[RFC][MIPS] What to do about DT_MIPS_RLD_MAP and PIE

[Maciej W. Rozycki]

On Thu, 30 Oct 2014, Roland McGrath wrote:

> >  Alternatively, we could cook up a generic DT_GNU_RLD_MAP tag for 
> > platforms that want to opt in to a read-only dynamic section/segment and 
> > start using it with the MIPS target first.  I think I like the latter a 
> > bit better, any thoughts, anyone?
> 
> What's the specification of this tag's semantics?

 Here's what the 32-bit MIPS psABI[1] says about it:

"DT_MIPS_RLD_MAP
                This member is used by debugging.  It contains the
                address of a 32-bit word in the .data section which is
                supplied by the compilation environment.  The word's
                contents are not specified and programs using this value
                are not ABI - compliant."

In a 64-bit ELF file the word is 64-bit instead; the 64-bit MIPS ELF 
specification[2] mentions the tag, but does not document it further.

 The GNU toolchain does not really use a location in the `.data' section; 
instead the BFD linker creates a separate `.rld_map' section that spans 
only this piece of data, and points DT_MIPS_RLD_MAP at it.  The section is 
then mapped to a writable segment.

 Our `ld.so' then puts the address of its link map there just as it puts 
it directly into the DT_DEBUG tag if present instead.  The value of the 
DT_MIPS_RLD_MAP tag is intepreted as a final virtual memory address and 
therefore does not work for PIE executables though.

 For a new DT_GNU_RLD_MAP to work universally, both for traditional and 
PIE executables, I propose that the contents of this tag were not an 
address of, but a relative offset from the location of the tag to the 
location referred.  This will be straightforward to handle in GDB too.

 References:

[1] "SYSTEM V APPLICATION BINARY INTERFACE, MIPS RISC Processor 
    Supplement, 3rd Edition"
    http://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf

[2] "64-bit ELF Object File Specification, Draft Version 2.5"
    http://techpubs.sgi.com/library/manuals/4000/007-4658-001/pdf/007-4658-001.pdf

  Maciej