[Bug tapsets/14325] New: for arm/s390x, the nd_syscall tapsets need improvement

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

http://sourceware.org/bugzilla/show_bug.cgi?id=14325

             Bug #: 14325
           Summary: for arm/s390x, the nd_syscall tapsets need improvement
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: tapsets
        AssignedTo: systemtap@sourceware.org
        ReportedBy: dsmith@redhat.com
    Classification: Unclassified


The nd_syscall tapsets use the *_arg() functions to get syscall arguments. 
However, arm can only get the 1st 4 arguments and s390x can only get the 1st 6
arguments.  The nd_syscall tapset tries to get the 7th argument in several
probe aliases, which will fail.

Commit 65ddca0 is a hack for s390x that supplies a dummy value for 2 probe
aliases that are used heavily by the nd_syscall testsuite.

But, a better fix is needed for arm and s390x.

Possible solutions include:

- Have the *_arg() functions return a dummy value when asked for an argument it
can't handle.

- Have each architecture provide a function, something like syscall_max_args(),
that would provide the maximum argument that this architecture supports.  This
function could be tested in each probe alias, like this:

  argstr = sprintf("%d %d %d %d %d", int_arg(1), int_arg(2), int_arg(3),
           int_arg(4), (syscall_max_args() >= 5 ? int_arg(5) : -1))

- Continue like commit 65ddca0 and just do architecture checks everywhere
needed.

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