Re: [ppc64-linux]: correctly find a BFD's code entry point address

[Jim Blandy <jimb@redhat.com>]

Kevin Buettner <kevinb@redhat.com> writes:

> On Jun 12,  6:12pm, Jim Blandy wrote:
> 
> > + /* Return the unrelocated code address at which execution begins for
> > +    ABFD, under the 64-bit PowerPC Linux ABI.  On that system, the ELF
> > +    header e_entry field (which is what bfd_get_start_address gives
> > +    you) is the address of the function descriptor for the startup
> > +    function, not the address of the actual machine instruction you
> > +    jump to.
> > + 
> > +    This function doesn't just go and read the entry point from the
> > +    function descriptor.  We need it to work when ABFD is the dynamic
> > +    linker, immediately after an exec.  But ld.so is a dynamic
> > +    executable itself on PPC64 Linux, so it appears in memory whereever
> > +    the kernel drops it; this means that bfd_get_start_address's result
> > +    needs to be adjusted --- by some offset we don't know.  So we can't
> > +    find the descriptor's address in memory to read the entry point
> > +    from it.
> > + 
> > +    Instead, we do it all based on ABFD's symbol table.  We take the
> > +    address from bfd_get_start_address, find each symbol at that
> > +    address, stick a '.' on the front of its name to get the entry
> > +    point symbol name, try to look that up, and return the value of
> > +    what we find, if anything.  We never touch memory, or talk with the
> > +    kernel about the inferior at all.
> > + 
> > +    Now, this address we return is straight from the symbol table, so
> > +    it hasn't been adjusted to take into account where ABFD was loaded.
> > +    But that's okay --- our job is just to return the unrelocated code
> > +    address.  */
> 
> This approach strikes me as somewhat more complicated (and fragile)
> than need be.  I think it would be preferable to simply fetch the
> necessary bytes from the address given by bfd_get_start_address in the
> executable (or object) file.
> 
> Nice description though; I really appreciate comments like this.

Thanks!  I redid the patch as you suggest, and it's much smaller and
simpler.  How's this:

2003-06-12  Jim Blandy  <jimb@redhat.com>

	* ppc-linux-tdep.c (ppc64_linux_bfd_entry_point): New function.
	(ppc_linux_init_abi): Register it as our bfd_entry_point method.

Index: gdb/ppc-linux-tdep.c
===================================================================
RCS file: /cvs/src/src/gdb/ppc-linux-tdep.c,v
retrieving revision 1.32
diff -c -r1.32 ppc-linux-tdep.c
*** gdb/ppc-linux-tdep.c	13 Jun 2003 00:06:11 -0000	1.32
--- gdb/ppc-linux-tdep.c	13 Jun 2003 22:46:59 -0000
***************
*** 884,889 ****
--- 884,935 ----
  }
  
  
+ /* Return the unrelocated code address at which execution begins for
+    ABFD, under the 64-bit PowerPC Linux ABI.
+ 
+    On that system, the ELF header's e_entry field (which is what
+    bfd_get_start_address gives you) is not the address of the actual
+    machine instruction you need to jump to, as it is on almost every
+    other target.  Instead, it's the address of a function descriptor
+    for the start function.  A function descriptor is a structure
+    containing three addresses: the entry point, the TOC pointer for
+    the function, and an environment pointer for the function.  The
+    first field is what we want to return.
+ 
+    So all we do is find the section containing the start address, read
+    the address-sized word there out of the BFD, and return that.  */
+ static CORE_ADDR
+ ppc64_linux_bfd_entry_point (struct gdbarch *gdbarch, bfd *abfd)
+ {
+   CORE_ADDR start_address = bfd_get_start_address (abfd);
+   CORE_ADDR addr_size = (bfd_arch_bits_per_address (abfd)
+                          / bfd_arch_bits_per_byte (abfd));
+   unsigned char *entry_pt_buf = alloca (addr_size);
+   asection *sec;
+ 
+   /* Find a data section containing an address word at the start
+      address.  */
+   for (sec = abfd->sections; sec; sec = sec->next)
+     if (bfd_get_section_vma (sec) <= start_address
+         && ((start_address + addr_size)
+             <= (bfd_get_section_vma (sec) + bfd_section_size (sec))))
+       break;
+   if (! sec)
+     return 0;
+ 
+   /* Seek to the start address, and read the address word there.  */
+   if (bfd_seek (abfd, 
+                 sec->filepos + (start_address - bfd_get_section_vma (sec)),
+                 SEEK_SET)
+       || bfd_bread (entry_pt_buf, addr_size, abfd) != addr_size)
+     return 0;
+       
+   /* That's the actual code entry point.  */
+   return (CORE_ADDR) bfd_get (bfd_arch_bits_per_address (abfd),
+                               abfd, entry_pt_buf);
+ }
+ 
+ 
  enum {
    ELF_NGREG = 48,
    ELF_NFPREG = 33,
***************
*** 1008,1013 ****
--- 1054,1061 ----
        set_gdbarch_in_solib_call_trampoline
          (gdbarch, ppc64_in_solib_call_trampoline);
        set_gdbarch_skip_trampoline_code (gdbarch, ppc64_skip_trampoline_code);
+       
+       set_gdbarch_bfd_entry_point (gdbarch, ppc64_linux_bfd_entry_point);
      }
  }