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Re: RFC: Use .plt section sh_entsize instead of GET_PLT_ENTRY_SIZE


On Mon, 19 Aug 2013, H.J. Lu wrote:

> >> We need to increase x86-64 PLT entry size to 32 bytes to
> >> support Intel MPX.
> >
> > I don't like this at all.  The suggestion from Ian or Alan (don't
> > remember) with multiple plts sounds much better.  Only 2 plt slots per
> > icache line seems quite horrible when not needed.  IIRC you said "it
> > sounds complicated" to that idea.  I say, "so what?".  Life is hard.
> >
> The main issue is the new shared libraries/executables must work with 
> the existing dynamic linker.

Explain what issue you see.  In the two-plt model the existing dynamic 
linker won't know about the second .plt, hence use the old non-bnd aware 
slots.  I.e. they will work, but bound checking will be ineffective.  I 
think requiring a new dynamic linker to make bounds checking work over 
PLT borders is sensible.

Halving the number of plt slots per cache line for everyone for a feature 
of questionable utility (*) OTOH doesn't seem nice.

(*) Sorry, I'm really not convinced that MPX is such a marvellous feature 
to warrant the plt bloat.

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