Re: Analysis of broken systemtap example scripts

["Frank Ch. Eigler" <fche at redhat dot com>]

Hi -

> [....]
> probe syscall.openat.return {
>   filename = @entry(user_string_quoted(pointer_arg(2)))
>   printf("%s %s (%x) %s\n", proc(), pp(), @entry(pointer_arg(2)), filename)
> }

> However, running the same script on a newer fedora kernel:
> [wcohen@cervelo io]$ stap open_args.stp -T 1
> 1335 (libvirtd) kprobe.function("__x64_sys_openat")? (7f331f4b66e0) "/sys/devices/system/node/node0/meminfo"
> [...]

Yes, this is expected, because @entry() expressions are evaluated at
synthetic probes at function.  The syscall.* probes for 4.17+ work by
first processing the incoming pt_regs pointer from a parameter.  The
@entry() part misses this step, so cannot work.

So, both @entry($var) and $var in syscall.*.return are not generally
available any more.  We'll improve @entry() to generalize to this sort
of situation, but in the mean time, user and samples will have to fend
for themselves with explicit globals etc.

- FChE