RFC: [PATCH] ELF: Don't require section header on ELF objects

[Alan Modra]

On Tue, Mar 10, 2020 at 12:24:51AM +1030, Kaylee Blake wrote:
> On 10/3/20 12:14 am, Alan Modra wrote:
> > On Mon, Mar 09, 2020 at 11:24:44PM +1030, Kaylee Blake wrote:
> >> On 9/3/20 6:43 pm, Florian Weimer wrote:
> >>> In my opinion, it should NOT be possible to link against objects
> >>> without section headers.  Lack of section headers clearly marks the
> >>> object as a run-time only object.  This is useful if you want to
> >>> prevent developers to create DT_NEEDED dependencies on internal
> >>> libraries, for example.
> > 
> > I agree.
> > 
> >> For shared objects without debug symbols, the section header table is
> >> ~2kB on average of redundant data. I'm also not a fan of the
> >> inconsistency of having shared libraries that the dynamic linker is
> >> perfectly happy to load, but ld can't link against, especially since
> >> this seems like an oversight rather than an intended design decision.
> > 
> > The ELF spec designed things that way.  See figure 4.1 which I'll try
> > to represent in text.
> > 
> > Figure 4-1: Object File Format
> > 
> > |----------------------|    |----------------------|    
> > |      ELF Header      |    |      ELF Header      |    
> > |----------------------|    |----------------------|    
> > | Program header table |    | Program header table |    
> > |       optional       |    |       required       |    
> > |----------------------|    |----------------------|    
> > |       Section 1      |    |       Segment 1      |    
> > |----------------------|    |----------------------|    
> > |          ...         |    |       Segment 2      |    
> > |----------------------|    |----------------------|    
> > |       Section n      |    |       Segment 3      |    
> > |----------------------|    |----------------------|    
> > |          ...         |    |          ...         |    
> > |----------------------|    |----------------------|    
> > | Section header table |    | Section header table |    
> > |       required       |    |       optional       |    
> > |----------------------|    |----------------------|    
> >       Linking View               Execution View      
> > 
> 
> I had interpreted that table in combination to various other references
> to which things are required vs optional in shared objects as meaning
> that the "execution view" applied to executables and shared objects, and
> the "linking view" applied to relocatable objects. You're saying that
> that table should be interpreted as saying that if a shared object is to
> be linkable, the spec is requiring it to have both sets of headers?

Yes.  Just below the table: "Files used during linking must have a
section header table".

-- 
Alan Modra
Australia Development Lab, IBM