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

["fche at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

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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