[PATCH v2 06/24] gdb: fix bugs in {get,put}_frame_register_bytes

[Simon Marchi <simon.marchi@efficios.com>]

I found this only by inspection: the myaddr pointer in
{get,put}_frame_register_bytes is reset to `buffer.data ()` at each
iteration.  This means that we will always use the bytes at the
beginning of `buffer` to read or write to the registers, instead of
progressing in `buffer`.

Fix this by re-writing the functions to chip away the beginning of the
buffer array_view as we progress in reading or writing the data.

These bugs was introduced almost 3 years ago [1], and yet nobody
complained.  I'm wondering which architecture relies on that register
"overflow" behavior (reading or writing multiple consecutive registers
with one {get,put}_frame_register_bytes calls), and in which situation.
I find these functions a bit dangerous, if a caller mis-calculates
things, it could end up silently reading or writing to the next
register, even if it's not the intent.

If I could change it, I would prefer to have functions specifically made
for that ({get,put}_frame_register_bytes_consecutive or something like
that) and make {get,put}_frame_register_bytes only able to write within
a single register (which I presume represents most of the use cases of
the current {get,put}_frame_register_bytes).  If a caller mis-calculates
things and an overflow occurs while calling
{get,put}_frame_register_bytes, it would hit an assert.  The problem is
knowing which callers rely on the overflow behavior and which don't.

[1] https://gitlab.com/gnutools/binutils-gdb/-/commit/bdec2917b1e94c7198ba39919f45060067952f43

Change-Id: I43bd4a9f7fa8419d388a2b20bdc57d652688ddf8
Reviewed-By: John Baldwin <jhb@FreeBSD.org>
Approved-By: Andrew Burgess <aburgess@redhat.com>
---
 gdb/frame.c | 63 +++++++++++++++++++----------------------------------
 1 file changed, 23 insertions(+), 40 deletions(-)

diff --git a/gdb/frame.c b/gdb/frame.c
index 4be9fc20b98c..288d0ef0d620 100644
--- a/gdb/frame.c
+++ b/gdb/frame.c
@@ -1483,9 +1483,6 @@ get_frame_register_bytes (frame_info_ptr frame, int regnum,
 			  int *optimizedp, int *unavailablep)
 {
   struct gdbarch *gdbarch = get_frame_arch (frame);
-  int i;
-  int maxsize;
-  int numregs;
 
   /* Skip registers wholly inside of OFFSET.  */
   while (offset >= register_size (gdbarch, regnum))
@@ -1496,9 +1493,9 @@ get_frame_register_bytes (frame_info_ptr frame, int regnum,
 
   /* Ensure that we will not read beyond the end of the register file.
      This can only ever happen if the debug information is bad.  */
-  maxsize = -offset;
-  numregs = gdbarch_num_cooked_regs (gdbarch);
-  for (i = regnum; i < numregs; i++)
+  int maxsize = -offset;
+  int numregs = gdbarch_num_cooked_regs (gdbarch);
+  for (int i = regnum; i < numregs; i++)
     {
       int thissize = register_size (gdbarch, i);
 
@@ -1507,20 +1504,15 @@ get_frame_register_bytes (frame_info_ptr frame, int regnum,
       maxsize += thissize;
     }
 
-  int len = buffer.size ();
-  if (len > maxsize)
+  if (buffer.size () > maxsize)
     error (_("Bad debug information detected: "
-	     "Attempt to read %d bytes from registers."), len);
+	     "Attempt to read %zu bytes from registers."), buffer.size ());
 
   /* Copy the data.  */
-  while (len > 0)
+  while (!buffer.empty ())
     {
-      int curr_len = register_size (gdbarch, regnum) - offset;
-
-      if (curr_len > len)
-	curr_len = len;
-
-      gdb_byte *myaddr = buffer.data ();
+      int curr_len = std::min<int> (register_size (gdbarch, regnum) - offset,
+				    buffer.size ());
 
       if (curr_len == register_size (gdbarch, regnum))
 	{
@@ -1528,8 +1520,8 @@ get_frame_register_bytes (frame_info_ptr frame, int regnum,
 	  CORE_ADDR addr;
 	  int realnum;
 
-	  frame_register (frame, regnum, optimizedp, unavailablep,
-			  &lval, &addr, &realnum, myaddr);
+	  frame_register (frame, regnum, optimizedp, unavailablep, &lval,
+			  &addr, &realnum, buffer.data ());
 	  if (*optimizedp || *unavailablep)
 	    return false;
 	}
@@ -1548,13 +1540,12 @@ get_frame_register_bytes (frame_info_ptr frame, int regnum,
 	      return false;
 	    }
 
-	  memcpy (myaddr, value->contents_all ().data () + offset,
-		  curr_len);
+	  copy (value->contents_all ().slice (offset, curr_len),
+		buffer.slice (0, curr_len));
 	  release_value (value);
 	}
 
-      myaddr += curr_len;
-      len -= curr_len;
+      buffer = buffer.slice (curr_len);
       offset = 0;
       regnum++;
     }
@@ -1579,36 +1570,28 @@ put_frame_register_bytes (frame_info_ptr frame, int regnum,
       regnum++;
     }
 
-  int len = buffer.size ();
   /* Copy the data.  */
-  while (len > 0)
+  while (!buffer.empty ())
     {
-      int curr_len = register_size (gdbarch, regnum) - offset;
+      int curr_len = std::min<int> (register_size (gdbarch, regnum) - offset,
+				    buffer.size ());
 
-      if (curr_len > len)
-	curr_len = len;
-
-      const gdb_byte *myaddr = buffer.data ();
       if (curr_len == register_size (gdbarch, regnum))
-	{
-	  put_frame_register (frame, regnum, myaddr);
-	}
+	put_frame_register (frame, regnum, buffer.data ());
       else
 	{
-	  struct value *value
+	  value *value
 	    = frame_unwind_register_value (frame_info_ptr (frame->next),
 					   regnum);
-	  gdb_assert (value != NULL);
+	  gdb_assert (value != nullptr);
 
-	  memcpy ((char *) value->contents_writeable ().data () + offset,
-		  myaddr, curr_len);
-	  put_frame_register (frame, regnum,
-			      value->contents_raw ().data ());
+	  copy (buffer.slice (0, curr_len),
+		value->contents_writeable ().slice (offset, curr_len));
+	  put_frame_register (frame, regnum, value->contents_raw ().data ());
 	  release_value (value);
 	}
 
-      myaddr += curr_len;
-      len -= curr_len;
+      buffer = buffer.slice (curr_len);
       offset = 0;
       regnum++;
     }
-- 
2.43.0