Return TARGET_XFER_EOF if reading at the end of TARGET_OBJECT_SIGNAL_INFO.

  This fixes an assertion failure if a request is made to read just past
the end of a native thread's signal information structure.

gdb/ChangeLog:

	* fbsd-nat.c (fbsd_nat_target::xfer_partial) [USE_SIGINFO]: Return
	TARGET_XFER_EOF if len is zero.
---
 gdb/ChangeLog  | 5 +++++
 gdb/fbsd-nat.c | 2 +-
 2 files changed, 6 insertions(+), 1 deletion(-)
  

>>>>> "John" == John Baldwin <jhb@FreeBSD.org> writes:

John> This fixes an assertion failure if a request is made to read just past
John> the end of a native thread's signal information structure.

I am curious to know where the assertion happens.

linux-nat.c doesn't seem to return TARGET_XFER_EOF here (see
linux_xfer_siginfo), so I wonder if either linux-nat.c needs a fix, or
if the bug is somewhere else.

Tom
  

On 5/10/19 10:40 AM, Tom Tromey wrote:
>>>>>> "John" == John Baldwin <jhb@FreeBSD.org> writes:
> 
> John> This fixes an assertion failure if a request is made to read just past
> John> the end of a native thread's signal information structure.
> 
> I am curious to know where the assertion happens.
> 
> linux-nat.c doesn't seem to return TARGET_XFER_EOF here (see
> linux_xfer_siginfo), so I wonder if either linux-nat.c needs a fix, or
> if the bug is somewhere else.

I had to try hard to provoke it.  I had a bug in out-of-tree patches to
fbsd-nat.c that resulted in getting the siginfo_size wrong (too small)
compared to the generated type for $_siginfo for the target architecture.
The size mismatch just so happened to result in a field that started on
the byte location of the end of the too-small size and so 'p $_siginfo'
tried to read a field at that exact offset and tripped the assertion at
the bottom of target_xfer_partial:

  /* Check implementations of to_xfer_partial update *XFERED_LEN
     properly.  Do assertion after printing debug messages, so that we
     can find more clues on assertion failure from debugging messages.  */
  if (retval == TARGET_XFER_OK || retval == TARGET_XFER_UNAVAILABLE)
    gdb_assert (*xfered_len > 0);