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[patch] linux: Report the faulting FPU instruction

To: Ralf Baechle <>
Subject: [patch] linux: Report the faulting FPU instruction
From: "Maciej W. Rozycki" <>
Date: Mon, 26 Nov 2001 16:28:34 +0100 (MET)
Organization: Technical University of Gdansk

 I believe it's desireable to point to the faulting instruction upon an
FPU trap and not the following one.  Why?  First, the FPU restores the
state from before attempting to exectute the instruction.  Second, with
the current approach state is lost -- consider instructions in branch/jump
delay slots.  Third, erroneous execution is possible if SIG_FPE's handler
is set to "ignore" by mistake.

 The following patch implements the described approach.  It should not
affect standard handlers which use setjmp()/longjmp(), but it should
enable a smarter interpreting handler or just better diagnostics.  Both
the hardware and the emulator are handled.  Tested successfully with gdb
on an R3k, an R4k and the emulator. 


+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+        e-mail:, PGP key available        +

diff -up --recursive --new-file 
--- linux-mips-2.4.14-20011123.macro/arch/mips/kernel/traps.c   Wed Nov 21 
05:26:46 2001
+++ linux-mips-2.4.14-20011123/arch/mips/kernel/traps.c Sun Nov 25 13:25:06 2001
@@ -497,9 +497,6 @@ asmlinkage void do_fpe(struct pt_regs *r
-       if (compute_return_epc(regs))
-               return;
        force_sig(SIGFPE, current);
        printk(KERN_DEBUG "Sent send SIGFPE to %s\n", current->comm);
diff -up --recursive --new-file 
--- linux-mips-2.4.14-20011123.macro/arch/mips/math-emu/cp1emu.c        Sun Oct 
14 04:26:36 2001
+++ linux-mips-2.4.14-20011123/arch/mips/math-emu/cp1emu.c      Sun Nov 25 
13:29:30 2001
@@ -1721,6 +1721,9 @@ int fpu_emulator_cop1Handler(struct pt_r
                /* but if epc has advanced, then ignore it */
                sig = 0;
+       if (sig)
+               xcp->cp0_epc = prevepc;
        return sig;

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