Re: PPC bn_div_words routine rewrite

[David Ho <davidkwho@gmail.com>]

Let's take first call to BN_div_word for example from BN_bn2dec, the
parameter being passed to BN_div_word is (a=3D35, w=3D1000000000) (decimal
numbers).  It then calls the bn_div_words with (h=3D0, l=3D35,
d=3D1000000000)  if you examine the code in linux_ppc32.s it will exit
early on because h is 0.  the routine returns a divide by 0, which is
undefined according to the manual.  In the case of ppc8xx the result
is 0x80000000.  So this is the return value from bn_div_words, as seen
in register R3.

So what happens next is BN_div_word modifies "a" (1st parameter) with
the result (0x80000000) and returns 23 as the remainder of the
division. So "a" is never zero as a result and hence the test for
BN_is_zero is always false.  The problem fails the very first time it
uses bn_div_words.

The next thing I did naturally was to fix the case when you have h=3D0,
which you can quite easy do it with the native divwu instruction.  Lo
and behold I was once again disappointed when h is not equal to 0.

More to come...


On 7/5/05, David Ho <davidkwho@gmail.com> wrote:
> I can tell you with certainty, with reference to the function
> BN_bn2dec, that since lp is a pointer, and within the while loop
> around bn_print.c:136 lp is being incremented.  Because the test
> BN_is_zero(t) is always false, you have a pointer that is going off
> into the stratosphere, hence the segfault on ppc8xx.
>=20
> More analysis to come.
>=20
> On 7/5/05, David Ho <davidkwho@gmail.com> wrote:
> > First pass debugging results from gdb on ppc8xx.  Executing ssh-keygen
> > with following arguments.
> >
> > (gdb) show args
> > Argument list to give program being debugged when it is started is
> >     "-t rsa1 -f /etc/ssh/ssh_host_key -N """.
> >
> > Program received signal SIGSEGV, Segmentation fault.
> > BN_bn2dec (a=3D0x1002d9f0) at bn_print.c:136
> > 136                             *lp=3DBN_div_word(t,BN_DEC_CONV);
> >
> > (gdb) i r
> > r0             0x0      0
> > r1             0x7fffd580       2147472768
> > r2             0x30012868       805382248
> > r3             0x80000000       2147483648
> > r4             0xfef33fc        267334652
> > r5             0x25     37
> > r6             0xfccdef8        265084664
> > r7             0x7fffd4c0       2147472576
> > r8             0xfbad2887       4222429319
> > r9             0x84044022       2214871074
> > r10            0x0      0
> > r11            0x2      2
> > r12            0xfef2054        267329620
> > r13            0x10030bc8       268635080
> > r14            0x0      0
> > r15            0x0      0
> > r16            0x0      0
> > r17            0x0      0
> > r18            0x0      0
> > r19            0x0      0
> > r20            0x0      0
> > r21            0x0      0
> > r22            0x0      0
> > r23            0x64     100
> > r24            0x5      5
> > r25            0x1002d438       268620856
> > r26            0x1002d9f0       268622320
> > r27            0x1002c578       268617080
> > r28            0x1      1
> > r29            0x10031000       268636160
> > r30            0xffbf7d0        268171216
> > r31            0x1002d9f0       268622320
> > pc             0xfef2058        267329624
> > ps             0xd032   53298
> > cr             0x24044022       604258338
> > lr             0xfef2054        267329620
> > ctr            0xfccefa0        265088928
> > xer            0x20000000       536870912
> > fpscr          0x0      0
> > vscr           0x0      0
> > vrsave         0x0      0
> >
> > (gdb) p/x $pc
> > $1 =3D 0xfef2058
> >
> > 0x0fef2058 <BN_bn2dec+472>:     stw     r3,0(r29)
> >
> > (gdb) x 0x10031000
> > 0x10031000:     Cannot access memory at address 0x10031000
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > On 7/5/05, David Ho <davidkwho@gmail.com> wrote:
> > > This is the second confirmed report of the same problem on the ppc8xx=
.
> > >
> > > After reading my email.  I must say I was the unfriendly one, I
> > > apologize for that.
> > >
> > > More debugging evidence to come.
> > >
> > > ---------- Forwarded message ----------
> > > From: Murch, Christopher <cmurch@mrv.com>
> > > Date: Jul 1, 2005 9:46 AM
> > > Subject: RE: PPC bn_div_words routine rewrite
> > > To: David Ho <davidkwho@gmail.com>
> > >
> > >
> > > David,
> > > I had observed the same issue on ppc 8xx machines after upgrading to =
the asm
> > > version of the BN routines.  Thank you very much for your work for th=
e fix.
> > > My question is, do you have high confidence in the other new asm ppc =
BN
> > > routines after observing this issue or do you think they might have s=
imiliar
> > > problems?
> > > Thanks.
> > > Chris
> > >
> > > -----Original Message-----
> > > From: David Ho [mailto:davidkwho@gmail.com]
> > > Sent: Thursday, June 30, 2005 6:22 PM
> > > To: openssl-dev@openssl.org; linuxppc-embedded@ozlabs.org
> > > Subject: Re: PPC bn_div_words routine rewrite
> > >
> > >
> > > The reason I had to redo this routine, in case anyone is wondering, i=
s
> > > because ssh-keygen  segfaults when this assembly routine returns junk
> > > to the BN_div_word function. On a ppc, if you issue the command
> > >
> > > ssh-keygen -t rsa1 -f /etc/ssh/ssh_host_key -N ""
> > >
> > > The program craps out when it tries to write the public key in ascii
> > > decimal.
> > >
> > > Regards,
> > > David
> > >
> > > On 6/30/05, David Ho <davidkwho@gmail.com> wrote:
> > > > Hi all,
> > > >
> > > > This is a rewrite of the bn_div_words routine for the PowerPC arch,
> > > > tested on a MPC8xx processor.
> > > > I initially thought there is maybe a small mistake in the code that
> > > > requires a one-liner change but it turns out I have to redo the
> > > > routine.
> > > > I guess this routine is not called very often as I see that most ot=
her
> > > > routines are hand-crafted, whereas this routine is compiled from a =
C
> > > > function that apparently has not gone through a whole lot of testin=
g.
> > > >
> > > > I wrote a C function to confirm correctness of the code.
> > > >
> > > > unsigned long div_words (unsigned long h,
> > > >                          unsigned long l,
> > > >                          unsigned long d)
> > > > {
> > > >   unsigned long i_h; /* intermediate dividend */
> > > >   unsigned long i_q; /* quotient of i_h/d */
> > > >   unsigned long i_r; /* remainder of i_h/d */
> > > >
> > > >   unsigned long i_cntr;
> > > >   unsigned long i_carry;
> > > >
> > > >   unsigned long ret_q; /* return quotient */
> > > >
> > > >   /* cannot divide by zero */
> > > >   if (d =3D=3D 0) return 0xffffffff;
> > > >
> > > >   /* do simple 32-bit divide */
> > > >   if (h =3D=3D 0) return l/d;
> > > >
> > > >   i_q =3D h/d;
> > > >   i_r =3D h - (i_q*d);
> > > >   ret_q =3D i_q;
> > > >
> > > >   i_cntr =3D 32;
> > > >
> > > >   while (i_cntr--)
> > > >   {
> > > >     i_carry =3D (l & 0x80000000) ? 1:0;
> > > >     l =3D l << 1;
> > > >
> > > >     i_h =3D (i_r << 1) | i_carry;
> > > >     i_q =3D i_h/d;
> > > >     i_r =3D i_h - (i_q*d);
> > > >
> > > >     ret_q =3D (ret_q << 1) | i_q;
> > > >   }
> > > >
> > > >   return ret_q;
> > > > }
> > > >
> > > >
> > > > Then I handcrafted the routine in PPC assembly.
> > > > The result is a 26 line assembly that is easy to understand and
> > > > predictable as opposed to a 81liner that I am still trying to
> > > > decipher...
> > > > If anyone is interested in incorporating this routine to the openss=
l
> > > > code I'll be happy to assist.
> > > > At this point I think I will be taking a bit of a break from this 3
> > > > day debugging/fixing marathon.
> > > >
> > > > Regards,
> > > > David Ho
> > > >
> > > >
> > > > #
> > > > #       Handcrafted version of bn_div_words
> > > > #
> > > > #       r3 =3D h
> > > > #       r4 =3D l
> > > > #       r5 =3D d
> > > >
> > > >         cmplwi  0,r5,0                  # compare r5 and 0
> > > >         bc      BO_IF_NOT,CR0_EQ,.Lppcasm_div1  # proceed if d!=3D0
> > > >         li      r3,-1                   # d=3D0 return -1
> > > >         bclr    BO_ALWAYS,CR0_LT
> > > > .Lppcasm_div1:
> > > >         cmplwi  0,r3,0                  # compare r3 and 0
> > > >         bc      BO_IF_NOT,CR0_EQ,.Lppcasm_div2  # proceed if h !=3D=
 0
> > > >         divwu   r3,r4,r5                # ret_q =3D l/d
> > > >         bclr    BO_ALWAYS,CR0_LT        # return result in r3
> > > > .Lppcasm_div2:
> > > >         divwu   r9,r3,r5                # i_q =3D h/d
> > > >         mullw   r10,r9,r5               # i_r =3D h - (i_q*d)
> > > >         subf    r10,r10,r3
> > > >         mr      r3,r9                   # req_q =3D i_q
> > > > .Lppcasm_set_ctr:
> > > >         li      r12,32                  # ctr =3D bitsizeof(d)
> > > >         mtctr   r12
> > > > .Lppcasm_div_loop:
> > > >         addc    r4,r4,r4                # l =3D l << 1 -> i_carry
> > > >         adde    r11,r10,r10             # i_h =3D (i_r << 1) | i_ca=
rry
> > > >         divwu   r9,r11,r5               # i_q =3D i_h/d
> > > >         mullw   r10,r9,r5               # i_r =3D i_h - (i_q*d)
> > > >         subf    r10,r10,r11
> > > >         add     r3,r3,r3                # ret_q =3D ret_q << 1 | i_=
q
> > > >         add     r3,r3,r9
> > > >         bc      BO_dCTR_NZERO,CR0_EQ,.Lppcasm_div_loop
> > > > .Lppc_div_end:
> > > >         bclr    BO_ALWAYS,CR0_LT        # return result in r3
> > > >         .long   0x00000000
> > > >
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