Re: SIGFPE trapping on HPPA

[dann frazier <dannf@debian.org>]

On Mon, May 03, 2010 at 10:44:21PM +0200, Helge Deller wrote:
> On 12/14/2009 11:08 PM, Thibaut Paumard wrote:
> > Le 14 déc. 09 à 16:06, Carlos O'Donell a écrit :
> > Attached is a rather small testcase.
> > 
> > Compile with
> >   gcc -lm -o fputest fputest.c
> > 
> > Run with
> >   ./fputest
> > or with any succession of 'i's and 'f's, e.g.
> >   ./fputest iffifi
> > 
> > The expected behaviour (checked on i386) is that for each f or i letter,
> > a line is output, for instance
> >      $ ./fputest iffi
> >      Triggering integer SIGFPE: 1/0=(SIGFPE trapped)0
> >      Triggering floating SIGFPE: 1./0.=(SIGFPE trapped)0.000000
> >      Triggering floating SIGFPE: 1./0.=(SIGFPE trapped)0.000000
> >      Triggering integer SIGFPE: 1/0=(SIGFPE trapped)0
> >      $ echo $?
> >      0
> > 
> > All the SIGFPEs should be trapped and the program should exit gracefully
> > (check $?).
> > 
> > Under HPPA, the integer SIGFPEs are trapped but the floating-point
> > SIGFPEs are not:
> >      $ ./fputest iffi
> >      Triggering integer SIGFPE: 1/0=(SIGFPE trapped)0
> >      Triggering floating SIGFPE: 1./0.=Floating point exception
> >      $ echo $?
> >      136
> 
> 
> The problem in this test case is, that a signal handler is called
> when SIGFPE happens. Then glibc uses a trampoline handler to call the
> signal handler, in which it touches the floating point registers again
> and finally gets another SIGFPE. In this case, the program just ends
> (since the original signal handler already trapped).
> Involved glibc function is _dl_runtime_resolve() at 
> ports/sysdeps/hppa/dl-trampoline.S:73
> 
> The solution is simple.
> Just clear the exception register in arch/parisc/math-emu/decode_exc.c
> in the linux kernel before sending the SIGFPE signal to user space.
> 
> Patch attached.
> 
> While at this, I think we need to clear the exception registers for
> the other cases like OVERFLOWEXCEPTION and INEXACTEXCEPTION at least
> as well (see arch/parisc/math-emu/decode_exc.c, line 286 ff).
> 
> Helge
> 
> Patch:
> -------------------------
> [PATCH] parisc: clear floating point exception flag on SIGFPE signal
> 
> Clear the floating point exception flag before returning to
> user space. This is needed, else the libc trampoline handler
> may hit the same SIGFPE again while building up a trampoline
> to a signal handler.
> 
> Fixes debian bug #559406.
> 
> Signed-off-by: Helge Deller <deller@gmx.de>

Thanks Helge! If you can poke me when this goes into Kyle's tree, I'll
get it added to Debian.

> diff --git a/arch/parisc/math-emu/decode_exc.c b/arch/parisc/math-emu/decode_exc.c
> index 3ca1c61..27a7492 100644
> --- a/arch/parisc/math-emu/decode_exc.c
> +++ b/arch/parisc/math-emu/decode_exc.c
> @@ -342,6 +342,7 @@ decode_fpu(unsigned int Fpu_register[], unsigned int trap_counts[])
>  		return SIGNALCODE(SIGFPE, FPE_FLTINV);
>  	  case DIVISIONBYZEROEXCEPTION:
>  		update_trap_counts(Fpu_register, aflags, bflags, trap_counts);
> +		Clear_excp_register(exception_index);
>  	  	return SIGNALCODE(SIGFPE, FPE_FLTDIV);
>  	  case INEXACTEXCEPTION:
>  		update_trap_counts(Fpu_register, aflags, bflags, trap_counts);
> 
> 

-- 
dann frazier