PR gdb/15236: gdbserver write to linux memory with zero length corrupts stack

PR gdb/15236: gdbserver write to linux memory with zero length corrupts stack

PROBLEM:

The function linux_write_memory () in linux-low.c allocates a buffer
on the stack to hold a copy of the data to be written.

  register PTRACE_XFER_TYPE *buffer = (PTRACE_XFER_TYPE *)
    alloca (count * sizeof (PTRACE_XFER_TYPE));

"count" is the number of bytes to be written, rounded up to the
nearest multiple of sizeof (PTRACE_XFER_TYPE) and allowing for not
being an aligned address. The function later uses

  buffer[0] = ptrace (PTRACE_PEEKTEXT, pid,
                      (PTRACE_ARG3_TYPE) (uintptr_t) addr, 0);

The problem is that this function can be called to write zero bytes on
an aligned address, for example when receiving an X packet of length 0
(used to test if 8-bit write is supported). Under these circumstances,
count can be zero.

Since in this case, buffer[0] may never have been allocated, the stack
is corrupted and gdbserver may crash.

SOLUTION:

Writing zero bytes should always succeed. The patch below returns
successfully early if the length is zero, so avoiding the stack
corruption.

Verified on the ARC GDB 7.5.1 port.

2013-03-07  Jeremy Bennett  <jeremy.bennett@embecosm.com>

	PR server/15236

	* linux-low.c (linux_write_memory): Return early success if LEN is
	zero.