[Bug tapsets/20618] New: Cannot get the real syscall return code value

[Bug tapsets/20618] New: Cannot get the real syscall return code value

[mysecondaccountabc at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=20618

            Bug ID: 20618
           Summary: Cannot get the real syscall return code value
           Product: systemtap
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: tapsets
          Assignee: systemtap at sourceware dot org
          Reporter: mysecondaccountabc at gmail dot com
  Target Milestone: ---

We need to get the syscall return value and the errno separately. However, when
for instance, the connect syscall is executed in a non-blocking socket and it
returns -1 and errno=115 (EINPROGRESS) all we get in a "probe syscall.*.return"
is:
  returnval() = -115
  retstr = "-115 (EINPROGRESS)"

We couldn't find in any part of the documentation that returnval() or the
syscall return value will be replaced with the errno value, so I wonder if it's
a bug or in any case, how can we get/recover the original syscall return code?

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[Bug tapsets/20618] Cannot get the real syscall return code value

[fche at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=20618

Frank Ch. Eigler <fche at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |codonell at redhat dot com,
                   |                            |fche at redhat dot com
         Resolution|---                         |WORKSFORME

--- Comment #1 from Frank Ch. Eigler <fche at redhat dot com> ---
> We couldn't find in any part of the documentation that returnval() or the
> syscall return value will be replaced with the errno value [...]

It turns out that this is an artefact of how linux system calls
work.  A positive/zero return value is success, a negative
return value usually is an error code that the glibc runtime will
shift into the global 'errno'.  At the low systemtap level, the
negative errno -is- the genuine return value from the syscall.

Unfortunately, we don't have convenient probes at the glibc level
to observe the C-level aftereffects properly.  There is a glibc
RFE open (for quite some time now) for them to insert some sys/sdt.h
markers, so that stap could present the cooked results properly.

(RHers, see also https://bugzilla.redhat.com/show_bug.cgi?id=742034 .)

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[Bug tapsets/20618] Cannot get the real syscall return code value

[mysecondaccountabc at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=20618

--- Comment #2 from Gustavo Moreira <mysecondaccountabc at gmail dot com> ---
Ouch I see, thank you for the quick response.

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