUS

[Russell King - ARM Linux]

On Wed, Jan 17, 2018 at 12:15:05PM +0000, Dave Martin wrote:
> On Wed, Jan 17, 2018 at 11:57:09AM +0000, Russell King - ARM Linux wrote:
> > On Tue, Jan 16, 2018 at 04:28:50PM -0600, Eric W. Biederman wrote:
> > > I will keep FPE_FIXME as a place holder until this gets sorted out.
> > > 
> > > There is a second issue I am looking at in this location,
> > > and maybe I don't have to address it now.  But it looks like the code is
> > > calling send_sig_info instead of force_sig_info for a synchronous
> > > exception.  Am I reading that correctly?
> > 
> > VFP used to use force_sig_info(), but it seems to be really the wrong
> > call to use.  force_sig_info() checks whether the program decided to
> > ignore or block the signal, and if it did, replaces the signal handler
> > with the default handler and unblocks the signal.
> > 
> > Are you really suggesting that FP all FP signals should get this
> > treatment?
> 
> feenableexcept(FE_OVERFLOW) kind of means "I can't run safely past
> an fp overflow exception, please signal me instead".
> 
> If the process also blocked SIGFPE, that could be taken to mean
> "I can't run safely past an fp overflow exception _and_ I can't
> take SIGFPE either" ... i.e., if an fp overflow happens there is
> no way to proceed and it's really fatal.
> 
> What SIG_IGN ought to mean is rather more debatable, but again,
> the process could be asking for two opposite things: guarantee a
> SIGFPE is delivered instead of running past an fp exception, and
> also guarantee that SIGFPE is _not_ delivered.
> 
> It looks like arm and arm64 are different from most other arches
> (including x86) here, but I'm not sure what is considered correct, and
> it looks like the answer is not standardised.  There's a possibility
> that some software goes subtly wrong on arm/arm64 where on other arches
> it would get terminated with SIGKILL.
> 
> Whether this matters depends on how harmless the fp exception is to
> the work of the program.  I think if an exception is set to trap
> via feenableexcept() then that's a strong hint the programmer thinks
> that exception is not harmless.  OTOH, trapping is not always
> available anyway...

Like many of these things, there is no clear answer.  It's a set of
conflicting requirements, and as you point out, even if you've called
feenableexcept(), you are not guaranteed to get a trap.

However, do remember that FP exceptions on ARM hardware are already
asynchronous - they get reported by the _next_ FP operation to the one
that caused them, which means they could be raised by a library function
sometime after it occured (when the library function decides to save the
FP registers to the stack before it makes use of them.)  It's entirely
possible that the library function has blocked FP signals temporarily
(not explicitly, just decided to block all signals while it does
something sensitive) and will unblock them again afterwards - at which
point we get the SIGFPE, and it would be quite right to deliver that
signal to the user SIGFPE handler, rather than forcing it onto the
program mid-library function.

It's also possible that SIGFPE could be blocked by another signal handler
having been invoked, and it triggers the latent generation of the SIGFPE.

I'd be more inclined to agree with you if VFP exceptions were synchronous
but they aren't.

> Was there some particular program being broken by the force_sig_info()
> here?

I don't recall.

-- 
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