Add code for relocating dynamic executables.

2 files changed, 117 insertions, 1 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index dced8de..2081283 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,10 @@
+2000-11-09  Kevin Buettner  <kevinb@redhat.com>
+
+	Changes based on a patch from Ulrich Drepper:
+	* solib-svr4.c (svr4_relocate_main_executable): New function.
+	(svr4_solib_create_inferior_hook):  Call
+	svr4_relocate_main_executable.
+	
 2000-11-09  J.T. Conklin  <jtc@redback.com>
 
 	* config/i386/nbsd.mh: Remove solib.o, solib-svr4.o from NATDEPFILES.
diff --git a/gdb/solib-svr4.c b/gdb/solib-svr4.c
index b5a0725..bbe90a6 100644
--- a/gdb/solib-svr4.c
+++ b/gdb/solib-svr4.c
@@ -1437,6 +1437,112 @@ svr4_special_symbol_handling (void)
 #endif /* !SVR4_SHARED_LIBS */
 }
 
+/* Relocate the main executable.  This function should be called upon
+   stopping the inferior process at the entry point to the program. 
+   The entry point from BFD is compared to the PC and if they are
+   different, the main executable is relocated by the proper amount. 
+   
+   As written it will only attempt to relocate executables which
+   lack interpreter sections.  It seems likely that only dynamic
+   linker executables will get relocated, though it should work
+   properly for a position-independent static executable as well.  */
+
+static void
+svr4_relocate_main_executable (void)
+{
+  asection *interp_sect;
+  CORE_ADDR pc = read_pc ();
+
+  /* Decide if the objfile needs to be relocated.  As indicated above,
+     we will only be here when execution is stopped at the beginning
+     of the program.  Relocation is necessary if the address at which
+     we are presently stopped differs from the start address stored in
+     the executable AND there's no interpreter section.  The condition
+     regarding the interpreter section is very important because if
+     there *is* an interpreter section, execution will begin there
+     instead.  When there is an interpreter section, the start address
+     is (presumably) used by the interpreter at some point to start
+     execution of the program.
+
+     If there is an interpreter, it is normal for it to be set to an
+     arbitrary address at the outset.  The job of finding it is
+     handled in enable_break().
+
+     So, to summarize, relocations are necessary when there is no
+     interpreter section and the start address obtained from the
+     executable is different from the address at which GDB is
+     currently stopped.
+     
+     [ The astute reader will note that we also test to make sure that
+       the executable in question has the DYNAMIC flag set.  It is my
+       opinion that this test is unnecessary (undesirable even).  It
+       was added to avoid inadvertent relocation of an executable
+       whose e_type member in the ELF header is not ET_DYN.  There may
+       be a time in the future when it is desirable to do relocations
+       on other types of files as well in which case this condition
+       should either be removed or modified to accomodate the new file
+       type.  (E.g, an ET_EXEC executable which has been built to be
+       position-independent could safely be relocated by the OS if
+       desired.  It is true that this violates the ABI, but the ABI
+       has been known to be bent from time to time.)  - Kevin, Nov 2000. ]
+     */
+
+  interp_sect = bfd_get_section_by_name (exec_bfd, ".interp");
+  if (interp_sect == NULL 
+      && (bfd_get_file_flags (exec_bfd) & DYNAMIC) != 0
+      && bfd_get_start_address (exec_bfd) != pc)
+    {
+      struct cleanup *old_chain;
+      struct section_offsets *new_offsets;
+      int i, changed;
+      CORE_ADDR displacement;
+      
+      /* It is necessary to relocate the objfile.  The amount to
+	 relocate by is simply the address at which we are stopped
+	 minus the starting address from the executable.
+
+	 We relocate all of the sections by the same amount.  This
+	 behavior is mandated by recent editions of the System V ABI. 
+	 According to the System V Application Binary Interface,
+	 Edition 4.1, page 5-5:
+
+	   ...  Though the system chooses virtual addresses for
+	   individual processes, it maintains the segments' relative
+	   positions.  Because position-independent code uses relative
+	   addressesing between segments, the difference between
+	   virtual addresses in memory must match the difference
+	   between virtual addresses in the file.  The difference
+	   between the virtual address of any segment in memory and
+	   the corresponding virtual address in the file is thus a
+	   single constant value for any one executable or shared
+	   object in a given process.  This difference is the base
+	   address.  One use of the base address is to relocate the
+	   memory image of the program during dynamic linking.
+
+	 The same language also appears in Edition 4.0 of the System V
+	 ABI and is left unspecified in some of the earlier editions.  */
+
+      displacement = pc - bfd_get_start_address (exec_bfd);
+      changed = 0;
+
+      new_offsets = xcalloc (sizeof (struct section_offsets),
+			     symfile_objfile->num_sections);
+      old_chain = make_cleanup (free, new_offsets);
+
+      for (i = 0; i < symfile_objfile->num_sections; i++)
+	{
+	  if (displacement != ANOFFSET (symfile_objfile->section_offsets, i))
+	    changed = 1;
+	  new_offsets->offsets[i] = displacement;
+	}
+
+      if (changed)
+	objfile_relocate (symfile_objfile, new_offsets);
+
+      do_cleanups (old_chain);
+    }
+}
+
 /*
 
    GLOBAL FUNCTION
@@ -1489,9 +1595,12 @@ svr4_special_symbol_handling (void)
    Also, what if child has exit()ed?  Must exit loop somehow.
  */
 
-void
+static void
 svr4_solib_create_inferior_hook (void)
 {
+  /* Relocate the main executable if necessary.  */
+  svr4_relocate_main_executable ();
+
   /* If we are using the BKPT_AT_SYMBOL code, then we don't need the base
      yet.  In fact, in the case of a SunOS4 executable being run on
      Solaris, we can't get it yet.  current_sos will get it when it needs