Bug in the _save_fp_context.

Quoted 55 lines · Carsten Langgaard

I posted the following problem on the list almost a year ago, but it still hasn't made into the SGI CVS.
I think there is a bug in the _save_fp_context function in arch/mips/kernel/r4k_fpu.S

The problem is the following piece of code:

jr ra
.set nomacro
EX(sw t0,SC_FPC_EIR(a0))
.set macro

We do not handle entries in the __ex_table which is located in a branch delay.
So we need to handle the situation where we take a page fault on an instruction which is located in a brach delay slot, or we don't put the "potential" faulting instruction in a delay slot.

This situation probably doesn't generally hit people since it hasn't been fix yet, but if you try run something nasty like Crashme, you will get hit by this problem.
I suggest the following patch.

/Carsten

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Index: arch/mips/kernel/r4k_fpu.S
===================================================================
RCS file: /cvs/linux/arch/mips/kernel/r4k_fpu.S,v
retrieving revision 1.12
diff -u -r1.12 r4k_fpu.S

mips/kernel/r4k_fpu.S 2002/01/17 14:21:09 renamed from mips/kernel/r4k_fpu.S 2001/04/11 05:19:46 1.12 +2 -3

@@ -50,11 +50,10 @@
      EX(sdc1 $f30,(SC_FPREGS+240)(a0))
      EX(sw   t1,SC_FPC_CSR(a0))
      cfc1    t0,$0                           # implementation/version
+     EX(sw   t0,SC_FPC_EIR(a0))
 
      jr      ra
-     .set    nomacro
-      EX(sw  t0,SC_FPC_EIR(a0))
-     .set    macro
+      nop
      END(_save_fp_context)
 

mips/kernel/r4k_fpu.S 2002/01/17 14:21:09 line 59

Quoted 1 line

mips/kernel/r4k_fpu.S 2002/01/17 14:21:09 renamed from mips/kernel/r4k_fpu.S 2001/04/11 05:19:46 1.12 +0 -0

 /*

Do we still need this patch for 2.4 on OSS?

H.J.