[ovs-dev] [PATCH] fatal-signal: Catch SIGSEGV and print backtrace.

Ben Pfaff blp at ovn.org
Wed Sep 18 23:27:46 UTC 2019


On Wed, Sep 18, 2019 at 01:16:37PM -0700, William Tu wrote:
> Thanks for the feedback.
> 
> On Wed, Sep 18, 2019 at 11:30 AM Ben Pfaff <blp at ovn.org> wrote:
> >
> > On Tue, Sep 17, 2019 at 04:13:24PM -0700, William Tu wrote:
> > > The patch catches the SIGSEGV signal and prints the backtrace
> > > using libunwind, hopefully makes it easier to debug.
> > >
> > > Signed-off-by: William Tu <u9012063 at gmail.com>
> >
> > I guess my experience is that this sort of thing is sometimes useful but
> > ultimately causes problems because it tries to call a lot of functions
> > that a signal handler is not supposed to call.
> 
> Do you mean the show_backtrace() tries to call too many functions?
> What's the concern about calling lots of functions in a signal handler?

There is no concern about the number of functions but about the specific
ones.  Most C library functions are not async-signal-safe, meaning that
it is not safe to call them from a signal handler.  There is a list of
functions guaranteed to be async-signal-safe in section 2.4.3 "Signal
Actions" of the System Interfaces volume of POSIX.  You can find it
here: https://pubs.opengroup.org/onlinepubs/9699919799/.  Notably, it
doesn't contain the stdio functions like printf() and fflush(), or
syslog, and those functions are likely to malfunction if the signal
handler interrupts some ongoing operation on a stream.  There's a risk
of being caught in some kind of SIGSEGV loop.

I think there needs to be a lot of care if we're going to do this by
default.


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