Re: [patch] Relocate phdr in read_program_header

[Aleksandar Ristovski <aristovski at qnx dot com>]

On 11-10-16 03:21 PM, Jan Kratochvil wrote:
> On Fri, 30 Sep 2011 21:31:33 +0200, Aleksandar Ristovski wrote:
> >      * solib-svr4.c (read_program_header): If PT_PHDR is present, use it
> >      to calculate correct address in case of PIE.
>
> FSF ChangeLog should describe the changes, not their reason.  Therefore:
> 	* solib-svr4.c (read_program_header): New variable pt_phdr, initialize
> 	it from target PT_PHDR p_vaddr, relocate sect_addr by it.


Change log changed (see below).


> > +  pt_phdr = (CORE_ADDR)-1; /* Address of the first entry. If not PIE,
> > +			      this will be zero.
> > +			      For PIE, it will be unrelocated at_phdr. */
>
> There is CORE_ADDR_MAX.  But I would prefer new `int pt_phdr_p' as "predicate"
> boolean flag for valid pt_phdr value, these special values are fragile and one
> has to think whether they can happen.


pt_phdr_p introduced. Also, pt_phdr initialized to 0 even though not 
strictly necessary, but gcc complained about possible uninitialized use.


> > +
> >     /* Find the requested segment.  */
> >     if (type == -1)
> >       {
> > @@ -427,6 +431,11 @@ read_program_header (int type, int *p_se
> >   	    return 0;
> >
> >   	  if (extract_unsigned_integer ((gdb_byte *)phdr.p_type,
> > +					4, byte_order) == PT_PHDR)
> > +	    pt_phdr = extract_unsigned_integer ((gdb_byte *)phdr.p_vaddr,
> > +						4, byte_order);
> > +
> > +	  if (extract_unsigned_integer ((gdb_byte *)phdr.p_type,
> >   					4, byte_order) == type)
>
> That p_type could be decoded only once.  Also any casta are `(type) val' with
> a space accoridng to GCS (GNU Coding Standards).

Cast fixed, p_type decoded only once.

> >     if (target_read_memory (sect_addr, buf, sect_size))
>
>
> Just about the style, I do not see any problems of the patch, thanks.
>
> Wrote the testcase, it is probably GNU/Linux-specific.

I was contemplating a testcase but I couldn't think of a trick to "make" 
gdb not find the executable... Thanks for the test.


> I would like to review yet the changes.

Revised patch attached.



Thanks,

Aleksandar


New change log:

<date>  Aleksandar Ristovski  <aristovski@qnx.com>

        * solib-svr4.c (read_program_header): New variable pt_phdr, 
initialize
        it from target PT_PHDR p_vaddr, relocate sect_addr by it.


Index: gdb/solib-svr4.c
===================================================================
RCS file: /cvs/src/src/gdb/solib-svr4.c,v
retrieving revision 1.157
diff -u -p -r1.157 solib-svr4.c
--- gdb/solib-svr4.c	14 Oct 2011 07:58:58 -0000	1.157
+++ gdb/solib-svr4.c	17 Oct 2011 14:48:31 -0000
@@ -364,10 +364,11 @@ static gdb_byte *
 read_program_header (int type, int *p_sect_size, int *p_arch_size)
 {
   enum bfd_endian byte_order = gdbarch_byte_order (target_gdbarch);
-  CORE_ADDR at_phdr, at_phent, at_phnum;
+  CORE_ADDR at_phdr, at_phent, at_phnum, pt_phdr = 0;
   int arch_size, sect_size;
   CORE_ADDR sect_addr;
   gdb_byte *buf;
+  int pt_phdr_p = 0;
 
   /* Get required auxv elements from target.  */
   if (target_auxv_search (&current_target, AT_PHDR, &at_phdr) <= 0)
@@ -401,12 +402,23 @@ read_program_header (int type, int *p_se
       /* Search for requested PHDR.  */
       for (i = 0; i < at_phnum; i++)
 	{
+	  int p_type;
+
 	  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, byte_order) == type)
+	  p_type = extract_unsigned_integer ((gdb_byte *) phdr.p_type,
+					     4, byte_order);
+
+	  if (p_type == PT_PHDR)
+	    {
+	      pt_phdr_p = 1;
+	      pt_phdr = extract_unsigned_integer ((gdb_byte *) phdr.p_vaddr,
+						  4, byte_order);
+	    }
+
+	  if (p_type == type)
 	    break;
 	}
 
@@ -427,12 +439,23 @@ read_program_header (int type, int *p_se
       /* Search for requested PHDR.  */
       for (i = 0; i < at_phnum; i++)
 	{
+	  int p_type;
+
 	  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, byte_order) == type)
+	  p_type = extract_unsigned_integer ((gdb_byte *) phdr.p_type,
+					     4, byte_order);
+
+	  if (p_type == PT_PHDR)
+	    {
+	      pt_phdr_p = 1;
+	      pt_phdr = extract_unsigned_integer ((gdb_byte *) phdr.p_vaddr,
+						  8, byte_order);
+	    }
+
+	  if (p_type == type)
 	    break;
 	}
 
@@ -446,6 +469,16 @@ read_program_header (int type, int *p_se
 					    8, byte_order);
     }
 
+  /* PT_PHDR is optional, but we really need it
+     for PIE to make this work in general.  */
+
+  if (pt_phdr_p)
+    {
+      /* at_phdr is real address in memory. pt_phdr is what pheader says it is.
+	 Relocation offset is the difference between the two. */
+      sect_addr = sect_addr + (at_phdr - pt_phdr);
+    }
+
   /* Read in requested program header.  */
   buf = xmalloc (sect_size);
   if (target_read_memory (sect_addr, buf, sect_size))