[uml-devel] NACK on shift-MM_COPY_SEGMENTS for API change

[uml-devel] NACK on shift-MM_COPY_SEGMENTS for API change

[Blaisorblade]

I understand the purpose of the patch; however, without any mention about the 
effects (which would be IMHO bad) we no more copy LDT segments on fork/clone, 
which is bad.

The patch moves that to the call of modify_ldt, but that's an hack, since 
currently it's perfectly legitimate that a child uses the father's LDT, 
though maybe uncommon.

It would be better to move LDT copying to an arch-specific helper, defaulting 
to null (i.e. an empty inline) and defined only for i386 and x86-64 (even the 
same def. is ok, probably).
-- 
Paolo Giarrusso, aka Blaisorblade
Skype user "PaoloGiarrusso"
Linux registered user n. 292729
http://www.user-mode-linux.org/~blaisorblade





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[uml-devel] Re: NACK on shift-MM_COPY_SEGMENTS for API change

[Bodo Stroesser]

Blaisorblade wrote:
> I understand the purpose of the patch; however, without any mention about the 
> effects (which would be IMHO bad) we no more copy LDT segments on fork/clone, 
> which is bad.
No. The patch shifts code only, but AFAICS it doesn't alter the bahavior of UML/i386.
If it does, that would be a bug in the patch!
> 
> The patch moves that to the call of modify_ldt, but that's an hack, since 
> currently it's perfectly legitimate that a child uses the father's LDT, 
> though maybe uncommon.
No. Let's read the code before and after the change:

Before the change, father's LDT is copied in new_mm() using MM_COPY_SEGMENTS.
new_mm() only has one caller, which is init_new_context_skas().

After the change, father's LDT is copied in init_new_ldt(), again
using MM_COPY_SEGMENTS.
init_new_ldt() also has exactly one caller, which again is init_new_context_skas().

In both cases, copying is done only, if /proc/mm is available.
So, nothing in the behavior is changed, but the code is shifted to arch-specific
files.
> 
> It would be better to move LDT copying to an arch-specific helper, defaulting 
> to null (i.e. an empty inline) and defined only for i386 and x86-64 (even the 
> same def. is ok, probably).
Yeah. init_new_ldt() exactly *is* such a arch-specific helper.
In UML/s390, this is a NOOP.

BTW: my UML/i386 uses a libc, that depends on modify_ldt() to manage the TLS.
If UML doesn't copy the LDT, this system crashes. And it works fine with the
changes!

Regards,
      Bodo


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[uml-devel] Re: NACK on shift-MM_COPY_SEGMENTS for API change

[Blaisorblade]

On Friday 06 May 2005 13:04, Bodo Stroesser wrote:
> Blaisorblade wrote:
> > I understand the purpose of the patch; however, without any mention about
> > the effects (which would be IMHO bad) we no more copy LDT segments on
> > fork/clone, which is bad.
>
> No. The patch shifts code only, but AFAICS it doesn't alter the bahavior of
> UML/i386. If it does, that would be a bug in the patch!
>
> > The patch moves that to the call of modify_ldt, but that's an hack, since
> > currently it's perfectly legitimate that a child uses the father's LDT,
> > though maybe uncommon.
>
> No. Let's read the code before and after the change:
>
> Before the change, father's LDT is copied in new_mm() using
> MM_COPY_SEGMENTS. new_mm() only has one caller, which is
> init_new_context_skas().

> After the change, father's LDT is copied in init_new_ldt(), again
> using MM_COPY_SEGMENTS.
Ok, it turns out that I was simply misreading the patch (I didn't apply the 
patch, since I don't have the time to work on Jeff's tree too, beyond the 
release tree).
-- 
Paolo Giarrusso, aka Blaisorblade
Skype user "PaoloGiarrusso"
Linux registered user n. 292729
http://www.user-mode-linux.org/~blaisorblade





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