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[Bug gdb/20851] New: Mach-specific exceptions aren't caught correctly

From: cosine at freesoft dot org
Subject: [Bug gdb/20851] New: Mach-specific exceptions aren't caught correctly
Date: Mon, 21 Nov 2016 23:02:46 +0000


            Bug ID: 20851
           Summary: Mach-specific exceptions aren't caught correctly
           Product: gdb
           Version: 7.10
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: cosine at freesoft dot org
                CC: bug-hurd at gnu dot org
  Target Milestone: ---

gdb doesn't behave correctly when a Mach/Hurd application receives a memory
access exception.  The memory faults get reported as unknown signals and the
program can't be restarted:

> Program received signal ?, Unknown signal.
> [Switching to Thread 812.8]
> 0x080eca46 in memcpy ()
> (gdb) cont
> Continuing.
> warning: Signal ? does not exist on this system.
> warning: Pid 812 died with unknown exit status, using SIGKILL.
> Program terminated with signal SIGKILL, Killed.
> The program no longer exists.
> (gdb)

Reproduce this bug by creating a ramdisk and mounting it with the /hurd/ext2fs
translator, then attach gdb to ext2fs and write data into the ramdisk until it
fills up.  ext2fs will then receive a memory access fault.

I've tracked the problem down to the file gdb/common/signals.c in the gdb
source tree.  This file includes a lot of statements like this:

>   /* Mach exceptions.  Assumes that the values for EXC_ are positive! */
> #if defined (EXC_BAD_ACCESS) && defined (_NSIG)
>   if (hostsig == _NSIG + EXC_BAD_ACCESS)
>     return GDB_EXC_BAD_ACCESS;
> #endif

The problem is that mach/exception.h isn't included at this point, so
EXC_BAD_ACCESS isn't defined and this code isn't included.

I've added #include <mach/exception.h> to signals.c, and produced a working

> Program received signal EXC_BAD_ACCESS, Could not access memory.
> [Switching to Thread 769.8]
> 0x080eca46 in memcpy ()
> (gdb) cont
> Continuing.

Obviously, tacking a Mach-specific include into signals.c isn't the right
solution, so can somebody suggest a proper fix?

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