[rfc] Use auxillary vector to retrieve .dynamic/.interp sections

["Ulrich Weigand" <uweigand at de dot ibm dot com>]

Hello,

here's a patch to solib-svr4.c that I originally did in the context of the
Cell combined debugger, but it could be more generally useful.

The shared library code currently requires certain information from the
main executable, and it needs the actual executable file to get it:
- the name of the program interpreter (from ".interp" section)
- various tags, in particular DT_DEBUG, from the ".dynamic" section

Now, on Linux (and other platforms), it would not actually be necessary
to use the main executable file to retrieve this information.  It is 
actually available directly from the target: the above sections are
present in memory, addressed via program headers, and the in-memory
copy of the program header table can be found via auxillary vector
entries AT_PHDR, AT_PHENT, and AT_PHNUM.

This is helpful on Cell, because we use a special loader (installed
via binfmt_misc) to load stand-alone SPU applications -- the "main
application" as far as shared library handling is concerned needs 
to be that loader, but the "main application" GDB knows about is
the SPU application.

However, it might as well be useful in other circumstances, e.g.
involving similar binfmt_misc setups, or simply situations where
the main application executable is not available to GDB (e.g. 
when attaching to a remote process).

The patch below does not change the behaviour on platforms that do
not provide the above auxv entries.  In fact, even on those that do,
we still try the current method first, and only fall back to using
the auxillary vector if that has failed.

(Note that the patch is against a tree that already has the patch to
retrieve libraries from the remote target applied.  That patch is not
really a pre-requiste, however.)

Tested on powerpc-linux and powerpc64-linux.

Does this seem useful / reasonble for mainline?

Bye,
Ulrich


ChangeLog:

	* solib-svr4.c (read_program_header): New function.
	(scan_dyntag_auxv): New function.
	(elf_locate_base): Use it if scan_dyntag fails.
	(find_program_interpreter): New function.
	(enable_break): Use it instead of .interp section.


diff -urNp gdb-orig/gdb/solib-svr4.c gdb-head/gdb/solib-svr4.c
--- gdb-orig/gdb/solib-svr4.c	2008-08-13 21:24:01.134851402 +0200
+++ gdb-head/gdb/solib-svr4.c	2008-08-13 21:32:32.443065824 +0200
@@ -374,6 +374,137 @@ bfd_lookup_symbol (bfd *abfd, char *symn
   return symaddr;
 }
 
+
+/* Read program header TYPE from inferior memory.  The header is found
+   by scanning the OS auxillary vector.
+
+   Return a pointer to allocated memory holding the program header contents,
+   or NULL on failure.  If sucessful, and unless P_SECT_SIZE is NULL, the
+   size of those contents is returned to P_SECT_SIZE.  Likewise, the target
+   architecture size (32-bit or 64-bit) is returned to P_ARCH_SIZE.  */
+
+static gdb_byte *
+read_program_header (int type, int *p_sect_size, int *p_arch_size)
+{
+  CORE_ADDR at_phdr, at_phent, at_phnum;
+  int arch_size, sect_size;
+  CORE_ADDR sect_addr;
+  gdb_byte *buf;
+
+  /* Get required auxv elements from target.  */
+  if (target_auxv_search (&current_target, AT_PHDR, &at_phdr) <= 0)
+    return 0;
+  if (target_auxv_search (&current_target, AT_PHENT, &at_phent) <= 0)
+    return 0;
+  if (target_auxv_search (&current_target, AT_PHNUM, &at_phnum) <= 0)
+    return 0;
+  if (!at_phdr || !at_phnum)
+    return 0;
+
+  /* Determine ELF architecture type.  */
+  if (at_phent == sizeof (Elf32_External_Phdr))
+    arch_size = 32;
+  else if (at_phent == sizeof (Elf64_External_Phdr))
+    arch_size = 64;
+  else
+    return 0;
+
+  /* Find .dynamic section via the PT_DYNAMIC PHDR.  */
+  if (arch_size == 32)
+    {
+      Elf32_External_Phdr phdr;
+      int i;
+
+      /* Search for requested PHDR.  */
+      for (i = 0; i < at_phnum; i++)
+	{
+	  if (target_read_memory (at_phdr + i * sizeof (phdr),
+				  (gdb_byte *)&phdr, sizeof (phdr)))
+	    return 0;
+
+	  if (extract_unsigned_integer ((gdb_byte *)phdr.p_type, 4) == type)
+	    break;
+	}
+
+      if (i == at_phnum)
+	return 0;
+
+      /* Retrieve address and size.  */
+      sect_addr = extract_unsigned_integer ((gdb_byte *)phdr.p_vaddr, 4);
+      sect_size = extract_unsigned_integer ((gdb_byte *)phdr.p_memsz, 4);
+    }
+  else
+    {
+      Elf64_External_Phdr phdr;
+      int i;
+
+      /* Search for requested PHDR.  */
+      for (i = 0; i < at_phnum; i++)
+	{
+	  if (target_read_memory (at_phdr + i * sizeof (phdr),
+				  (gdb_byte *)&phdr, sizeof (phdr)))
+	    return 0;
+
+	  if (extract_unsigned_integer ((gdb_byte *)phdr.p_type, 4) == type)
+	    break;
+	}
+
+      if (i == at_phnum)
+	return 0;
+
+      /* Retrieve address and size.  */
+      sect_addr = extract_unsigned_integer ((gdb_byte *)phdr.p_vaddr, 8);
+      sect_size = extract_unsigned_integer ((gdb_byte *)phdr.p_memsz, 8);
+    }
+
+  /* Read in requested program header.  */
+  buf = xmalloc (sect_size);
+  if (target_read_memory (sect_addr, buf, sect_size))
+    {
+      xfree (buf);
+      return NULL;
+    }
+
+  if (p_arch_size)
+    *p_arch_size = arch_size;
+  if (p_sect_size)
+    *p_sect_size = sect_size;
+
+  return buf;
+}
+
+
+/* Return program interpreter string.  */
+static gdb_byte *
+find_program_interpreter (void)
+{
+  gdb_byte *buf = NULL;
+
+  /* If we have an exec_bfd, use its section table.  */
+  if (exec_bfd
+      && bfd_get_flavour (exec_bfd) == bfd_target_elf_flavour)
+   {
+     struct bfd_section *interp_sect;
+
+     interp_sect = bfd_get_section_by_name (exec_bfd, ".interp");
+     if (interp_sect != NULL)
+      {
+	CORE_ADDR sect_addr = bfd_section_vma (exec_bfd, interp_sect);
+	int sect_size = bfd_section_size (exec_bfd, interp_sect);
+
+	buf = xmalloc (sect_size);
+	bfd_get_section_contents (exec_bfd, interp_sect, buf, 0, sect_size);
+      }
+   }
+
+  /* If we didn't find it, use the target auxillary vector.  */
+  if (!buf)
+    buf = read_program_header (PT_INTERP, NULL, NULL);
+
+  return buf;
+}
+
+
 /* Scan for DYNTAG in .dynamic section of ABFD. If DYNTAG is found 1 is
    returned and the corresponding PTR is set.  */
 
@@ -451,6 +582,59 @@ scan_dyntag (int dyntag, bfd *abfd, CORE
   return 0;
 }
 
+/* Scan for DYNTAG in .dynamic section of the target's main executable,
+   found by consulting the OS auxillary vector.  If DYNTAG is found 1 is
+   returned and the corresponding PTR is set.  */
+
+static int
+scan_dyntag_auxv (int dyntag, CORE_ADDR *ptr)
+{
+  int sect_size, arch_size, step;
+  long dyn_tag;
+  CORE_ADDR dyn_ptr;
+  gdb_byte *bufend, *bufstart, *buf;
+
+  /* Read in .dynamic section.  */
+  buf = bufstart = read_program_header (PT_DYNAMIC, &sect_size, &arch_size);
+  if (!buf)
+    return 0;
+
+  /* Iterate over BUF and scan for DYNTAG.  If found, set PTR and return.  */
+  step = (arch_size == 32) ? sizeof (Elf32_External_Dyn)
+			   : sizeof (Elf64_External_Dyn);
+  for (bufend = buf + sect_size;
+       buf < bufend;
+       buf += step)
+  {
+    if (arch_size == 32)
+      {
+	Elf32_External_Dyn *dynp = (Elf32_External_Dyn *) buf;
+	dyn_tag = extract_unsigned_integer ((gdb_byte *) dynp->d_tag, 4);
+	dyn_ptr = extract_unsigned_integer ((gdb_byte *) dynp->d_un.d_ptr, 4);
+      }
+    else
+      {
+	Elf64_External_Dyn *dynp = (Elf64_External_Dyn *) buf;
+	dyn_tag = extract_unsigned_integer ((gdb_byte *) dynp->d_tag, 8);
+	dyn_ptr = extract_unsigned_integer ((gdb_byte *) dynp->d_un.d_ptr, 8);
+      }
+    if (dyn_tag == DT_NULL)
+      break;
+
+    if (dyn_tag == dyntag)
+      {
+	if (ptr)
+	  *ptr = dyn_ptr;
+
+	xfree (bufstart);
+	return 1;
+      }
+  }
+
+  xfree (bufstart);
+  return 0;
+}
+
 
 /*
 
@@ -485,7 +669,8 @@ elf_locate_base (void)
   /* Look for DT_MIPS_RLD_MAP first.  MIPS executables use this
      instead of DT_DEBUG, although they sometimes contain an unused
      DT_DEBUG.  */
-  if (scan_dyntag (DT_MIPS_RLD_MAP, exec_bfd, &dyn_ptr))
+  if (scan_dyntag (DT_MIPS_RLD_MAP, exec_bfd, &dyn_ptr)
+      || scan_dyntag_auxv (DT_MIPS_RLD_MAP, &dyn_ptr))
     {
       gdb_byte *pbuf;
       int pbuf_size = TYPE_LENGTH (builtin_type_void_data_ptr);
@@ -498,7 +683,8 @@ elf_locate_base (void)
     }
 
   /* Find DT_DEBUG.  */
-  if (scan_dyntag (DT_DEBUG, exec_bfd, &dyn_ptr))
+  if (scan_dyntag (DT_DEBUG, exec_bfd, &dyn_ptr)
+      || scan_dyntag_auxv (DT_DEBUG, &dyn_ptr))
     return dyn_ptr;
 
   /* This may be a static executable.  Look for the symbol
@@ -964,6 +1150,7 @@ enable_break (void)
   struct minimal_symbol *msymbol;
   char **bkpt_namep;
   asection *interp_sect;
+  gdb_byte *interp_name;
   CORE_ADDR sym_addr;
 
   /* First, remove all the solib event breakpoints.  Their addresses
@@ -1026,13 +1213,11 @@ enable_break (void)
 	}
     }
 
-  /* Find the .interp section; if not found, warn the user and drop
+  /* Find the program interpreter; if not found, warn the user and drop
      into the old breakpoint at symbol code.  */
-  interp_sect = bfd_get_section_by_name (exec_bfd, ".interp");
-  if (interp_sect)
+  interp_name = find_program_interpreter ();
+  if (interp_name)
     {
-      unsigned int interp_sect_size;
-      char *buf;
       CORE_ADDR load_addr = 0;
       int load_addr_found = 0;
       int loader_found_in_list = 0;
@@ -1041,13 +1226,7 @@ enable_break (void)
       struct target_ops *tmp_bfd_target;
       volatile struct gdb_exception ex;
 
-      /* Read the contents of the .interp section into a local buffer;
-         the contents specify the dynamic linker this program uses.  */
       sym_addr = 0;
-      interp_sect_size = bfd_section_size (exec_bfd, interp_sect);
-      buf = alloca (interp_sect_size);
-      bfd_get_section_contents (exec_bfd, interp_sect,
-				buf, 0, interp_sect_size);
 
       /* Now we need to figure out where the dynamic linker was
          loaded so that we can load its symbols and place a breakpoint
@@ -1060,7 +1239,7 @@ enable_break (void)
 
       TRY_CATCH (ex, RETURN_MASK_ALL)
         {
-	  tmp_bfd = solib_bfd_open (buf);
+	  tmp_bfd = solib_bfd_open (interp_name);
 	}
       if (tmp_bfd == NULL)
 	goto bkpt_at_symbol;
@@ -1075,7 +1254,7 @@ enable_break (void)
       so = master_so_list ();
       while (so)
 	{
-	  if (svr4_same_1 (buf, so->so_original_name))
+	  if (svr4_same_1 (interp_name, so->so_original_name))
 	    {
 	      load_addr_found = 1;
 	      loader_found_in_list = 1;
@@ -1104,7 +1283,7 @@ enable_break (void)
 
       if (!loader_found_in_list)
 	{
-	  debug_loader_name = xstrdup (buf);
+	  debug_loader_name = xstrdup (interp_name);
 	  debug_loader_offset_p = 1;
 	  debug_loader_offset = load_addr;
 	  solib_add (NULL, 0, &current_target, auto_solib_add);
@@ -1152,12 +1331,14 @@ enable_break (void)
       if (sym_addr != 0)
 	{
 	  create_solib_event_breakpoint (load_addr + sym_addr);
+	  xfree (interp_name);
 	  return 1;
 	}
 
       /* For whatever reason we couldn't set a breakpoint in the dynamic
          linker.  Warn and drop into the old code.  */
     bkpt_at_symbol:
+      xfree (interp_name);
       warning (_("Unable to find dynamic linker breakpoint function.\n"
                "GDB will be unable to debug shared library initializers\n"
                "and track explicitly loaded dynamic code."));
-- 
  Dr. Ulrich Weigand
  GNU Toolchain for Linux on System z and Cell BE
  Ulrich.Weigand@de.ibm.com