Problem after hitting breakpoint on Windows (with GDBserver)

When debugging on Windows with GDBserver, the debugger starts
failing after hitting a breakpoint.  For instance:

    (gdb) b foo
    Breakpoint 1 at 0x40177e: file foo.adb, line 5.
    (gdb) cont
    Continuing.

    Breakpoint 1, foo () at foo.adb:5
    5          Put_Line ("Hello World.");  -- STOP
    (gdb) n

    Program received signal SIGSEGV, Segmentation fault.
    0x00401782 in foo () at foo.adb:5
    5          Put_Line ("Hello World.");  -- STOP

There are two issues:

  1. While trying to re-insert a breakpoint that is still inserted
     in memory, insert_bp_location wipes out the breakpoint location's
     shadow_contents.  As a consequence, we cannot restore the proper
     instruction when removing the breakpoint anymore.  That's why
     the inferior's behavior changes when trying to resume after
     the breakpoint was hit.

  2. mem-break.c:default_memory_insert_breakpoint passes a breakpoint
     location's shadow_contents as the buffer for a memory read.
     This reveals a limitation of the various memory-read target
     functions.  This patch documents this limitation and adjust
     the two calls that seem to hit that limitation.

gdb/ChangeLog:

        * breakpoint.c (breakpoint_xfer_memory): Add assertion.
        Update function description.
        (insert_bp_location): Do not wipe bl->target_info out.
        * mem-break.c: #include "gdb_string.h".
        (default_memory_insert_breakpoint): Do not call target_read_memory
        with a pointer to the breakpoint's shadow_contents buffer.  Use
        a local buffer instead.
        * m32r-tdep.c (m32r_memory_insert_breakpoint): Ditto.

4 files changed, 43 insertions, 9 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 1a75508..875d675 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,14 @@
+2012-03-15  Joel Brobecker  <brobecker@adacore.com>
+
+	* breakpoint.c (breakpoint_xfer_memory): Add assertion.
+	Update function description.
+	(insert_bp_location): Do not wipe bl->target_info out.
+	* mem-break.c: #include "gdb_string.h".
+	(default_memory_insert_breakpoint): Do not call target_read_memory
+	with a pointer to the breakpoint's shadow_contents buffer.  Use
+	a local buffer instead.
+	* m32r-tdep.c (m32r_memory_insert_breakpoint): Ditto.
+
 2012-03-15  Tom Tromey  <tromey@redhat.com>
 
 	* NEWS: Mention "info vtbl", not "info vtable".
diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index d35704d..debf2b2 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -1318,6 +1318,10 @@ bp_location_has_shadow (struct bp_location *bl)
 /* Update BUF, which is LEN bytes read from the target address MEMADDR,
    by replacing any memory breakpoints with their shadowed contents.
 
+   If READBUF is not NULL, this buffer must not overlap with any of
+   the breakpoint location's shadow_contents buffers.  Otherwise,
+   a failed assertion internal error will be raised.
+
    The range of shadowed area by each bp_location is:
      bl->address - bp_location_placed_address_before_address_max
      up to bl->address + bp_location_shadow_len_after_address_max
@@ -1446,6 +1450,12 @@ breakpoint_xfer_memory (gdb_byte *readbuf, gdb_byte *writebuf,
 
     if (readbuf != NULL)
       {
+	/* Verify that the readbuf buffer does not overlap with
+	   the shadow_contents buffer.  */
+	gdb_assert (bl->target_info.shadow_contents >= readbuf + len
+		    || readbuf >= (bl->target_info.shadow_contents
+				   + bl->target_info.shadow_len));
+
 	/* Update the read buffer with this inserted breakpoint's
 	   shadow.  */
 	memcpy (readbuf + bp_addr - memaddr,
@@ -2082,8 +2092,15 @@ insert_bp_location (struct bp_location *bl,
   if (!should_be_inserted (bl) || (bl->inserted && !bl->needs_update))
     return 0;
 
-  /* Initialize the target-specific information.  */
-  memset (&bl->target_info, 0, sizeof (bl->target_info));
+  /* Note we don't initialize bl->target_info, as that wipes out
+     the breakpoint location's shadow_contents if the breakpoint
+     is still inserted at that location.  This in turn breaks
+     target_read_memory which depends on these buffers when
+     a memory read is requested at the breakpoint location:
+     Once the target_info has been wiped, we fail to see that
+     we have a breakpoint inserted at that address and thus
+     read the breakpoint instead of returning the data saved in
+     the breakpoint location's shadow contents.  */
   bl->target_info.placed_address = bl->address;
   bl->target_info.placed_address_space = bl->pspace->aspace;
   bl->target_info.length = bl->length;
diff --git a/gdb/m32r-tdep.c b/gdb/m32r-tdep.c
index 72872bd..d504eb3 100644
--- a/gdb/m32r-tdep.c
+++ b/gdb/m32r-tdep.c
@@ -85,7 +85,7 @@ m32r_memory_insert_breakpoint (struct gdbarch *gdbarch,
   CORE_ADDR addr = bp_tgt->placed_address;
   int val;
   gdb_byte buf[4];
-  gdb_byte *contents_cache = bp_tgt->shadow_contents;
+  gdb_byte contents_cache[4];
   gdb_byte bp_entry[] = { 0x10, 0xf1 };	/* dpt */
 
   /* Save the memory contents.  */
@@ -93,6 +93,7 @@ m32r_memory_insert_breakpoint (struct gdbarch *gdbarch,
   if (val != 0)
     return val;			/* return error */
 
+  memcpy (bp_tgt->shadow_contents, contents_cache, 4);
   bp_tgt->placed_size = bp_tgt->shadow_len = 4;
 
   /* Determine appropriate breakpoint contents and size for this address.  */
diff --git a/gdb/mem-break.c b/gdb/mem-break.c
index 7d0e3f1..bd34fb2 100644
--- a/gdb/mem-break.c
+++ b/gdb/mem-break.c
@@ -29,6 +29,7 @@
 #include "breakpoint.h"
 #include "inferior.h"
 #include "target.h"
+#include "gdb_string.h"
 
 
 /* Insert a breakpoint on targets that don't have any better
@@ -46,6 +47,7 @@ default_memory_insert_breakpoint (struct gdbarch *gdbarch,
 {
   int val;
   const unsigned char *bp;
+  gdb_byte *readbuf;
 
   /* Determine appropriate breakpoint contents and size for this address.  */
   bp = gdbarch_breakpoint_from_pc
@@ -53,15 +55,18 @@ default_memory_insert_breakpoint (struct gdbarch *gdbarch,
   if (bp == NULL)
     error (_("Software breakpoints not implemented for this target."));
 
-  /* Save the memory contents.  */
+  /* Save the memory contents in the shadow_contents buffer and then
+     write the breakpoint instruction.  */
   bp_tgt->shadow_len = bp_tgt->placed_size;
-  val = target_read_memory (bp_tgt->placed_address, bp_tgt->shadow_contents,
+  readbuf = alloca (bp_tgt->placed_size);
+  val = target_read_memory (bp_tgt->placed_address, readbuf,
 			    bp_tgt->placed_size);
-
-  /* Write the breakpoint.  */
   if (val == 0)
-    val = target_write_raw_memory (bp_tgt->placed_address, bp,
-				   bp_tgt->placed_size);
+    {
+      memcpy (bp_tgt->shadow_contents, readbuf, bp_tgt->placed_size);
+      val = target_write_raw_memory (bp_tgt->placed_address, bp,
+				     bp_tgt->placed_size);
+    }
 
   return val;
 }