Re: vdso feature requests from the Go people

[Stijn Volckaert]

Andy Lutomirski schreef op 13/06/2014 7:23:
> On Thu, Jun 12, 2014 at 10:15 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> > On 06/12/2014 09:36 PM, Andy Lutomirski wrote:
> > > If we were to implement both, maybe we'd actually want to provide
> > > something like:
> > >
> > > struct vdso_entry {
> > >    unsigned long vdso_entry_struct_size;  /* so we can add fields later on */
> > >    void *func;
> > >    unsigned int max_stack;  /* zero if not known */
> > > };
> >
> > How different is this really from the .dynsym table -- other than that
> > you have to find the latter via the dynamic segment?
> >
> > The hash part is just to speed up lookup...
>
> As far as I know, there's no reliable way to just read the dynsym
> table -- the thing doesn't have a specified length, which is what
> broke Go in the first place.
>
> Parsing the ELF dynamic tables is kind of annoyingly complicated, and
> understanding the format is a real PITA -- the documentation I've been
> able to find is outright terrible.
>
> --Andy

Why does it need a specified length? It's pretty easy to find, the 
entries have a fixed length and the last entry in the table is NULLed 
out (i.e. the d_tag is DT_NULL).

Just look for the PT_DYNAMIC segment in the program headers and iterate 
over the fixed length Elf_Dyn entries to find what you're looking for.

As for ELF documentation, I've never had any problems understanding the
official specification:
http://refspecs.linuxbase.org/elf/elf.pdf

--Stijn

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