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Re: Next over function with Secure PLT


On 12/07/2011 08:01 PM, Alan Modra wrote:
On Wed, Dec 07, 2011 at 04:58:22PM -0800, Michael Eager wrote:
When using PowerPC Secure PLT, trying to "next" over a
library function in a shared library does not work correctly.
Instead of skipping over the function, gdb steps through
the PLT entry which shows up as code in
call___do_global_ctors_aux.

As an aside, if you use a newer linker symbols will be emitted on each plt stub by default for -shared.

Or I might have to backport the binutils patch. Another work- around is to force --emit-stub-sims.

This doesn't happen when "nexting" over a library function
in the executable.  Next works the same as with a local function.

When reading the executable, ppc_elf_get_synthetic_symtab()
calls is_nonpic_glink_stub() to recognizes the PLT stub and
then generates internal symbols for each PLT stub like foo@plt.
It also creates an entry for __glink and __glink_PLTresolve.

If I modify is_nonpic_glink_stub() to recognize the shared
library PLT stub format, similar internal symbols are created
and gdb seems to work correctly.

Don't do that. You can't easily know which plt entry is loaded by a PIC stub.

There's a comment before the call:
  /* If the stubs are those for -shared/-pie then we might have
      multiple stubs for each plt entry.  If that is the case then
      there is no way to associate stubs with their plt entries short
      of figuring out the GOT pointer value used in the stub.  */
   if (!is_nonpic_glink_stub (abfd, glink,
			     glink_vma - GLINK_ENTRY_SIZE - glink->vma))

What is this trying to tell me?  What are the circumstances where
there would be multiple stubs for each PLT entry?  If there are
multiple stubs, then this might create multiple foo@plt symbols
with different values.  Would this cause any problems?

There can be multiple stubs using the same PLT entry when linking -fPIC code. -fPIC effectively gives you a 64k GOT per file, with the result that r30 may differ from one function to another in the executable. Since r30 is used in a PIC stub to calculate the PLT entry address, you need a different stub for different values of r30.

OK. That was the detail I was missing.


gdb ought to just pattern match the stub code to recognize when the pc
is in a stub.

I wanted to avoid pattern matching code because of the dependency it creates between code generation and debugging. But that shouldn't be much of an issue or hard to implement.

--
Michael Eager	 eager@eagercon.com
1960 Park Blvd., Palo Alto, CA 94306  650-325-8077


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