RE: [Mips}Using DT tags for handling local ifuncs

[Jack Carter <Jack dot Carter at imgtec dot com>]

Richard,

You got me digging in my old code ;-)

> > I also have a hard time with how the GOT is used for binutils. In my
> > experience and world view, sections have attributes that make them gp
> > relative or not. All these sections get gathered in gp relative
> > regions that are 64k from a value that will be in their $GP. If there
> > are GOT elements that are not gp relative, they should be in another
> > .got that is not marked SHF_MIPS_GPREL. It will not get laid out and
> > calibrated with any of the other GOTs.  Other sections in my life that
> > get bundled up in the equation for multigot are .sbss, .sdata,
> > .lit[4,8,16], .srdata, but only if they are marked SHF_MIPS_GPREL.
> 
> Just so I understand, do you think that the ABI GOT should always be 64k
> or smaller?  I.e. DT_MIPS_LOCAL_GOTNO + (DT_MIPS_SYMTABNO - DT_MIPS_GOTSYM)
> should be <= 64 * 1024 / sizeof (void *)?  If so, what should happen
> (under the original or IRIX n32/n64 ABIs) if the number of symbols
> involved in .rel.dyn relocations exceeds the 64k limit?  Is that a
> link error?
> 
> Yes, because in sgi's case you count all the SHF_MIPS_GPREL sections as 
> the GP area. .got is only one of them and sgi just put gp-relative entries in it.
> 
> > The DT_MIPS_LOCAL_GOTNO describes local got entries. Not other
> > partitions that we reserve the right to put non-local got entries.
> 
> I'm still not sure which part you're describing as the local GOT here.
> Let's go back to the original 32-bit GOT layout, without any GNU extensions:
> 
>       +------------+   +    <--- DT_PLTGOT
>       |   entry 0  |   |
>       +------------+ + B
>       |  ........  | A |
>       +------------+ + +    <--- DT_PLTGOT + DT_MIPS_LOCAL_GOTNO * 4
>       | Global GOT |
>       +------------+
> 
> where:
> 
>   The zero entry in the global offset table is reserved to hold the
>   address of the entry point in the dynamic linker to call when lazy
>   resolving text symbols. The dynamic linker must always initialize this
>   entry regardless of whether lazy binding is or is not enabled.
> 
> Do you see the local GOT as being A or B?  I.e. does it include
> the zero entry?

It is by definition A and B, here is the quote from the pre-sgi System V 
Application Binary Interface Mips Processor Supplement:

Global Offset Table (5-9, second paragraph)
"The global offset tables split into two locally separate subtables: local and 
externals. Local entries reside in the first part of the global offset table. The 
value of the dynamic tag DT_MIPS_LOCAL_GOTNO holds the number of 
local global offset table entries."

The sgi edition is essentially the same but it includes:
"It (the GOT" is essentially two tables. The first (with DT_MIPS_LOCAL_GOTNO 
entries) consists of local GOT addresses, i.e. non-preemptible (protected) 
addresses defined within the executable/DSO."

> > Dealing with the ifunc "local" entries implicitly will save a
> > relocation lookup, a tiny blip of time in relation to the other costs
> > of calling the resolver. So I am arguing about how many angels can
> > dance on a pin.
> 
> Yeah, maybe this is one we'll have to agree to disagree on.  I think the
> benefit of having an implicitly-relocated irelative region is small at best.
> I like the generality of including the GOT R_MIPS_IRELATIVE GOT
> relocations in the general .rel.dyn pool and sorting them accordingly,
> because it feels more future-proof.  I also think an implicit region is
> harder to handle in a backward-compatible way, since if we just add new
> tags, older ld.sos would ignore them and not flag an error.

Then go the way sgi did and have .dynsym indexed regions for:

    DT_MIPS_LOCAL_GOTIDX
    DT_MIPS_INTERNAL_GOTIDX
    DT_MIPS_HIDDEN_GOTIDX
    DT_MIPS_PROTECTED_GOTIDX

For entertainment sake here is the comment in my private elf dumper wrote back then:

/**
    @internal

    Function:	mips_print_got

    MIPS has 2 different GOT table variants that are 
    pretty much the same except one depends on symbol
    table to got table symmetry for runtime fixup purposes
    and the other uses runtime relocations.
    
    If there is multigot there will be entries in the first dynamic section
    of type DT_MIPS_AUX_DYNAMIC which point to the other
    dynamic sections which in turn point to and describe their
    associated gots.
    
    DT_MIPS_LOCAL_GOTNO     	Starting point for DEFAULT symbols
    DT_MIPS_GOTSYM  	    	Index into dsymtab matching DT_MIPS_LOCAL_GOTNO
    DT_MIPS_HIPAGENO		Number of page table entries.
    DT_MIPS_LOCALPAGE_GOTIDX	Starting point for a local got page table
    DT_MIPS_LOCAL_GOTIDX    	Starting point for local full addresses
    DT_MIPS_HIDDEN_GOTIDX   	Starting point for HIDDEN symbols
    DT_MIPS_PROTECTED_GOTIDX	Starting point for PROTECTED symbols

    If DT_MIPS_LOCAL_GOTIDX == DT_HIDDEN_GOT_IDX ||
    	    	    	       DT_PROTECTED_GOT_IDX ||
			       DT_MIPS_LOCAL_GOTNO
    then there are no local entries. Local in this sense
    means addresses that may or may not have associated
    entries in the symbol table or relocation table. If
    they are present in the symbol table they will be marked
    as STO_INTERNAL and must not be referenced outside of the
    defining dso/a.out in any form.

    If DT_HIDDEN_GOT_IDX == DT_PROTECTED_GOT_IDX ||
    	    	    	    DT_MIPS_LOCAL_GOTNO
    then there are no hidden entries. Hidden symbols
    are those that are marked STO_HIDDEN in the dynamic
    symbol table and are accessable from outside the defining
    dso only non-symbolicly such as through pointers.


    If DT_PROTECTED_GOT_IDX == DT_MIPS_LOCAL_GOTNO
    then there are no protected entries. Protected symbols
    are those that are marked STO_PROTECTED in the dynamic
    symbol table and are accessable from the outside, but
    cannot be preempted during runtime loading and thus are
    "protected".
    
    @return  void.
 */

Note, for multigot this resulted in multiple dynamic sections, dynsyms and 
relocation fixups for the got entries.

I am not proposing that we go down this route, but it may give a sense of
the world I came from. I liked it because (other than that I designed a lot of
it :-)) of the structure in symbol visibility and that I could dump the entries 
symbolically. Also, each GP region was described by its dynamic section.

This is not a trivial change and goes beyond the ifunc scope, but it would resolve
the fixup by relocation issues and usher in GP rel areas that go beyond the GOT.

I really just want to get ifunc done without messing up future goodness in ld/ld.so.

Jack

> 
> If someone else has any opinions about implicit irelative relocs vs.
> explicit irelative relocs though, please shout.
> 
> Thanks,
> Richard