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.

1 files changed, 11 insertions, 6 deletions

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;
 }