Re: [BUG] NULL pointer deref with rcutorture

Eric Sesterhenn
2009-01-05T20:32:27Z

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> On Mon, Jan 05, 2009 at 09:01:45PM +0100, Eric Sesterhenn wrote:
> > hi,
> >
> > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > > On Mon, Jan 05, 2009 at 07:56:55PM +0100, Eric Sesterhenn wrote:
> > >
> > > Wow!!! Am I reading this correctly? Does the above "call" instruction
> > > -really- call one byte into itself? That is what the hex for the x86
> > > instruction -looks- like it is doing, but I cannot see what would have
> > > possessed the compiler to generate this code.
> >
> > Compiler is gcc version 4.2.4 (Ubuntu 4.2.4-1ubuntu3)
>
> I am using 4.1.3, for whatever it is worth. (Ancient, I know!)
>
> > > When I compile on a 32-bit x86 machine, I don't see the above "call"
> > > instruction. Other than that, the code I see looks consistent.
> > >
> > > > 9f0: eb 1d jmp a0f <rcu_stutter_wait+0x27>
> > > > 9f2: 83 3d 00 00 00 00 00 cmpl $0x0,0x0
> > > > 9f9: b8 01 00 00 00 mov $0x1,%eax
> > > > 9fe: 75 0a jne a0a <rcu_stutter_wait+0x22>
> > > > a00: b8 e8 03 00 00 mov $0x3e8,%eax
> > > > a05: e8 fc ff ff ff call a06 <rcu_stutter_wait+0x1e>
> > > > a0a: e8 fc ff ff ff call a0b <rcu_stutter_wait+0x23>
> > > > a0f: 83 3d 6c 00 00 00 00 cmpl $0x0,0x6c
> > > > ^---------- this line
> > >
> > > This looks like the first test in the "while" loop.
> > >
> > > > a16: 75 09 jne a21 <rcu_stutter_wait+0x39>
> > > > a18: 83 3d 00 00 00 00 00 cmpl $0x0,0x0
> > > > a1f: 75 09 jne a2a <rcu_stutter_wait+0x42>
> > > > a21: 83 3d 50 1a 00 00 00 cmpl $0x0,0x1a50
> > > > a28: 74 c8 je 9f2 <rcu_stutter_wait+0xa>
> > > > a2a: 5d pop %ebp
> > > > a2b: c3 ret
> > >
> > > The corresponding C code is as follows:
> > >
> > > static void
> > > rcu_stutter_wait(void)
> > > {
> > > while ((stutter_pause_test || !rcutorture_runnable) && !fullstop) {
> > > if (rcutorture_runnable)
> > > schedule_timeout_interruptible(1);
> > > else
> > > schedule_timeout_interruptible(round_jiffies_relative(HZ));
> > > }
> > > }
> > >
> > > I don't see much opportunity for a page fault here... This is the
> > > binary I get when I compile it, though not as a module:
> > >
> > > 0000085a <rcu_stutter_wait>:
> > > 85a: 55 push %ebp
> > > 85b: 89 e5 mov %esp,%ebp
> > > 85d: eb 1d jmp 87c <rcu_stutter_wait+0x22>
> > > 85f: 83 3d 00 00 00 00 00 cmpl $0x0,0x0
> > > 866: b8 01 00 00 00 mov $0x1,%eax
> > > 86b: 75 0a jne 877 <rcu_stutter_wait+0x1d>
> > > 86d: b8 e8 03 00 00 mov $0x3e8,%eax
> > > 872: e8 fc ff ff ff call 873 <rcu_stutter_wait+0x19>
> > > 877: e8 fc ff ff ff call 878 <rcu_stutter_wait+0x1e>
> > > 87c: 83 3d 14 00 00 00 00 cmpl $0x0,0x14
> > > 883: 75 09 jne 88e <rcu_stutter_wait+0x34>
> > > 885: 83 3d 00 00 00 00 00 cmpl $0x0,0x0
> > > 88c: 75 09 jne 897 <rcu_stutter_wait+0x3d>
> > > 88e: 83 3d 08 1a 00 00 00 cmpl $0x0,0x1a08
> > > 895: 74 c8 je 85f <rcu_stutter_wait+0x5>
> > > 897: 5d pop %ebp
> > > 898: c3 ret
> > >
> > > I confess, I am confused!!!
> >
> > on the other box with a different gcc version
> >
> > gcc version 4.3.2 (Ubuntu 4.3.2-1ubuntu11)
> >
> > d1902e90 is the start of rcu_stutter_wait
> >
> > [ 533.391719] d087e000 d1902e90
> > [ 533.392294] rcu-torture:--- Start of test: nreaders=2 nfakewriters=4 stat_interval=0 verbose=0 test_no_idle_hz=0 shuffle_interval=3 stutter=5 irqreader=1
> > [ 541.000139] BUG: unable to handle kernel paging request at d1902efd
> > [ 541.000423] IP: [<d1902efd>] 0xd1902efd
> > [ 541.000660] *pde = 0f08f067 *pte = 00000000
> > [ 541.000867] Oops: 0000 [#1] DEBUG_PAGEALLOC
> > [ 541.001126] last sysfs file: /sys/block/sda/size
> > [ 541.001246] Modules linked in: nfsd exportfs nfs lockd nfs_acl auth_rpcgss sunrpc ipv6 fuse unix [last unloaded: rcutorture]
> > [ 541.002235]
> > [ 541.002334] Pid: 5292, comm: rcu_torture_wri Not tainted (2.6.28 #84)
> > [ 541.002470] EIP: 0060:[<d1902efd>] EFLAGS: 00010296 CPU: 0
> > [ 541.002598] EIP is at 0xd1902efd
> > [ 541.002767] EAX: 00000000 EBX: d19073c0 ECX: 00000000 EDX: 00000000
> > [ 541.002900] ESI: 0000000a EDI: 00000000 EBP: c7b63fb8 ESP: c7b63fb8
> > [ 541.003033] DS: 007b ES: 007b FS: 0000 GS: 0000 SS: 0068
> > [ 541.003160] Process rcu_torture_wri (pid: 5292, ti=c7b63000 task=c7b09710 task.ti=c7b63000)
> > [ 541.003400] Stack:
> > [ 541.003497] c7b63fd0 d19032c1 00000000 00000000 00000000 d1903200 c7b63fe0 c013d80a
> > [ 541.004022] c013d7d0 00000000 00000000 c0103cf3 cef6ee70 00000000 00000000 00000000
> > [ 541.004022] 00000201 000004b4
> > [ 541.004022] Call Trace:
> > [ 541.004022] [<c013d80a>] ? kthread+0x3a/0x70
> > [ 541.004022] [<c013d7d0>] ? kthread+0x0/0x70
> > [ 541.004022] [<c0103cf3>] ? kernel_thread_helper+0x7/0x14
> > [ 541.004022] Code: Bad EIP value.
> > [ 541.004022] EIP: [<d1902efd>] 0xd1902efd SS:ESP 0068:c7b63fb8
> > [ 541.004022] ---[ end trace cb3b10c2bb94b4e3 ]---
> >
> >
> > 00000e90 <rcu_stutter_wait>:
> > e90: 55 push %ebp
> > e91: 89 e5 mov %esp,%ebp
> > e93: 90 nop
> > e94: 8d 74 26 00 lea 0x0(%esi,%eiz,1),%esi
> > e98: a1 98 00 00 00 mov 0x98,%eax
> > e9d: 85 c0 test %eax,%eax
> > e9f: 75 09 jne eaa <rcu_stutter_wait+0x1a>
> > ea1: a1 00 00 00 00 mov 0x0,%eax
> > ea6: 85 c0 test %eax,%eax
> > ea8: 75 36 jne ee0 <rcu_stutter_wait+0x50>
> > eaa: a1 88 1a 00 00 mov 0x1a88,%eax
> > eaf: 85 c0 test %eax,%eax
> > eb1: 75 2d jne ee0 <rcu_stutter_wait+0x50>
> > eb3: 8b 15 00 00 00 00 mov 0x0,%edx
> > eb9: 85 d2 test %edx,%edx
> > ebb: 74 2b je ee8 <rcu_stutter_wait+0x58>
> > ebd: b8 01 00 00 00 mov $0x1,%eax
> > ec2: e8 fc ff ff ff call ec3 <rcu_stutter_wait+0x33>
> > ec7: a1 98 00 00 00 mov 0x98,%eax
> > ecc: 85 c0 test %eax,%eax
> > ece: 74 d1 je ea1 <rcu_stutter_wait+0x11>
> > ed0: a1 88 1a 00 00 mov 0x1a88,%eax
> > ed5: 85 c0 test %eax,%eax
> > ed7: 74 da je eb3 <rcu_stutter_wait+0x23>
> > ed9: 8d b4 26 00 00 00 00 lea 0x0(%esi,%eiz,1),%esi
> > ee0: 5d pop %ebp
> > ee1: c3 ret
> > ee2: 8d b6 00 00 00 00 lea 0x0(%esi),%esi
> > ee8: b8 fa 00 00 00 mov $0xfa,%eax
> > eed: e8 fc ff ff ff call eee <rcu_stutter_wait+0x5e>
>
> Here we are again calling one byte into the current instruction!!!
>
> Or am I misinterpreting this code?
>
> > ef2: 8d b6 00 00 00 00 lea 0x0(%esi),%esi
> > ef8: e8 fc ff ff ff call ef9 <rcu_stutter_wait+0x69>
> > efd: 8d 76 00 lea 0x0(%esi),%esi
> > ^------------- here
> >
> > This one looks more like it can explain a page fault
>
> I don't understand why there are indirections in the assembly given the
> C code for rcu_stutter_wait().
>
> > f00: eb 96 jmp e98 <rcu_stutter_wait+0x8>
> > f02: 8d b4 26 00 00 00 00 lea 0x0(%esi,%eiz,1),%esi
> > f09: 8d bc 27 00 00 00 00 lea 0x0(%edi,%eiz,1),%edi

ok, after trying to find out if the ubuntu gccs are broken, i stumbled
upon this:
http://forum.soft32.com/linux/Strange-problem-disassembling-shared-lib-ftopict439936.html

Seems the difference is that you dont compile it as a module and the
jump is perfectly normal, it gets overwritten when the stuff is loaded
objdump -dr gives me

00000e90 <rcu_stutter_wait>:
e90: 55 push %ebp
e91: 89 e5 mov %esp,%ebp
e93: 90 nop
e94: 8d 74 26 00 lea 0x0(%esi,%eiz,1),%esi
e98: a1 98 00 00 00 mov 0x98,%eax
e99: R_386_32 .bss
e9d: 85 c0 test %eax,%eax
e9f: 75 09 jne eaa <rcu_stutter_wait+0x1a>
ea1: a1 00 00 00 00 mov 0x0,%eax
ea2: R_386_32 rcutorture_runnable
ea6: 85 c0 test %eax,%eax
ea8: 75 36 jne ee0 <rcu_stutter_wait+0x50>
eaa: a1 88 1a 00 00 mov 0x1a88,%eax
eab: R_386_32 .bss
eaf: 85 c0 test %eax,%eax
eb1: 75 2d jne ee0 <rcu_stutter_wait+0x50>
eb3: 8b 15 00 00 00 00 mov 0x0,%edx
eb5: R_386_32 rcutorture_runnable
eb9: 85 d2 test %edx,%edx
ebb: 74 2b je ee8 <rcu_stutter_wait+0x58>
ebd: b8 01 00 00 00 mov $0x1,%eax
ec2: e8 fc ff ff ff call ec3 <rcu_stutter_wait+0x33>
ec3: R_386_PC32 schedule_timeout_interruptible
ec7: a1 98 00 00 00 mov 0x98,%eax
ec8: R_386_32 .bss
ecc: 85 c0 test %eax,%eax
ece: 74 d1 je ea1 <rcu_stutter_wait+0x11>
ed0: a1 88 1a 00 00 mov 0x1a88,%eax
ed1: R_386_32 .bss
ed5: 85 c0 test %eax,%eax
ed7: 74 da je eb3 <rcu_stutter_wait+0x23>
ed9: 8d b4 26 00 00 00 00 lea 0x0(%esi,%eiz,1),%esi
ee0: 5d pop %ebp
ee1: c3 ret
ee2: 8d b6 00 00 00 00 lea 0x0(%esi),%esi
ee8: b8 fa 00 00 00 mov $0xfa,%eax
eed: e8 fc ff ff ff call eee <rcu_stutter_wait+0x5e>
eee: R_386_PC32 round_jiffies_relative
ef2: 8d b6 00 00 00 00 lea 0x0(%esi),%esi
ef8: e8 fc ff ff ff call ef9 <rcu_stutter_wait+0x69>
ef9: R_386_PC32 schedule_timeout_interruptible
efd: 8d 76 00 lea 0x0(%esi),%esi

here is the deref ------------------------^

f00: eb 96 jmp e98 <rcu_stutter_wait+0x8>
f02: 8d b4 26 00 00 00 00 lea 0x0(%esi,%eiz,1),%esi
f09: 8d bc 27 00 00 00 00 lea 0x0(%edi,%eiz,1),%edi