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Re: Sigcontext->sc_pc Passed to User

To: "Kevin D. Kissell" <kevink@mips.com>
Subject: Re: Sigcontext->sc_pc Passed to User
From: Ralf Baechle <ralf@oss.sgi.com>
Date: Fri, 12 Jul 2002 03:40:15 +0200
Cc: linux-mips@oss.sgi.com
In-reply-to: <00b401c228ba$88b29bf0$10eca8c0@grendel>; from kevink@mips.com on Thu, Jul 11, 2002 at 11:08:21AM +0200
References: <00b401c228ba$88b29bf0$10eca8c0@grendel>
Sender: owner-linux-mips@oss.sgi.com
User-agent: Mutt/1.2.5.1i
On Thu, Jul 11, 2002 at 11:08:21AM +0200, Kevin D. Kissell wrote:

> In responding to an enquiry from one of MIPS' third-party
> software vendors, I noted something that seems a little
> broken to me in the current (and maybe all historical)
> MIPS/Linux kernels.  Please forgive me for opening
> old wounds if this has been beaten to death in the past.
> 
> When a user catches a signal, such as SIGBUS, the
> signal "payload" includes a pointer to a sigcontext
> structure on the stack, containing the state of the
> CPU when the exception associated with the signal
> occurred.  But not exactly.  We seem to consistently
> call compute_return_epc() before send_sig() or
> force_sig().  This results in the user being passed
> an indication of the faulting PC that is one instruction
> past the true location.  That would be no problem,
> except that the faulting instruction may have been 
> in a branch delay slot, such that there is no practical
> and reliable way for the signal handler to determine
> which instruction failed on the basis of the sigcontext
> data.
> 
> It is, of course, important that execution resume
> at the instruction following any instruction generating
> an exception/signal.  But that's not the same thing
> as saying that the sigcontext should report the resumption
> EPC instead of the faulting EPC.  There are various
> ways of dealing with this, but before going into any
> of them, I'm curious as to whether this has been 
> discussed before, and whether anyone thinks that 
> things really should be the way they are.

Our signal stackframe is almost the same as on IRIX5 which is what
some software expects.  Maybe time to checkout what IRIX does ...

  Ralf


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