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a REALLY, REALLY nasty bug

To: linux-mips@oss.sgi.com
Subject: a REALLY, REALLY nasty bug
From: Jun Sun <jsun@mvista.com>
Date: Wed, 25 Oct 2000 20:03:20 -0700
Sender: owner-linux-mips@oss.sgi.com
Nasty degree - 3 days of tracking.

The symptom was pthread cannot be created.  In the end the caller will
get a BUS error.

What exactly happened has to do with how registers are saved.  Below
attached is the beginning part of sys_sigsuspend() function.  It is easy
to see that s0 is saved into stack frame AFTER its modified.  Next time
when process returns to userland, the s0 reg will be wrong!

So the bug is either 

1) that we need to save s0 register in SAVE_SOME and not save it in
save_static; or that

2) we fix compiler so that it does not use s0 register in that case (it
does the same thing for sys_rt_sigsuspend)

I am sure Ralf will have something to say about it.  :-)  In any case, I
attached a patch for 1) fix.

Jun

------------



sys_sigsuspend(struct pt_regs regs)
{
    8008e280:   27bdffc0        addiu   $sp,$sp,-64
    8008e284:   afb00030        sw      $s0,48($sp)
        sigset_t *uset, saveset, newset;

        save_static(&regs);
    8008e288:   27b00040        addiu   $s0,$sp,64
    8008e28c:   afbf003c        sw      $ra,60($sp)
    8008e290:   afb20038        sw      $s2,56($sp)
    8008e294:   afb10034        sw      $s1,52($sp)
    8008e298:   afa40040        sw      $a0,64($sp)
    8008e29c:   afa50044        sw      $a1,68($sp)
    8008e2a0:   afa60048        sw      $a2,72($sp)
    8008e2a4:   afa7004c        sw      $a3,76($sp)
    8008e2a8:   ae100058        sw      $s0,88($s0)
    8008e2ac:   ae11005c        sw      $s1,92($s0)
    .....
diff -Nru linux/include/asm-mips/stackframe.h.orig 
linux/include/asm-mips/stackframe.h
--- linux/include/asm-mips/stackframe.h.orig    Wed Oct 25 19:45:50 2000
+++ linux/include/asm-mips/stackframe.h Wed Oct 25 19:47:54 2000
@@ -49,7 +49,6 @@
 
 #define save_static(frame)                               \
        __asm__ __volatile__(                            \
-               "sw\t$16,"__str(PT_R16)"(%0)\n\t"        \
                "sw\t$17,"__str(PT_R17)"(%0)\n\t"        \
                "sw\t$18,"__str(PT_R18)"(%0)\n\t"        \
                "sw\t$19,"__str(PT_R19)"(%0)\n\t"        \
@@ -90,6 +89,7 @@
                mfc0    v1, CP0_EPC;                     \
                sw      $7, PT_R7(sp);                   \
                sw      v1, PT_EPC(sp);                  \
+               sw      $16, PT_R16(sp);                 \
                sw      $25, PT_R25(sp);                 \
                sw      $28, PT_R28(sp);                 \
                sw      $31, PT_R31(sp);                 \

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