Re: [patch] Fix a glitch in debugging 32-bit process with 64-bit GDB.

[Paul Pluzhnikov <ppluzhnikov@google.com>]

[-- Attachment #1: Type: text/plain, Size: 1174 bytes --]

On Mon, Dec 15, 2008 at 12:58 AM, Joel Brobecker <brobecker@adacore.com> wrote:

> I'd like to see the comment detail a little the situation that we are
> trying to solve (64bit gdb debugging a 32bit app). Just as a starting
> point, I suggest for instance:
>
>  if (sizeof (nlist.n_value) > 4)
>    /* We are a 64bit debugger debugging a 32bit program.
>       We have to be a little careful with the n_value in the case
>       of N_LSYM and N_PSYM entries, because they are signed offsets
>       that we actually read as an unsigned 32bit value. This is
>       not a problem with 32bit debuggers where negative values end
>       up being interpreted correctly.  But we need to sign-extend
>       the sign bit on 64bit debuggers.  Otherwise, we'll end up
>       interpreting negative values as very large positive values...  */
>    if (type == N_LSYM || type == N_PSYM)
>      nlist.n_value = (nlist.n_value ^ 0x80000000) - 0x80000000;

Thanks, I slightly re-worded this.

OK to commit?


-- 
Paul Pluzhnikov

2008-12-15  Paul Pluzhnikov  <ppluzhnikov@google.com>

	* dbxread.c (read_ofile_symtab): Sign-extend 32-bit N_LSYM and
	N_PSYM STABS values for 64-bit GDB.

[-- Attachment #2: gdb-1526195-20081215.txt --]
[-- Type: text/plain, Size: 1163 bytes --]

Index: dbxread.c
===================================================================
RCS file: /cvs/src/src/gdb/dbxread.c,v
retrieving revision 1.98
diff -u -r1.98 dbxread.c
--- dbxread.c	1 Oct 2008 16:41:27 -0000	1.98
+++ dbxread.c	15 Dec 2008 21:42:41 -0000
@@ -2597,6 +2597,19 @@
 
       if (type & N_STAB)
 	{
+	  if (sizeof (nlist.n_value) > 4
+	      /* We are a 64-bit debugger debugging a 32-bit program.  */
+	      && (type == N_LSYM || type == N_PSYM))
+	      /* We have to be careful with the n_value in the case of N_LSYM
+		 and N_PSYM entries, because they are signed offsets from frame
+		 pointer, but we actually read them as unsigned 32-bit values.
+		 This is not a problem for 32-bit debuggers, for which negative
+		 values end up being interpreted correctly (as negative
+		 offsets) due to integer overflow.
+		 But we need to sign-extend the value for 64-bit debuggers,
+		 or we'll end up interpreting negative values as very large
+		 positive offsets.  */
+	    nlist.n_value = (nlist.n_value ^ 0x80000000) - 0x80000000;
 	  process_one_symbol (type, nlist.n_desc, nlist.n_value,
 			      namestring, section_offsets, objfile);
 	}