[PATCH 1/1] [RFC] gdb: corelow: make possible to modify (set) registers for a corefile

[Roman Pen <roman.penyaev@profitbricks.com>]

Despite the fact that this is a hairy hack this change eases debugging
of a jmp_buf (setjmp()) and user contexts (makecontext()), which are
highly used in QEMU project as a part of coroutines.

This change allows setting registers for a corefile, thus QEMU gdb
script (qemu/scripts/qemugdb/coroutine.py) is allowed to investigate
backtrace of a preempted coroutine context.  Previously only debugging
of a live process was allowed.

This patch caches all register on a first attempt to modify register
'(gdb) set $REG = ADDR' and then cached copy is always returned from
get_core_registers().

This change should not break previous behaviour if nobody sets any
register, i.e. on each get_core_registers() call registers from a
corefile will be reread.

Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
Cc: Pedro Alves <palves@redhat.com>
Cc: Daniel Jacobowitz <dan@codesourcery.com>
Cc: Jan Kratochvil <jan.kratochvil@redhat.com>
Cc: gdb-patches@sourceware.org

QEMU guys who can be interested in this new gdb behaviour:

Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
---
 gdb/corelow.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/gdb/corelow.c b/gdb/corelow.c
index c46af0a8a59d..4a4b20ed57a0 100644
--- a/gdb/corelow.c
+++ b/gdb/corelow.c
@@ -74,6 +74,15 @@ static struct gdbarch *core_gdbarch = NULL;
    unix child targets.  */
 static struct target_section_table *core_data;
 
+/* Cached registers. Once registers are modified (set) for a corefile,
+   they are cached and then are always fetched from get_core_registers().
+   This hairy hack is used only for one purpose: give a possibility to
+   debug jmp_buf (setjmp()) and user contexts (makecontext()). */
+
+static struct cached_reg {
+	char regp[MAX_REGISTER_SIZE];
+} *core_cachedregs;
+
 static void core_files_info (struct target_ops *);
 
 static struct core_fns *sniff_core_bfd (bfd *);
@@ -213,6 +222,11 @@ core_close (struct target_ops *self)
     }
   core_vec = NULL;
   core_gdbarch = NULL;
+  if (core_cachedregs)
+    {
+      xfree(core_cachedregs);
+      core_cachedregs = NULL;
+    }
 }
 
 static void
@@ -620,6 +634,18 @@ get_core_registers (struct target_ops *ops,
     }
 
   gdbarch = get_regcache_arch (regcache);
+
+  if (core_cachedregs)
+    {
+      /* If registers were once modified (set) for a corefile,
+         follow this path and always return cached registers */
+
+      for (i = 0; i < gdbarch_num_regs(gdbarch); i++)
+        regcache_raw_supply(regcache, i, &core_cachedregs[i]);
+
+      return;
+    }
+
   if (gdbarch_iterate_over_regset_sections_p (gdbarch))
     gdbarch_iterate_over_regset_sections (gdbarch,
 					  get_core_registers_cb,
@@ -639,6 +665,29 @@ get_core_registers (struct target_ops *ops,
 }
 
 static void
+set_core_registers (struct target_ops *self, struct regcache *regcache,
+                    int regnum)
+{
+  struct gdbarch *gdbarch;
+  int i;
+
+  gdbarch = get_regcache_arch(regcache);
+
+  if (core_cachedregs == NULL)
+    core_cachedregs = (struct cached_reg*)xcalloc(gdbarch_num_regs(gdbarch),
+                                                  sizeof(*core_cachedregs));
+
+  for (i = 0; i < gdbarch_num_regs(gdbarch); i++)
+    regcache_raw_collect(regcache, i, &core_cachedregs[i]);
+}
+
+static void
+prepare_core_registers (struct target_ops *self, struct regcache *arg1)
+{
+    /* nothing here */
+}
+
+static void
 core_files_info (struct target_ops *t)
 {
   print_section_info (core_data, core_bfd);
@@ -1050,6 +1099,8 @@ init_core_ops (void)
   core_ops.to_close = core_close;
   core_ops.to_detach = core_detach;
   core_ops.to_fetch_registers = get_core_registers;
+  core_ops.to_store_registers = set_core_registers;
+  core_ops.to_prepare_to_store = prepare_core_registers;
   core_ops.to_xfer_partial = core_xfer_partial;
   core_ops.to_files_info = core_files_info;
   core_ops.to_insert_breakpoint = ignore;
-- 
2.11.0