[lttng-dev] [LTTNG-TOOLS PATCH v2] ABI with support for compat 32/64 bits

[mathieu.desnoyers@efficios.com (Mathieu Desnoyers)]

* David Goulet (dgoulet at efficios.com) wrote:
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> 
> 
> Julien Desfossez:
> >>>>>> 
> >>>>>> +/* + * This flag indicates if lttng-tools must use the
> >>>>>> new or the old kernel ABI + * (without compat support for
> >>>>>> 32/64 bits). It is set by + * kernctl_tracer_version()
> >>>>> 
> >>>>> Hrm. I don't like that this is only set by 
> >>>>> kernctl_tracer_version(). What if, for an unforeseen
> >>>>> reason (e.g. code change), sessiond starts using "create
> >>>>> session" before checking the version ? The change you
> >>>>> propose assumes a behavior on the sessiond side that is not
> >>>>> cast in stone (no ABI requires it), so it might change.
> >>>>> This brings coupling between this otherwise self-contained
> >>>>> wrapper and the entire sessiond code base, which I don't
> >>>>> like.
> >>>>> 
> >>>>> We should put the check in a wrapper macro around the every
> >>>>> new ioctl call.
> >>>>> 
> >>>>> e.g.
> >>>>> 
> >>>>> /* * Cache whether we need to use the old or new ABI. */
> >>>>> static lttng_kernel_use_old_abi = -1;
> >>>>> 
> >>>>> 
> >>>>> if (lttng_kernel_use_old_abi == -1) { ret = ioctl(fd,
> >>>>> newname, args); if (!ret) { lttng_kernel_use_old_abi = 0; }
> >>>>> } else { if (!lttng_kernel_use_old_abi) { ret = ioctl(fd,
> >>>>> oldname, args); } else { ret = ioctl(fd, newname, args); }
> >>>>> } return ret;
> >>>>> 
> >>>>> Thoughts ?
> >>>> 
> >>>> Ok I will add this wrapper, I am wondering if we could just
> >>>> limit to these ioctl since they are the only one called on
> >>>> the "top-level" fd (/proc/lttng) instead of adding it
> >>>> everywhere : LTTNG_KERNEL_SESSION LTTNG_KERNEL_TRACER_VERSION
> >>>>  LTTNG_KERNEL_TRACEPOINT_LIST LTTNG_KERNEL_WAIT_QUIESCENT 
> >>>> LTTNG_KERNEL_CALIBRATE
> >>>> 
> >>>> All other ioctl require at least a session so is it OK if I
> >>>> limit the test to only these ?
> >> 
> >>> yes, it makes sense. And given that this code is very much 
> >>> localized, I don't think it would be worth the effort to create
> >>> a macro wrapper. Duplicating the checks at each 5 sites, all
> >>> within the same file, seems good enough. But it's David's
> >>> call.
> >> 
> >> I don't know... I can easily see some other use cases in the
> >> future that requires us to wrap the ioctl according to the
> >> kernel.
> >> 
> >> I remember having this discussion way back when we started
> >> lttng-tools code and here we are almost three years later fixing
> >> the "ioctl wrapper issue" :P.
> >> 
> >> I don't think adding a macro is a lot of work and it will be
> >> really easier for us to scale and adapt over time. Please, if I'm
> >> wrong, speak now or shut up to the end of eternity! :P
> > 
> > For this particular case, we need to make special treatment when
> > the ioctl takes an argument depending on the ABI version (alloc and
> > assign the old struct values from the new one), so we won't be able
> > to call a generic wrapper (except if we do this treatment all the
> > time which I doubt we want). For ioctls that don't take an argument
> > the wrapper is easy though.
> > 
> > So I think I should do a wrapper like compat_ioctl_no_arg(int fd,
> > unsigned long oldname, unsigned long newname) and for the ioctls
> > that take an argument make the checks locally.
> 
> Hmmm it's that or a compat call for each ioctl listed above... or
> using va_list but I really don't like that :P
> 
> Either way works for me. Mathieu?

Let's make it simple for those few sites, and not overengineer this. I
like Julien's solution.

Thanks,

Mathieu

> 
> David
> 
> > 
> > Thoughts ?
> > 
> > Thanks,
> > 
> > Julien
> > 
> > 
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-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com