This is a warn that I added to fail more gracefully (sorry for
whitespace damage):
diff --git a/drivers/nvdimm/dimm_devs.c b/drivers/nvdimm/dimm_devs.c
index 4890310df874..1161b994b1ec 100644
--- a/drivers/nvdimm/dimm_devs.c
+++ b/drivers/nvdimm/dimm_devs.c
@@ -516,6 +516,8 @@ static umode_t nvdimm_visible(struct kobject
*kobj, struct attribute *a, int n)
return a->mode;
if (nvdimm->sec.state < 0)
return 0;
+ if (WARN_ON_ONCE(!nvdimm->sec.ops))
+ return 0;
/* Are there any state mutation ops? */
if (nvdimm->sec.ops->freeze || nvdimm->sec.ops->disable
|| nvdimm->sec.ops->change_key
Without it, I would crash at boot due to the sec.ops dereference. It's
not clear to me if there is a better solution than just the sec.ops
NULL test (i.e. should it ever be NULL?)
[ 1.393599] WARNING: CPU: 3 PID: 484 at
drivers/nvdimm/dimm_devs.c:519 nvdimm_visible+0x79/0x80
[ 1.393858] Modules linked in:
[ 1.393858] CPU: 3 PID: 484 Comm: kworker/u8:3 Not tainted 5.0.0-rc1+ #926
[ 1.393858] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS 1.10.2-1ubuntu1 04/01/2014
[ 1.396781] Workqueue: events_unbound async_run_entry_fn
[ 1.396781] RIP: 0010:nvdimm_visible+0x79/0x80
[ 1.396781] Code: e8 4c fc ff ff eb c7 48 83 78 20 00 75 e6 48 83
78 10 00 75 df 48 83 78 28 00 75 d8 48 83 78 30 00 75 d1 b8 24 01 00
00 eb b1 <0f> 0b eb ad 0f 1f 00 0f 1f 44 00 00 55 48 89 e5 41 57 41 56
41 55
[ 1.396781] RSP: 0000:ffffb911803abd00 EFLAGS: 00010246
[ 1.396781] RAX: 0000000000000000 RBX: ffffffff98cf5a80 RCX: 00000000000001a4
[ 1.396781] RDX: 0000000000000004 RSI: ffffffff98cf5a80 RDI: ffff94e7ed088028
[ 1.396781] RBP: ffffb911803abd10 R08: 0000000000000000 R09: 0000000000000001
[ 1.396781] R10: ffffb911803abaf8 R11: 0000000000000000 R12: ffff94e7ed088028
[ 1.396781] R13: ffff94e7ed088028 R14: ffffffff98cf5a60 R15: 0000000000000000
[ 1.396781] FS: 0000000000000000(0000) GS:ffff94e7efb80000(0000)
knlGS:0000000000000000
[ 1.396781] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1.396781] CR2: 00000000ffffffff CR3: 0000000150822001 CR4: 00000000001606e0
[ 1.396781] Call Trace:
[ 1.396781] internal_create_group+0xf4/0x380
[ 1.396781] sysfs_create_groups+0x46/0xb0
[ 1.396781] device_add+0x331/0x680
[ 1.396781] nd_async_device_register+0x15/0x60
[ 1.396781] async_run_entry_fn+0x38/0x100
[ 1.396781] process_one_work+0x22b/0x5a0
[ 1.396781] worker_thread+0x3f/0x3b0
[ 1.396781] kthread+0x12b/0x150
[ 1.396781] ? process_one_work+0x5a0/0x5a0
[ 1.396781] ? kthread_park+0xa0/0xa0
[ 1.396781] ret_from_fork+0x24/0x30
[ 1.396781] irq event stamp: 952
[ 1.396781] hardirqs last enabled at (951): [<ffffffff973f5cb4>]
__slab_alloc.constprop.79+0x44/0x70
[ 1.396781] hardirqs last disabled at (952): [<ffffffff97201cf0>]
trace_hardirqs_off_thunk+0x1a/0x1c
[ 1.396781] softirqs last enabled at (0): [<ffffffff97267ae3>]
copy_process.part.55+0x413/0x1f10
[ 1.396781] softirqs last disabled at (0): [<0000000000000000>]
(null)
[ 1.396781] ---[ end trace 5608ce056f09564f ]---
I assume this crash is due to be using nvdimm without any special
markings (i.e. I'm using it crudely with pstore), in KVM:
RAM_SIZE=16384
NVDIMM_SIZE=128
MAX_SIZE=$(( RAM_SIZE + NVDIMM_SIZE ))
sudo qemu-system-x86_64 \
...
-machine pc,nvdimm \
-m ${RAM_SIZE}M,slots=2,maxmem=${MAX_SIZE}M \
-object
memory-backend-file,id=mem1,share=on,mem-path=nvdimm.img,size=${NVDIMM_SIZE}M,align=128M
\
-device nvdimm,id=nvdimm1,memdev=mem1 \
...
-append '... ramoops.mem_size=1048576 ramoops.ecc=1
ramoops.mem_address=0x440000000 ramoops.console_size=16384
ramoops.ftrace_size=16384 ramoops.pmsg_size=16384
ramoops.record_size=32768'
I assume 37833fb7989a9 ("acpi/nfit, libnvdimm: Add freeze security
support to Intel nvdimm") was where it started, but I didn't actually
bisect.
--
Kees Cook
On Tue, Jan 8, 2019 at 3:28 PM Dan Williams <[email protected]> wrote:
> Ah, thanks for the report! The key difference is that you don't define
> a "label area", so the driver bails out early and never initializes
> the security state.
>
> This should fix it up.
>
> diff --git a/drivers/nvdimm/dimm_devs.c b/drivers/nvdimm/dimm_devs.c
> index 4890310df874..636cdb06ee17 100644
> --- a/drivers/nvdimm/dimm_devs.c
> +++ b/drivers/nvdimm/dimm_devs.c
> @@ -514,7 +514,7 @@ static umode_t nvdimm_visible(struct kobject
> *kobj, struct attribute *a, int n)
>
> if (a != &dev_attr_security.attr)
> return a->mode;
> - if (nvdimm->sec.state < 0)
> + if (!nvdimm->sec.ops || nvdimm->sec.state < 0)
> return 0;
> /* Are there any state mutation ops? */
> if (nvdimm->sec.ops->freeze || nvdimm->sec.ops->disable
Okay, cool. I wasn't sure if that test needed a deeper check. :)
Fixes: 37833fb7989a9 ("acpi/nfit, libnvdimm: Add freeze security
support to Intel nvdimm")
Tested-by: Kees Cook <[email protected]>
Thanks!
--
Kees Cook
On Tue, Jan 8, 2019 at 3:10 PM Kees Cook <[email protected]> wrote:
>
> This is a warn that I added to fail more gracefully (sorry for
> whitespace damage):
>
> diff --git a/drivers/nvdimm/dimm_devs.c b/drivers/nvdimm/dimm_devs.c
> index 4890310df874..1161b994b1ec 100644
> --- a/drivers/nvdimm/dimm_devs.c
> +++ b/drivers/nvdimm/dimm_devs.c
> @@ -516,6 +516,8 @@ static umode_t nvdimm_visible(struct kobject
> *kobj, struct attribute *a, int n)
> return a->mode;
> if (nvdimm->sec.state < 0)
> return 0;
> + if (WARN_ON_ONCE(!nvdimm->sec.ops))
> + return 0;
> /* Are there any state mutation ops? */
> if (nvdimm->sec.ops->freeze || nvdimm->sec.ops->disable
> || nvdimm->sec.ops->change_key
>
> Without it, I would crash at boot due to the sec.ops dereference. It's
> not clear to me if there is a better solution than just the sec.ops
> NULL test (i.e. should it ever be NULL?)
It will always be NULL for anything other than real nvdimms with
security support.
>
> [ 1.393599] WARNING: CPU: 3 PID: 484 at
> drivers/nvdimm/dimm_devs.c:519 nvdimm_visible+0x79/0x80
> [ 1.393858] Modules linked in:
> [ 1.393858] CPU: 3 PID: 484 Comm: kworker/u8:3 Not tainted 5.0.0-rc1+ #926
> [ 1.393858] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
> BIOS 1.10.2-1ubuntu1 04/01/2014
> [ 1.396781] Workqueue: events_unbound async_run_entry_fn
> [ 1.396781] RIP: 0010:nvdimm_visible+0x79/0x80
> [ 1.396781] Code: e8 4c fc ff ff eb c7 48 83 78 20 00 75 e6 48 83
> 78 10 00 75 df 48 83 78 28 00 75 d8 48 83 78 30 00 75 d1 b8 24 01 00
> 00 eb b1 <0f> 0b eb ad 0f 1f 00 0f 1f 44 00 00 55 48 89 e5 41 57 41 56
> 41 55
> [ 1.396781] RSP: 0000:ffffb911803abd00 EFLAGS: 00010246
> [ 1.396781] RAX: 0000000000000000 RBX: ffffffff98cf5a80 RCX: 00000000000001a4
> [ 1.396781] RDX: 0000000000000004 RSI: ffffffff98cf5a80 RDI: ffff94e7ed088028
> [ 1.396781] RBP: ffffb911803abd10 R08: 0000000000000000 R09: 0000000000000001
> [ 1.396781] R10: ffffb911803abaf8 R11: 0000000000000000 R12: ffff94e7ed088028
> [ 1.396781] R13: ffff94e7ed088028 R14: ffffffff98cf5a60 R15: 0000000000000000
> [ 1.396781] FS: 0000000000000000(0000) GS:ffff94e7efb80000(0000)
> knlGS:0000000000000000
> [ 1.396781] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 1.396781] CR2: 00000000ffffffff CR3: 0000000150822001 CR4: 00000000001606e0
> [ 1.396781] Call Trace:
> [ 1.396781] internal_create_group+0xf4/0x380
> [ 1.396781] sysfs_create_groups+0x46/0xb0
> [ 1.396781] device_add+0x331/0x680
> [ 1.396781] nd_async_device_register+0x15/0x60
> [ 1.396781] async_run_entry_fn+0x38/0x100
> [ 1.396781] process_one_work+0x22b/0x5a0
> [ 1.396781] worker_thread+0x3f/0x3b0
> [ 1.396781] kthread+0x12b/0x150
> [ 1.396781] ? process_one_work+0x5a0/0x5a0
> [ 1.396781] ? kthread_park+0xa0/0xa0
> [ 1.396781] ret_from_fork+0x24/0x30
> [ 1.396781] irq event stamp: 952
> [ 1.396781] hardirqs last enabled at (951): [<ffffffff973f5cb4>]
> __slab_alloc.constprop.79+0x44/0x70
> [ 1.396781] hardirqs last disabled at (952): [<ffffffff97201cf0>]
> trace_hardirqs_off_thunk+0x1a/0x1c
> [ 1.396781] softirqs last enabled at (0): [<ffffffff97267ae3>]
> copy_process.part.55+0x413/0x1f10
> [ 1.396781] softirqs last disabled at (0): [<0000000000000000>]
> (null)
> [ 1.396781] ---[ end trace 5608ce056f09564f ]---
>
> I assume this crash is due to be using nvdimm without any special
> markings (i.e. I'm using it crudely with pstore), in KVM:
>
> RAM_SIZE=16384
> NVDIMM_SIZE=128
> MAX_SIZE=$(( RAM_SIZE + NVDIMM_SIZE ))
>
> sudo qemu-system-x86_64 \
> ...
> -machine pc,nvdimm \
> -m ${RAM_SIZE}M,slots=2,maxmem=${MAX_SIZE}M \
> -object
> memory-backend-file,id=mem1,share=on,mem-path=nvdimm.img,size=${NVDIMM_SIZE}M,align=128M
> \
> -device nvdimm,id=nvdimm1,memdev=mem1 \
Ah, thanks for the report! The key difference is that you don't define
a "label area", so the driver bails out early and never initializes
the security state.
This should fix it up.
diff --git a/drivers/nvdimm/dimm_devs.c b/drivers/nvdimm/dimm_devs.c
index 4890310df874..636cdb06ee17 100644
--- a/drivers/nvdimm/dimm_devs.c
+++ b/drivers/nvdimm/dimm_devs.c
@@ -514,7 +514,7 @@ static umode_t nvdimm_visible(struct kobject
*kobj, struct attribute *a, int n)
if (a != &dev_attr_security.attr)
return a->mode;
- if (nvdimm->sec.state < 0)
+ if (!nvdimm->sec.ops || nvdimm->sec.state < 0)
return 0;
/* Are there any state mutation ops? */
if (nvdimm->sec.ops->freeze || nvdimm->sec.ops->disable
On Tue, Jan 8, 2019 at 3:34 PM Kees Cook <[email protected]> wrote:
>
> On Tue, Jan 8, 2019 at 3:28 PM Dan Williams <[email protected]> wrote:
> > Ah, thanks for the report! The key difference is that you don't define
> > a "label area", so the driver bails out early and never initializes
> > the security state.
> >
> > This should fix it up.
> >
> > diff --git a/drivers/nvdimm/dimm_devs.c b/drivers/nvdimm/dimm_devs.c
> > index 4890310df874..636cdb06ee17 100644
> > --- a/drivers/nvdimm/dimm_devs.c
> > +++ b/drivers/nvdimm/dimm_devs.c
> > @@ -514,7 +514,7 @@ static umode_t nvdimm_visible(struct kobject
> > *kobj, struct attribute *a, int n)
> >
> > if (a != &dev_attr_security.attr)
> > return a->mode;
> > - if (nvdimm->sec.state < 0)
> > + if (!nvdimm->sec.ops || nvdimm->sec.state < 0)
> > return 0;
> > /* Are there any state mutation ops? */
> > if (nvdimm->sec.ops->freeze || nvdimm->sec.ops->disable
>
> Okay, cool. I wasn't sure if that test needed a deeper check. :)
>
> Fixes: 37833fb7989a9 ("acpi/nfit, libnvdimm: Add freeze security
> support to Intel nvdimm")
> Tested-by: Kees Cook <[email protected]>
>
Actually, looking closer this should have been avoided by the fact
that __nvdimm_create() initializes the security state early and that
nvdimm->sec.state should have saved us.
I'll dig a bit deeper with your qemu config.
On Tue, Jan 8, 2019 at 3:55 PM Kees Cook <[email protected]> wrote:
>
> On Tue, Jan 8, 2019 at 3:54 PM Dan Williams <[email protected]> wrote:
> >
> > On Tue, Jan 8, 2019 at 3:34 PM Kees Cook <[email protected]> wrote:
> > >
> > > On Tue, Jan 8, 2019 at 3:28 PM Dan Williams <[email protected]> wrote:
> > > > Ah, thanks for the report! The key difference is that you don't define
> > > > a "label area", so the driver bails out early and never initializes
> > > > the security state.
> > > >
> > > > This should fix it up.
> > > >
> > > > diff --git a/drivers/nvdimm/dimm_devs.c b/drivers/nvdimm/dimm_devs.c
> > > > index 4890310df874..636cdb06ee17 100644
> > > > --- a/drivers/nvdimm/dimm_devs.c
> > > > +++ b/drivers/nvdimm/dimm_devs.c
> > > > @@ -514,7 +514,7 @@ static umode_t nvdimm_visible(struct kobject
> > > > *kobj, struct attribute *a, int n)
> > > >
> > > > if (a != &dev_attr_security.attr)
> > > > return a->mode;
> > > > - if (nvdimm->sec.state < 0)
> > > > + if (!nvdimm->sec.ops || nvdimm->sec.state < 0)
> > > > return 0;
> > > > /* Are there any state mutation ops? */
> > > > if (nvdimm->sec.ops->freeze || nvdimm->sec.ops->disable
> > >
> > > Okay, cool. I wasn't sure if that test needed a deeper check. :)
> > >
> > > Fixes: 37833fb7989a9 ("acpi/nfit, libnvdimm: Add freeze security
> > > support to Intel nvdimm")
> > > Tested-by: Kees Cook <[email protected]>
> > >
> >
> > Actually, looking closer this should have been avoided by the fact
> > that __nvdimm_create() initializes the security state early and that
> > nvdimm->sec.state should have saved us.
> >
> > I'll dig a bit deeper with your qemu config.
>
> Maybe something goes weird with pstore stealing the region?
No, pstore is off the hook. I was just able to reproduce locally and
I'm not doing anything with pstore.
On Tue, Jan 8, 2019 at 3:54 PM Dan Williams <[email protected]> wrote:
>
> On Tue, Jan 8, 2019 at 3:34 PM Kees Cook <[email protected]> wrote:
> >
> > On Tue, Jan 8, 2019 at 3:28 PM Dan Williams <[email protected]> wrote:
> > > Ah, thanks for the report! The key difference is that you don't define
> > > a "label area", so the driver bails out early and never initializes
> > > the security state.
> > >
> > > This should fix it up.
> > >
> > > diff --git a/drivers/nvdimm/dimm_devs.c b/drivers/nvdimm/dimm_devs.c
> > > index 4890310df874..636cdb06ee17 100644
> > > --- a/drivers/nvdimm/dimm_devs.c
> > > +++ b/drivers/nvdimm/dimm_devs.c
> > > @@ -514,7 +514,7 @@ static umode_t nvdimm_visible(struct kobject
> > > *kobj, struct attribute *a, int n)
> > >
> > > if (a != &dev_attr_security.attr)
> > > return a->mode;
> > > - if (nvdimm->sec.state < 0)
> > > + if (!nvdimm->sec.ops || nvdimm->sec.state < 0)
> > > return 0;
> > > /* Are there any state mutation ops? */
> > > if (nvdimm->sec.ops->freeze || nvdimm->sec.ops->disable
> >
> > Okay, cool. I wasn't sure if that test needed a deeper check. :)
> >
> > Fixes: 37833fb7989a9 ("acpi/nfit, libnvdimm: Add freeze security
> > support to Intel nvdimm")
> > Tested-by: Kees Cook <[email protected]>
> >
>
> Actually, looking closer this should have been avoided by the fact
> that __nvdimm_create() initializes the security state early and that
> nvdimm->sec.state should have saved us.
>
> I'll dig a bit deeper with your qemu config.
Maybe something goes weird with pstore stealing the region?
--
Kees Cook
On Tue, Jan 8, 2019 at 4:02 PM Dan Williams <[email protected]> wrote:
>
> On Tue, Jan 8, 2019 at 3:55 PM Kees Cook <[email protected]> wrote:
> >
> > On Tue, Jan 8, 2019 at 3:54 PM Dan Williams <[email protected]> wrote:
> > >
> > > On Tue, Jan 8, 2019 at 3:34 PM Kees Cook <[email protected]> wrote:
> > > >
> > > > On Tue, Jan 8, 2019 at 3:28 PM Dan Williams <[email protected]> wrote:
> > > > > Ah, thanks for the report! The key difference is that you don't define
> > > > > a "label area", so the driver bails out early and never initializes
> > > > > the security state.
> > > > >
> > > > > This should fix it up.
> > > > >
> > > > > diff --git a/drivers/nvdimm/dimm_devs.c b/drivers/nvdimm/dimm_devs.c
> > > > > index 4890310df874..636cdb06ee17 100644
> > > > > --- a/drivers/nvdimm/dimm_devs.c
> > > > > +++ b/drivers/nvdimm/dimm_devs.c
> > > > > @@ -514,7 +514,7 @@ static umode_t nvdimm_visible(struct kobject
> > > > > *kobj, struct attribute *a, int n)
> > > > >
> > > > > if (a != &dev_attr_security.attr)
> > > > > return a->mode;
> > > > > - if (nvdimm->sec.state < 0)
> > > > > + if (!nvdimm->sec.ops || nvdimm->sec.state < 0)
> > > > > return 0;
> > > > > /* Are there any state mutation ops? */
> > > > > if (nvdimm->sec.ops->freeze || nvdimm->sec.ops->disable
> > > >
> > > > Okay, cool. I wasn't sure if that test needed a deeper check. :)
> > > >
> > > > Fixes: 37833fb7989a9 ("acpi/nfit, libnvdimm: Add freeze security
> > > > support to Intel nvdimm")
> > > > Tested-by: Kees Cook <[email protected]>
> > > >
> > >
> > > Actually, looking closer this should have been avoided by the fact
> > > that __nvdimm_create() initializes the security state early and that
> > > nvdimm->sec.state should have saved us.
> > >
> > > I'll dig a bit deeper with your qemu config.
> >
> > Maybe something goes weird with pstore stealing the region?
>
> No, pstore is off the hook. I was just able to reproduce locally and
> I'm not doing anything with pstore.
Huh, this fixes it:
diff --git a/include/linux/libnvdimm.h b/include/linux/libnvdimm.h
index 5440f11b0907..7315977b64da 100644
--- a/include/linux/libnvdimm.h
+++ b/include/linux/libnvdimm.h
@@ -160,6 +160,7 @@ static inline struct nd_blk_region_desc *to_blk_region_desc(
}
enum nvdimm_security_state {
+ NVDIMM_SECURITY_ERROR = -1,
NVDIMM_SECURITY_DISABLED,
NVDIMM_SECURITY_UNLOCKED,
NVDIMM_SECURITY_LOCKED,
Apparently I was wrong to think an enum was a signed int without
actually making a signed value a possibility. I would have a expected
the compiler to give me a "statement has no effect" for testing for a
negative value against an effectively unsigned quantity.
On Tue, Jan 8, 2019 at 4:49 PM Dan Williams <[email protected]> wrote:
>
> On Tue, Jan 8, 2019 at 4:02 PM Dan Williams <[email protected]> wrote:
> >
> > On Tue, Jan 8, 2019 at 3:55 PM Kees Cook <[email protected]> wrote:
> > >
> > > On Tue, Jan 8, 2019 at 3:54 PM Dan Williams <[email protected]> wrote:
> > > >
> > > > On Tue, Jan 8, 2019 at 3:34 PM Kees Cook <[email protected]> wrote:
> > > > >
> > > > > On Tue, Jan 8, 2019 at 3:28 PM Dan Williams <[email protected]> wrote:
> > > > > > Ah, thanks for the report! The key difference is that you don't define
> > > > > > a "label area", so the driver bails out early and never initializes
> > > > > > the security state.
> > > > > >
> > > > > > This should fix it up.
> > > > > >
> > > > > > diff --git a/drivers/nvdimm/dimm_devs.c b/drivers/nvdimm/dimm_devs.c
> > > > > > index 4890310df874..636cdb06ee17 100644
> > > > > > --- a/drivers/nvdimm/dimm_devs.c
> > > > > > +++ b/drivers/nvdimm/dimm_devs.c
> > > > > > @@ -514,7 +514,7 @@ static umode_t nvdimm_visible(struct kobject
> > > > > > *kobj, struct attribute *a, int n)
> > > > > >
> > > > > > if (a != &dev_attr_security.attr)
> > > > > > return a->mode;
> > > > > > - if (nvdimm->sec.state < 0)
> > > > > > + if (!nvdimm->sec.ops || nvdimm->sec.state < 0)
> > > > > > return 0;
> > > > > > /* Are there any state mutation ops? */
> > > > > > if (nvdimm->sec.ops->freeze || nvdimm->sec.ops->disable
> > > > >
> > > > > Okay, cool. I wasn't sure if that test needed a deeper check. :)
> > > > >
> > > > > Fixes: 37833fb7989a9 ("acpi/nfit, libnvdimm: Add freeze security
> > > > > support to Intel nvdimm")
> > > > > Tested-by: Kees Cook <[email protected]>
> > > > >
> > > >
> > > > Actually, looking closer this should have been avoided by the fact
> > > > that __nvdimm_create() initializes the security state early and that
> > > > nvdimm->sec.state should have saved us.
> > > >
> > > > I'll dig a bit deeper with your qemu config.
> > >
> > > Maybe something goes weird with pstore stealing the region?
> >
> > No, pstore is off the hook. I was just able to reproduce locally and
> > I'm not doing anything with pstore.
>
> Huh, this fixes it:
>
> diff --git a/include/linux/libnvdimm.h b/include/linux/libnvdimm.h
> index 5440f11b0907..7315977b64da 100644
> --- a/include/linux/libnvdimm.h
> +++ b/include/linux/libnvdimm.h
> @@ -160,6 +160,7 @@ static inline struct nd_blk_region_desc *to_blk_region_desc(
> }
>
> enum nvdimm_security_state {
> + NVDIMM_SECURITY_ERROR = -1,
> NVDIMM_SECURITY_DISABLED,
> NVDIMM_SECURITY_UNLOCKED,
> NVDIMM_SECURITY_LOCKED,
>
> Apparently I was wrong to think an enum was a signed int without
> actually making a signed value a possibility. I would have a expected
> the compiler to give me a "statement has no effect" for testing for a
> negative value against an effectively unsigned quantity.
Thanks for the one-line patch! It fixed the same crash for me.
-- Dexuan