[rfc] Displaced stepping with wrong entry point address

Ulrich Weigand
2008-08-22T17:49:37Z

Hello,

displaced stepping caused a regression for the Cell multi-arch debugger.
This is because the default displaced_step_at_entry_point routine uses
entry_point_address to determine where to put the displaced instruction.

This is a problem when debugging a stand-alone SPU executable using the
multi-arch debugger, because entry_point_address will point to the SPU
entry point, while we need a place in PowerPC address space to execute
displaced PowerPC code.

(SPU currently does not use displaced stepping, and this probably does
not really matter, as a single SPU context is always single-threaded.)

The following patch makes the multi-arch debugger work again, by using
the AT_ENTRY auxiliary vector to find the entry point address. This
will always point to the PowerPC-side entry point.

I'm wondering whether this fix would be good for the general case too
-- there may be situations where entry_point_address does not work
(e.g. because the main executable file could not be loaded). The
auxiliary vector, on targets where it is present, will probably be
more reliable ...

What do you think?

Tested on powerpc-linux and powerpc64-linux.

Bye,
Ulrich

ChangeLog:

* arch-utils.c: Include "target.h", "auxv.h" and "elf/common.h".
(displaced_step_at_entry_point): Use AT_ENTRY auxiliary vector
to determine entry point address.

diff -urNp src-orig/gdb/arch-utils.c src/gdb/arch-utils.c
--- src-orig/gdb/arch-utils.c 2008-08-18 02:34:56.000000000 +0200
+++ src/gdb/arch-utils.c 2008-08-18 02:37:42.288892324 +0200
@@ -32,6 +32,9 @@
#include "osabi.h"
#include "target-descriptions.h"
#include "objfiles.h"
+#include "target.h"
+#include "auxv.h"
+#include "elf/common.h"

#include "version.h"

@@ -74,7 +77,10 @@ displaced_step_at_entry_point (struct gd
CORE_ADDR addr;
int bp_len;

- addr = entry_point_address ();
+ /* Determine entry point from target auxiliary vector. Fall back
+ to entry point from symbol file if not found. */
+ if (target_auxv_search (&current_target, AT_ENTRY, &addr) <= 0)
+ addr = entry_point_address ();

/* Make certain that the address points at real code, and not a
function descriptor. */