gcc 4.6.3 miscompile on ppc32 (was Re: Regression in kernel 4.12-rc1 for Powerpc 32 - bisected to commit 3448890c32c3)
Al Viro
viro at ZenIV.linux.org.uk
Mon Jun 26 06:53:24 AEST 2017
On Sun, Jun 25, 2017 at 12:14:04PM +0100, Al Viro wrote:
> On Sun, Jun 25, 2017 at 10:53:58AM +0100, Al Viro wrote:
> > On Sat, Jun 24, 2017 at 12:29:23PM -0500, Larry Finger wrote:
> >
> > > I made a break through. If I turn off inline copy to/from users for 32-bit
> > > ppc with the following patch, then the system boots:
> >
> > OK... So it's 4.6.3 miscompiling something - it is hardware-independent,
> > reproduced in qemu. I'd like to get more self-contained example of
> > miscompile, though; should be done by tonight...
>
> OK, it's the call in rw_copy_check_uvector(); with INLINE_COPY_FROM_USER
> it's miscompiled by 4.6.3. I hadn't looked through the generated code
> yet; will do that after I grab some sleep.
Confirmed. It manages to bugger the loop immediately after the (successful)
copying of iovec array in rw_copy_check_uvector(); both with and without
INLINE_COPY_FROM_USER it has (just before the call of copy_from_user()) r27
set to nr_segs * sizeof(struct iovec). The call is made, we check that it
has succeeded and that's when it hits the fan: without INLINE_COPY_FROM_USER
we have (interleaved with unrelated insns)
addi 27,27,-8
srwi 27,27,3
addi 27,27,1
mtctr 27
Weird, but manages to pass nr_segs to mtctr. _With_ INLINE_COPY_FROM_USER we
get this:
lis 9,0x2000
mtctr 9
In other words, the loop will try to go through 8192 iterations. No idea where
that number has come from, but it sure as hell is wrong. That's where those
-EINVAL, etc. are coming from - we run into something negative in iov[seg].len,
after having run out of on-stack iovec array.
Assembler generated out of rw_copy_check_uvector() with and without
INLINE_COPY_FROM_USER is attached; it's a definite miscompile. Neither 4.4.5
nor 6.3.0 use mtctr/bdnz for that loop.
The bottom line is, ppc cross-toolchain on kernel.org happens to be
the version that miscompiles rw_copy_check_uvector() with INLINE_COPY_FROM_USER
and hell knows what else. Said that, I would rather have ppc32 drop the
INLINE_COPY_{TO,FROM}_USER anyway; that won't fix any other places where
the same 4.6.3 bug hits, but I seriously suspect that it will end up being
faster even on non^Wless buggy gcc versions. Could powerpc folks check
what does removing those two defines from arch/powerpc/include/asm/uaccess.h
do to performance? If there's no slowdown, I would strongly recommend just
removing those as in the patch Larry has posted upthread.
Fixing whatever it is in gcc 4.6.3 that triggers that behaviour is
IMO pointless - it might make sense to switch kernel.org cross-toolchain to
something more recent, but that's it.
-------------- next part --------------
.globl rw_copy_check_uvector
.type rw_copy_check_uvector, @function
rw_copy_check_uvector:
.LFB2683:
.loc 1 773 0
stwu 1,-32(1) #,,
.LCFI142:
mflr 0 #,
.LCFI143:
stmw 27,12(1) #,
.LCFI144:
.loc 1 783 0
mr. 27,5 # nr_segs, nr_segs
.loc 1 773 0
mr 30,3 # type, type
stw 0,36(1) #,
.LCFI145:
.loc 1 773 0
mr 31,4 # uvector, uvector
mr 29,8 # ret_pointer, ret_pointer
.loc 1 776 0
mr 28,7 # iov, fast_pointer
.loc 1 784 0
li 0,0 # ret,
.loc 1 783 0
beq- 0,.L495 #
.loc 1 792 0
cmplwi 7,27,1024 #, tmp160, nr_segs
.loc 1 793 0
li 0,-22 # ret,
.loc 1 792 0
bgt- 7,.L495 #
.loc 1 796 0
cmplw 7,27,6 # fast_segs, tmp161, nr_segs
ble- 7,.L496 #
.LBB1538:
.LBB1539:
.file 21 "./include/linux/slab.h"
.loc 21 495 0
lis 4,0x140 # tmp190,
slwi 3,27,3 #, nr_segs,
ori 4,4,192 #,, tmp190,
bl __kmalloc #
.LBE1539:
.LBE1538:
.loc 1 799 0
li 0,-12 # ret,
.loc 1 798 0
mr. 28,3 # iov,
beq- 0,.L495 #
.L496:
.LBB1540:
.LBB1541:
.LBB1542:
.LBB1543:
.loc 19 113 0
lwz 0,1128(2) # current.192_185->thread.fs.seg, D.39493
.LBE1543:
.LBE1542:
.LBE1541:
.LBE1540:
.loc 1 803 0
slwi 27,27,3 # n, nr_segs,
.LBB1549:
.LBB1548:
.LBB1547:
.LBB1546:
mr 5,27 # n, n
.loc 19 113 0
cmplw 7,31,0 # D.39493, tmp165, uvector
bgt- 7,.L497 #
addi 9,27,-1 # tmp166, n,
subf 0,31,0 # tmp167, uvector, D.39493
cmplw 7,9,0 # tmp167, tmp168, tmp166
bgt- 7,.L497 #
.LBB1544:
.LBB1545:
.file 22 "./arch/powerpc/include/asm/uaccess.h"
.loc 22 305 0
mr 3,28 #, iov
mr 4,31 #, uvector
bl __copy_tofrom_user #
.LBE1545:
.LBE1544:
.loc 19 115 0
mr. 5,3 # n,
beq+ 0,.L498 #
.L497:
.loc 19 116 0
subf 3,5,27 # tmp170, n, n
li 4,0 #,
add 3,28,3 #, iov, tmp170
bl memset #
b .L510 #
.L498:
.LBE1546:
.LBE1547:
.LBE1548:
.LBE1549:
.LBB1550:
.loc 1 833 0
lis 9,0x2000 #,
.loc 1 828 0
cmpwi 6,30,0 #, tmp186, type
.loc 1 833 0
lis 6,0x7fff # tmp189,
mtctr 9 # tmp188,
.loc 1 829 0
mr 5,2 # current.121, current
li 8,0 # ivtmp.533,
li 0,0 # ret,
.loc 1 833 0
ori 6,6,61440 #, tmp187, tmp189,
.L501:
.loc 1 819 0
mr 11,28 # D.40168, iov
lwzux 10,11,8 # MEM[base: iov_4, index: ivtmp.533_176, offset: 0B], buf
.loc 1 820 0
lwz 9,4(11) # MEM[base: D.40168_211, offset: 4B], len
.loc 1 824 0
cmpwi 7,9,0 #, tmp175, len
blt- 7,.L508 #
.loc 1 828 0
blt- 6,.L499 #
.loc 1 829 0
lwz 7,1128(5) # current.121_33->thread.fs.seg, D.36573
cmplw 1,10,7 # D.36573, tmp177, buf
bgt- 1,.L510 #
.loc 1 829 0 is_stmt 0 discriminator 1
beq- 7,.L499 #
.loc 1 829 0 discriminator 4
addi 4,9,-1 # tmp179, len,
subf 10,10,7 # tmp180, buf, D.36573
cmplw 7,4,10 # tmp180, tmp181, tmp179
bgt- 7,.L510 #
.L499:
.loc 1 833 0 is_stmt 1
subf 10,0,6 # len, ret, tmp187
cmpw 7,9,10 # len, tmp183, len
ble- 7,.L500 #
.loc 1 835 0
stw 10,4(11) # MEM[base: D.40168_211, offset: 4B], len
mr 9,10 # len, len
.L500:
.loc 1 837 0
add 0,0,9 # ret, ret, len
addi 8,8,8 # ivtmp.533, ivtmp.533,
.LBE1550:
.loc 1 818 0
bdnz .L501 #
b .L495 #
.L508:
.LBB1551:
.loc 1 825 0
li 0,-22 # ret,
b .L495 #
.L510:
.loc 1 830 0
li 0,-14 # ret,
.L495:
.LBE1551:
.loc 1 842 0
addi 11,1,32 #,,
.loc 1 840 0
stw 28,0(29) # *ret_pointer_53(D), iov
.loc 1 842 0
mr 3,0 #, ret
b _restgpr_27_x #
.LFE2683:
.size rw_copy_check_uvector,.-rw_copy_check_uvector
-------------- next part --------------
.globl rw_copy_check_uvector
.type rw_copy_check_uvector, @function
rw_copy_check_uvector:
.LFB2683:
.loc 1 773 0
stwu 1,-32(1) #,,
.LCFI142:
mflr 0 #,
.LCFI143:
stmw 27,12(1) #,
.LCFI144:
.loc 1 783 0
mr. 27,5 # nr_segs, nr_segs
.loc 1 773 0
mr 31,3 # type, type
stw 0,36(1) #,
.LCFI145:
.loc 1 773 0
mr 30,4 # uvector, uvector
mr 29,8 # ret_pointer, ret_pointer
.loc 1 776 0
mr 28,7 # iov, fast_pointer
.loc 1 784 0
li 0,0 # ret,
.loc 1 783 0
beq- 0,.L495 #
.loc 1 792 0
cmplwi 7,27,1024 #, tmp151, nr_segs
.loc 1 793 0
li 0,-22 # ret,
.loc 1 792 0
bgt- 7,.L495 #
.loc 1 796 0
cmplw 7,27,6 # fast_segs, tmp152, nr_segs
ble- 7,.L496 #
.LBB1516:
.LBB1517:
.file 21 "./include/linux/slab.h"
.loc 21 495 0
lis 4,0x140 # tmp175,
slwi 3,27,3 #, nr_segs,
ori 4,4,192 #,, tmp175,
bl __kmalloc #
.LBE1517:
.LBE1516:
.loc 1 799 0
li 0,-12 # ret,
.loc 1 798 0
mr. 28,3 # iov,
beq- 0,.L495 #
.L496:
.loc 1 803 0
slwi 27,27,3 # n, nr_segs,
.LBB1518:
.LBB1519:
.loc 19 153 0
mr 3,28 #, iov
mr 4,30 #, uvector
mr 5,27 #, n
bl _copy_from_user #
.LBE1519:
.LBE1518:
.loc 1 804 0
li 0,-14 # ret,
.loc 1 803 0
cmpwi 7,3,0 #, tmp156,
bne- 7,.L495 #
.LBB1520:
.loc 1 833 0
addi 27,27,-8 # tmp172, n,
.loc 1 828 0
cmpwi 6,31,0 #, tmp168, type
.loc 1 833 0
srwi 27,27,3 # tmp173, tmp172,
lis 6,0x7fff # tmp174,
addi 27,27,1 #, tmp173,
.loc 1 829 0
mr 5,2 # current.121, current
.loc 1 833 0
mtctr 27 # tmp170,
.loc 1 829 0
li 8,0 # ivtmp.528,
li 0,0 # ret,
.loc 1 833 0
ori 6,6,61440 #, tmp169, tmp174,
.L499:
.loc 1 819 0
mr 11,28 # D.40034, iov
lwzux 10,11,8 # MEM[base: iov_4, index: ivtmp.528_176, offset: 0B], buf
.loc 1 820 0
lwz 9,4(11) # MEM[base: D.40034_183, offset: 4B], len
.loc 1 824 0
cmpwi 7,9,0 #, tmp157, len
blt- 7,.L505 #
.loc 1 828 0
blt- 6,.L497 #
.loc 1 829 0
lwz 7,1128(5) # current.121_33->thread.fs.seg, D.36573
cmplw 1,10,7 # D.36573, tmp159, buf
bgt- 1,.L507 #
.loc 1 829 0 is_stmt 0 discriminator 1
beq- 7,.L497 #
.loc 1 829 0 discriminator 4
addi 4,9,-1 # tmp161, len,
subf 10,10,7 # tmp162, buf, D.36573
cmplw 7,4,10 # tmp162, tmp163, tmp161
bgt- 7,.L507 #
.L497:
.loc 1 833 0 is_stmt 1
subf 10,0,6 # len, ret, tmp169
cmpw 7,9,10 # len, tmp165, len
ble- 7,.L498 #
.loc 1 835 0
stw 10,4(11) # MEM[base: D.40034_183, offset: 4B], len
mr 9,10 # len, len
.L498:
.loc 1 837 0
add 0,0,9 # ret, ret, len
addi 8,8,8 # ivtmp.528, ivtmp.528,
.LBE1520:
.loc 1 818 0
bdnz .L499 #
b .L495 #
.L505:
.LBB1521:
.loc 1 825 0
li 0,-22 # ret,
b .L495 #
.L507:
.loc 1 830 0
li 0,-14 # ret,
.L495:
.LBE1521:
.loc 1 842 0
addi 11,1,32 #,,
.loc 1 840 0
stw 28,0(29) # *ret_pointer_53(D), iov
.loc 1 842 0
mr 3,0 #, ret
b _restgpr_27_x #
.LFE2683:
.size rw_copy_check_uvector,.-rw_copy_check_uvector
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