Re: [PATCH,resubmit] Add static probes to runtime linker

[Roland McGrath <roland at hack dot frob dot com>]

> As I said before, if the stap probe compiles to a single-byte NOP,
> then it's hostile to in-process debuggers.  If {un,}map_complete
> probes could be made sufficiently "wide" for an in-process debugger to
> be able to place a JMP there, that would go a long way to help.

As Tom said, that is entirely orthogonal to the subject at hand.  It's
a generic issue with the utility of sdt.h probes and it needs to be
addressed in sdt.h (i.e. on the systemtap mailing list).  When the
assembly code sdt.h generates improves, then you'll just need to
recompile libc (and whatever else you have probes in) with the new
sdt.h installed.

It so happens that I am the person who (re)wrote the current version
of sdt.h and did consider the x86 long-nop issue, though I am no
longer really involved with that work on the systemtap side.  At the
time, we decided to leave that as future work, but you're welcome to
declare that the future is now and pursue it.  The only real
difficulty is that there is no 5-byte nop instruction that is
available on all x86-32 vintages.  If you're only concerned with
x86-64, then it's easy (and there's no good reason that we didn't
bother with it for x86-64 in the first place, but we didn't).  Feel
free to start that discussion on systemtap@sourceware.org, and you
can CC me personally if you like.  But there is nothing for libc
folks to discuss about that whole subject.


Thanks,
Roland