[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 Wed, Jun 05, 2019 at 11:33:22AM +0200, Paul Chaignon wrote:
> On Tue, Jun 04, 2019 at 12:31AM, Dmitry V. Levin wrote:
> > 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.
> 
> I thought SIGKILL would be such a case.  The ptrace man page does mention
> PTRACE_EVENT_EXIT shouldn't happen in case of SIGKILL.  However, it also
> mentions that there's currently a bug:
> 
>   A SIGKILL signal may still cause a PTRACE_EVENT_EXIT stop before actual
>   signal death.  This may be changed in the future; SIGKILL is meant to
>   always immediately kill tasks even under ptrace.  Last confirmed on
>   Linux 3.13.
> 
> I was able to confirm the bug under Linux 5.1...
> 
> I also checked tests/threads-execve.c which tests different behaviors
> under execve.  All such behaviors seem to be covered by the exited status
> already.  Any other tests/functions(/signals?) I should look into that you
> think highlight the disappearance case?

I don't think we have a test that covers the case of disappearance without
PTRACE_EVENT_EXIT, but we have a piece of code that handles a very odd
situation: in printleader() " <unfinished ...>\n" is printed in case of
(printing_tcp->curcol != 0 && followfork >= 2 && printing_tcp == tcp).

OK, lets introduce just one "unfinished" state for now as it makes little
sense to handle this elusive "unfinished without PTRACE_EVENT_EXIT"
condition differently.


-- 
ldv
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