[PATCH v3 2/3] The -z and -Z options print only successful and failing syscalls respectively. However, failure of syscall is only known after syscall return. Thus, we end up with something like this on, e.g., ENOENT:

[Dmitry V. Levin]

On Sun, Jun 02, 2019 at 10:42:01PM +0200, Paul Chaignon wrote:
> On Wed, May 22, 2019 at 12:13 PM Dmitry V. Levin <ldv at altlinux.org> wrote:
> >
> > On Mon, May 13, 2019 at 04:00:31PM +0000, Kohl, Burkhard wrote:
> > [...]
> > > > > diff --git a/strace.1.in b/strace.1.in index 76a74119..6ab95836 100644
> > > > > --- a/strace.1.in
> > > > > +++ b/strace.1.in
> > > > > @@ -771,6 +771,13 @@ Print unabbreviated versions of environment, stat, termios, etc.
> > > > >  calls.  These structures are very common in calls and so the default
> > > > > behavior displays a reasonable subset of structure members.  Use  this
> > > > > option to get all of the gory details.
> > > > > +.TP
> > > > > +.B \-z
> > > > > +Print successful syscalls only.
> > > >
> > > > It might be better to be precise here regarding what we mean by "successful" exactly.  Maybe "Print only
> > > > syscalls that did not return an error code"?
> > >
> > > Seems there are 3 possible outcomes for a syscall:
> > > - zero/successful
> > > - non-zero/not successful
> > > - void
> >
> > Strictly speaking, there are more than 3 possible outcomes for a syscall:
> > - syscall returns with an error
> > - syscall returns without an error
> > - syscall returns but strace fails to fetch error status (see e.g. <unavailable>)
> > - syscall does not return because of PTRACE_EVENT_EXIT (see print_event_exit)
> > - syscall does not return because of process disappearance (e.g. due to execve in a neighbour thread)
> 
> Wouldn't execve in a neightbour thread result in PTRACE_EVENT_EXIT? Are
> the exited and unfinished cases really different?
> 
> It looks like PTRACE_O_TRACEEXIT is set in all cases (both PTRACE_SEIZE
> and PTRACE_ATTACH), so when a neighbour thread does an execve, the tracee
> thread should be torn down and stopped with PTRACE_EVENT_EXIT before
> actual death. What am I missing?

Yes, your interpretation seems to be correct.  If I'm not mistaken,
we even rely on this behaviour in one of strace tests.

Nevertheless, I'm pretty sure processes can be killed in a way that would
look like disappearance.


-- 
ldv
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