* [PATCH kcsan 0/3] KCSAN updates for 5.11
@ 2020-11-05 22:03 Paul E. McKenney
2020-11-05 22:03 ` [PATCH kcsan 1/3] kcsan: selftest: Ensure that address is at least PAGE_SIZE paulmck
` (2 more replies)
0 siblings, 3 replies; 10+ messages in thread
From: Paul E. McKenney @ 2020-11-05 22:03 UTC (permalink / raw)
To: linux-kernel, kasan-dev, kernel-team, mingo
Cc: elver, andreyknvl, glider, dvyukov, cai, boqun.feng
Hello!
This series provides KCSAN updates:
1. Ensure that selftest address is at least PAGE_SIZE.
2. Never set up watchpoints on NULL pointers.
3. Fix encoding masks and regain address bit.
Thanx, Paul
------------------------------------------------------------------------
encoding.h | 20 +++++++++++---------
selftest.c | 3 +++
2 files changed, 14 insertions(+), 9 deletions(-)
^ permalink raw reply [flat|nested] 10+ messages in thread
* [PATCH kcsan 1/3] kcsan: selftest: Ensure that address is at least PAGE_SIZE
2020-11-05 22:03 [PATCH kcsan 0/3] KCSAN updates for 5.11 Paul E. McKenney
@ 2020-11-05 22:03 ` paulmck
2020-11-05 22:03 ` [PATCH kcsan 2/3] kcsan: Never set up watchpoints on NULL pointers paulmck
2020-11-05 22:03 ` [PATCH kcsan 3/3] kcsan: Fix encoding masks and regain address bit paulmck
2 siblings, 0 replies; 10+ messages in thread
From: paulmck @ 2020-11-05 22:03 UTC (permalink / raw)
To: linux-kernel, kasan-dev, kernel-team, mingo
Cc: elver, andreyknvl, glider, dvyukov, cai, boqun.feng, Paul E . McKenney
From: Marco Elver <elver@google.com>
In preparation of supporting only addresses not within the NULL page,
change the selftest to never use addresses that are less than PAGE_SIZE.
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/kcsan/selftest.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/kernel/kcsan/selftest.c b/kernel/kcsan/selftest.c
index d98bc20..9014a3a 100644
--- a/kernel/kcsan/selftest.c
+++ b/kernel/kcsan/selftest.c
@@ -33,6 +33,9 @@ static bool test_encode_decode(void)
unsigned long addr;
prandom_bytes(&addr, sizeof(addr));
+ if (addr < PAGE_SIZE)
+ addr = PAGE_SIZE;
+
if (WARN_ON(!check_encodable(addr, size)))
return false;
--
2.9.5
^ permalink raw reply related [flat|nested] 10+ messages in thread
* [PATCH kcsan 2/3] kcsan: Never set up watchpoints on NULL pointers
2020-11-05 22:03 [PATCH kcsan 0/3] KCSAN updates for 5.11 Paul E. McKenney
2020-11-05 22:03 ` [PATCH kcsan 1/3] kcsan: selftest: Ensure that address is at least PAGE_SIZE paulmck
@ 2020-11-05 22:03 ` paulmck
2020-11-05 22:03 ` [PATCH kcsan 3/3] kcsan: Fix encoding masks and regain address bit paulmck
2 siblings, 0 replies; 10+ messages in thread
From: paulmck @ 2020-11-05 22:03 UTC (permalink / raw)
To: linux-kernel, kasan-dev, kernel-team, mingo
Cc: elver, andreyknvl, glider, dvyukov, cai, boqun.feng, Paul E . McKenney
From: Marco Elver <elver@google.com>
Avoid setting up watchpoints on NULL pointers, as otherwise we would
crash inside the KCSAN runtime (when checking for value changes) instead
of the instrumented code.
Because that may be confusing, skip any address less than PAGE_SIZE.
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/kcsan/encoding.h | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/kernel/kcsan/encoding.h b/kernel/kcsan/encoding.h
index 1a6db2f..4f73db6 100644
--- a/kernel/kcsan/encoding.h
+++ b/kernel/kcsan/encoding.h
@@ -48,7 +48,11 @@
static inline bool check_encodable(unsigned long addr, size_t size)
{
- return size <= MAX_ENCODABLE_SIZE;
+ /*
+ * While we can encode addrs<PAGE_SIZE, avoid crashing with a NULL
+ * pointer deref inside KCSAN.
+ */
+ return addr >= PAGE_SIZE && size <= MAX_ENCODABLE_SIZE;
}
static inline long
--
2.9.5
^ permalink raw reply related [flat|nested] 10+ messages in thread
* [PATCH kcsan 3/3] kcsan: Fix encoding masks and regain address bit
2020-11-05 22:03 [PATCH kcsan 0/3] KCSAN updates for 5.11 Paul E. McKenney
2020-11-05 22:03 ` [PATCH kcsan 1/3] kcsan: selftest: Ensure that address is at least PAGE_SIZE paulmck
2020-11-05 22:03 ` [PATCH kcsan 2/3] kcsan: Never set up watchpoints on NULL pointers paulmck
@ 2020-11-05 22:03 ` paulmck
2020-11-06 1:23 ` Boqun Feng
2020-11-06 9:34 ` [PATCH v2] " Marco Elver
2 siblings, 2 replies; 10+ messages in thread
From: paulmck @ 2020-11-05 22:03 UTC (permalink / raw)
To: linux-kernel, kasan-dev, kernel-team, mingo
Cc: elver, andreyknvl, glider, dvyukov, cai, boqun.feng, Paul E . McKenney
From: Marco Elver <elver@google.com>
The watchpoint encoding masks for size and address were off-by-one bit
each, with the size mask using 1 unnecessary bit and the address mask
missing 1 bit. However, due to the way the size is shifted into the
encoded watchpoint, we were effectively wasting and never using the
extra bit.
For example, on x86 with PAGE_SIZE==4K, we have 1 bit for the is-write
bit, 14 bits for the size bits, and then 49 bits left for the address.
Prior to this fix we would end up with this usage:
[ write<1> | size<14> | wasted<1> | address<48> ]
Fix it by subtracting 1 bit from the GENMASK() end and start ranges of
size and address respectively. The added static_assert()s verify that
the masks are as expected. With the fixed version, we get the expected
usage:
[ write<1> | size<14> | address<49> ]
Functionally no change is expected, since that extra address bit is
insignificant for enabled architectures.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/kcsan/encoding.h | 14 ++++++--------
1 file changed, 6 insertions(+), 8 deletions(-)
diff --git a/kernel/kcsan/encoding.h b/kernel/kcsan/encoding.h
index 4f73db6..b50bda9 100644
--- a/kernel/kcsan/encoding.h
+++ b/kernel/kcsan/encoding.h
@@ -37,14 +37,12 @@
*/
#define WATCHPOINT_ADDR_BITS (BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
-/*
- * Masks to set/retrieve the encoded data.
- */
-#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1)
-#define WATCHPOINT_SIZE_MASK \
- GENMASK(BITS_PER_LONG-2, BITS_PER_LONG-2 - WATCHPOINT_SIZE_BITS)
-#define WATCHPOINT_ADDR_MASK \
- GENMASK(BITS_PER_LONG-3 - WATCHPOINT_SIZE_BITS, 0)
+/* Bitmasks for the encoded watchpoint access information. */
+#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1)
+#define WATCHPOINT_SIZE_MASK GENMASK(BITS_PER_LONG-2, BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
+#define WATCHPOINT_ADDR_MASK GENMASK(BITS_PER_LONG-2 - WATCHPOINT_SIZE_BITS, 0)
+static_assert(WATCHPOINT_ADDR_MASK == (1UL << WATCHPOINT_ADDR_BITS) - 1);
+static_assert((WATCHPOINT_WRITE_MASK ^ WATCHPOINT_SIZE_MASK ^ WATCHPOINT_ADDR_MASK) == ~0UL);
static inline bool check_encodable(unsigned long addr, size_t size)
{
--
2.9.5
^ permalink raw reply related [flat|nested] 10+ messages in thread
* Re: [PATCH kcsan 3/3] kcsan: Fix encoding masks and regain address bit
2020-11-05 22:03 ` [PATCH kcsan 3/3] kcsan: Fix encoding masks and regain address bit paulmck
@ 2020-11-06 1:23 ` Boqun Feng
2020-11-06 9:03 ` Marco Elver
2020-11-06 9:34 ` [PATCH v2] " Marco Elver
1 sibling, 1 reply; 10+ messages in thread
From: Boqun Feng @ 2020-11-06 1:23 UTC (permalink / raw)
To: paulmck
Cc: linux-kernel, kasan-dev, kernel-team, mingo, elver, andreyknvl,
glider, dvyukov, cai
Hi Marco,
On Thu, Nov 05, 2020 at 02:03:24PM -0800, paulmck@kernel.org wrote:
> From: Marco Elver <elver@google.com>
>
> The watchpoint encoding masks for size and address were off-by-one bit
> each, with the size mask using 1 unnecessary bit and the address mask
> missing 1 bit. However, due to the way the size is shifted into the
> encoded watchpoint, we were effectively wasting and never using the
> extra bit.
>
> For example, on x86 with PAGE_SIZE==4K, we have 1 bit for the is-write
> bit, 14 bits for the size bits, and then 49 bits left for the address.
> Prior to this fix we would end up with this usage:
>
> [ write<1> | size<14> | wasted<1> | address<48> ]
>
> Fix it by subtracting 1 bit from the GENMASK() end and start ranges of
> size and address respectively. The added static_assert()s verify that
> the masks are as expected. With the fixed version, we get the expected
> usage:
>
> [ write<1> | size<14> | address<49> ]
>
> Functionally no change is expected, since that extra address bit is
> insignificant for enabled architectures.
>
> Signed-off-by: Marco Elver <elver@google.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---
> kernel/kcsan/encoding.h | 14 ++++++--------
> 1 file changed, 6 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/kcsan/encoding.h b/kernel/kcsan/encoding.h
> index 4f73db6..b50bda9 100644
> --- a/kernel/kcsan/encoding.h
> +++ b/kernel/kcsan/encoding.h
> @@ -37,14 +37,12 @@
> */
> #define WATCHPOINT_ADDR_BITS (BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
>
> -/*
> - * Masks to set/retrieve the encoded data.
> - */
> -#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1)
> -#define WATCHPOINT_SIZE_MASK \
> - GENMASK(BITS_PER_LONG-2, BITS_PER_LONG-2 - WATCHPOINT_SIZE_BITS)
> -#define WATCHPOINT_ADDR_MASK \
> - GENMASK(BITS_PER_LONG-3 - WATCHPOINT_SIZE_BITS, 0)
> +/* Bitmasks for the encoded watchpoint access information. */
> +#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1)
> +#define WATCHPOINT_SIZE_MASK GENMASK(BITS_PER_LONG-2, BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
> +#define WATCHPOINT_ADDR_MASK GENMASK(BITS_PER_LONG-2 - WATCHPOINT_SIZE_BITS, 0)
> +static_assert(WATCHPOINT_ADDR_MASK == (1UL << WATCHPOINT_ADDR_BITS) - 1);
Nit:
Since you use the static_assert(), why not define WATCHPOINT_ADDR_MASK
as:
#define WATCHPOINT_ADDR_MASK (BIT(WATCHPOINT_SIZE_BITS) - 1)
Besides, WATCHPOINT_SIZE_MASK can also be defined as:
#define WATCHPOINT_SIZE_MASK GENMASK(BITS_PER_LONG - 2, WATCHPOINT_SIZE_BITS)
Regards,
Boqun
> +static_assert((WATCHPOINT_WRITE_MASK ^ WATCHPOINT_SIZE_MASK ^ WATCHPOINT_ADDR_MASK) == ~0UL);
>
> static inline bool check_encodable(unsigned long addr, size_t size)
> {
> --
> 2.9.5
>
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH kcsan 3/3] kcsan: Fix encoding masks and regain address bit
2020-11-06 1:23 ` Boqun Feng
@ 2020-11-06 9:03 ` Marco Elver
2020-11-06 10:18 ` Boqun Feng
0 siblings, 1 reply; 10+ messages in thread
From: Marco Elver @ 2020-11-06 9:03 UTC (permalink / raw)
To: Boqun Feng
Cc: Paul E. McKenney, LKML, kasan-dev, kernel-team, Ingo Molnar,
Andrey Konovalov, Alexander Potapenko, Dmitry Vyukov, Qian Cai
On Fri, 6 Nov 2020 at 02:23, Boqun Feng <boqun.feng@gmail.com> wrote:
> Hi Marco,
>
> On Thu, Nov 05, 2020 at 02:03:24PM -0800, paulmck@kernel.org wrote:
> > From: Marco Elver <elver@google.com>
> >
> > The watchpoint encoding masks for size and address were off-by-one bit
> > each, with the size mask using 1 unnecessary bit and the address mask
> > missing 1 bit. However, due to the way the size is shifted into the
> > encoded watchpoint, we were effectively wasting and never using the
> > extra bit.
> >
> > For example, on x86 with PAGE_SIZE==4K, we have 1 bit for the is-write
> > bit, 14 bits for the size bits, and then 49 bits left for the address.
> > Prior to this fix we would end up with this usage:
> >
> > [ write<1> | size<14> | wasted<1> | address<48> ]
> >
> > Fix it by subtracting 1 bit from the GENMASK() end and start ranges of
> > size and address respectively. The added static_assert()s verify that
> > the masks are as expected. With the fixed version, we get the expected
> > usage:
> >
> > [ write<1> | size<14> | address<49> ]
> >
> > Functionally no change is expected, since that extra address bit is
> > insignificant for enabled architectures.
> >
> > Signed-off-by: Marco Elver <elver@google.com>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > ---
> > kernel/kcsan/encoding.h | 14 ++++++--------
> > 1 file changed, 6 insertions(+), 8 deletions(-)
> >
> > diff --git a/kernel/kcsan/encoding.h b/kernel/kcsan/encoding.h
> > index 4f73db6..b50bda9 100644
> > --- a/kernel/kcsan/encoding.h
> > +++ b/kernel/kcsan/encoding.h
> > @@ -37,14 +37,12 @@
> > */
> > #define WATCHPOINT_ADDR_BITS (BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
> >
> > -/*
> > - * Masks to set/retrieve the encoded data.
> > - */
> > -#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1)
> > -#define WATCHPOINT_SIZE_MASK \
> > - GENMASK(BITS_PER_LONG-2, BITS_PER_LONG-2 - WATCHPOINT_SIZE_BITS)
> > -#define WATCHPOINT_ADDR_MASK \
> > - GENMASK(BITS_PER_LONG-3 - WATCHPOINT_SIZE_BITS, 0)
> > +/* Bitmasks for the encoded watchpoint access information. */
> > +#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1)
> > +#define WATCHPOINT_SIZE_MASK GENMASK(BITS_PER_LONG-2, BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
> > +#define WATCHPOINT_ADDR_MASK GENMASK(BITS_PER_LONG-2 - WATCHPOINT_SIZE_BITS, 0)
> > +static_assert(WATCHPOINT_ADDR_MASK == (1UL << WATCHPOINT_ADDR_BITS) - 1);
>
> Nit:
>
> Since you use the static_assert(), why not define WATCHPOINT_ADDR_MASK
> as:
>
> #define WATCHPOINT_ADDR_MASK (BIT(WATCHPOINT_SIZE_BITS) - 1)
This is incorrect, as the static_assert()s would have indicated. It
should probably be (BIT(WATCHPOINT_ADDR_BITS) - 1)?
As an aside, I explicitly did *not* want to use additional arithmetic
to generate the masks but purely rely on BIT(), and GENMASK(), as it
would be inconsistent otherwise. The static_assert()s then sanity
check everything without BIT+GENMASK (because I've grown slightly
paranoid about off-by-1s here). So I'd rather not start bikeshedding
about which way around things should go.
In general, GENMASK() is safer, because subtracting 1 to get the mask
doesn't always work, specifically e.g. (BIT(BITS_PER_LONG) - 1) does
not work.
> Besides, WATCHPOINT_SIZE_MASK can also be defined as:
No, sorry it cannot.
> #define WATCHPOINT_SIZE_MASK GENMASK(BITS_PER_LONG - 2, WATCHPOINT_SIZE_BITS)
GENMASK(BITS_PER_LONG - 2, WATCHPOINT_SIZE_BITS)
is not equivalent to the current
GENMASK(BITS_PER_LONG-2, BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
Did you mean GENMASK(BITS_PER_LONG-2, WATCHPOINT_ADDR_BITS)? I can
send a v2 for this one.
Thanks,
-- Marco
^ permalink raw reply [flat|nested] 10+ messages in thread
* [PATCH v2] kcsan: Fix encoding masks and regain address bit
2020-11-05 22:03 ` [PATCH kcsan 3/3] kcsan: Fix encoding masks and regain address bit paulmck
2020-11-06 1:23 ` Boqun Feng
@ 2020-11-06 9:34 ` Marco Elver
2020-11-06 10:19 ` Boqun Feng
1 sibling, 1 reply; 10+ messages in thread
From: Marco Elver @ 2020-11-06 9:34 UTC (permalink / raw)
To: paulmck, boqun.feng
Cc: linux-kernel, kasan-dev, kernel-team, mingo, andreyknvl, glider,
dvyukov, cai
The watchpoint encoding masks for size and address were off-by-one bit
each, with the size mask using 1 unnecessary bit and the address mask
missing 1 bit. However, due to the way the size is shifted into the
encoded watchpoint, we were effectively wasting and never using the
extra bit.
For example, on x86 with PAGE_SIZE==4K, we have 1 bit for the is-write
bit, 14 bits for the size bits, and then 49 bits left for the address.
Prior to this fix we would end up with this usage:
[ write<1> | size<14> | wasted<1> | address<48> ]
Fix it by subtracting 1 bit from the GENMASK() end and start ranges of
size and address respectively. The added static_assert()s verify that
the masks are as expected. With the fixed version, we get the expected
usage:
[ write<1> | size<14> | address<49> ]
Functionally no change is expected, since that extra address bit is
insignificant for enabled architectures.
Signed-off-by: Marco Elver <elver@google.com>
---
v2:
* Use WATCHPOINT_ADDR_BITS to avoid duplicating "BITS_PER_LONG-1 -
WATCHPOINT_SIZE_BITS" per Boqun's suggestion.
---
kernel/kcsan/encoding.h | 14 ++++++--------
1 file changed, 6 insertions(+), 8 deletions(-)
diff --git a/kernel/kcsan/encoding.h b/kernel/kcsan/encoding.h
index 4f73db6d1407..7ee405524904 100644
--- a/kernel/kcsan/encoding.h
+++ b/kernel/kcsan/encoding.h
@@ -37,14 +37,12 @@
*/
#define WATCHPOINT_ADDR_BITS (BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
-/*
- * Masks to set/retrieve the encoded data.
- */
-#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1)
-#define WATCHPOINT_SIZE_MASK \
- GENMASK(BITS_PER_LONG-2, BITS_PER_LONG-2 - WATCHPOINT_SIZE_BITS)
-#define WATCHPOINT_ADDR_MASK \
- GENMASK(BITS_PER_LONG-3 - WATCHPOINT_SIZE_BITS, 0)
+/* Bitmasks for the encoded watchpoint access information. */
+#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1)
+#define WATCHPOINT_SIZE_MASK GENMASK(BITS_PER_LONG-2, WATCHPOINT_ADDR_BITS)
+#define WATCHPOINT_ADDR_MASK GENMASK(WATCHPOINT_ADDR_BITS-1, 0)
+static_assert(WATCHPOINT_ADDR_MASK == (1UL << WATCHPOINT_ADDR_BITS) - 1);
+static_assert((WATCHPOINT_WRITE_MASK ^ WATCHPOINT_SIZE_MASK ^ WATCHPOINT_ADDR_MASK) == ~0UL);
static inline bool check_encodable(unsigned long addr, size_t size)
{
--
2.29.2.222.g5d2a92d10f8-goog
^ permalink raw reply related [flat|nested] 10+ messages in thread
* Re: [PATCH kcsan 3/3] kcsan: Fix encoding masks and regain address bit
2020-11-06 9:03 ` Marco Elver
@ 2020-11-06 10:18 ` Boqun Feng
2020-11-06 10:47 ` Marco Elver
0 siblings, 1 reply; 10+ messages in thread
From: Boqun Feng @ 2020-11-06 10:18 UTC (permalink / raw)
To: Marco Elver
Cc: Paul E. McKenney, LKML, kasan-dev, kernel-team, Ingo Molnar,
Andrey Konovalov, Alexander Potapenko, Dmitry Vyukov, Qian Cai
On Fri, Nov 06, 2020 at 10:03:21AM +0100, Marco Elver wrote:
> On Fri, 6 Nov 2020 at 02:23, Boqun Feng <boqun.feng@gmail.com> wrote:
> > Hi Marco,
> >
> > On Thu, Nov 05, 2020 at 02:03:24PM -0800, paulmck@kernel.org wrote:
> > > From: Marco Elver <elver@google.com>
> > >
> > > The watchpoint encoding masks for size and address were off-by-one bit
> > > each, with the size mask using 1 unnecessary bit and the address mask
> > > missing 1 bit. However, due to the way the size is shifted into the
> > > encoded watchpoint, we were effectively wasting and never using the
> > > extra bit.
> > >
> > > For example, on x86 with PAGE_SIZE==4K, we have 1 bit for the is-write
> > > bit, 14 bits for the size bits, and then 49 bits left for the address.
> > > Prior to this fix we would end up with this usage:
> > >
> > > [ write<1> | size<14> | wasted<1> | address<48> ]
> > >
> > > Fix it by subtracting 1 bit from the GENMASK() end and start ranges of
> > > size and address respectively. The added static_assert()s verify that
> > > the masks are as expected. With the fixed version, we get the expected
> > > usage:
> > >
> > > [ write<1> | size<14> | address<49> ]
> > >
> > > Functionally no change is expected, since that extra address bit is
> > > insignificant for enabled architectures.
> > >
> > > Signed-off-by: Marco Elver <elver@google.com>
> > > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > > ---
> > > kernel/kcsan/encoding.h | 14 ++++++--------
> > > 1 file changed, 6 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/kernel/kcsan/encoding.h b/kernel/kcsan/encoding.h
> > > index 4f73db6..b50bda9 100644
> > > --- a/kernel/kcsan/encoding.h
> > > +++ b/kernel/kcsan/encoding.h
> > > @@ -37,14 +37,12 @@
> > > */
> > > #define WATCHPOINT_ADDR_BITS (BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
> > >
> > > -/*
> > > - * Masks to set/retrieve the encoded data.
> > > - */
> > > -#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1)
> > > -#define WATCHPOINT_SIZE_MASK \
> > > - GENMASK(BITS_PER_LONG-2, BITS_PER_LONG-2 - WATCHPOINT_SIZE_BITS)
> > > -#define WATCHPOINT_ADDR_MASK \
> > > - GENMASK(BITS_PER_LONG-3 - WATCHPOINT_SIZE_BITS, 0)
> > > +/* Bitmasks for the encoded watchpoint access information. */
> > > +#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1)
> > > +#define WATCHPOINT_SIZE_MASK GENMASK(BITS_PER_LONG-2, BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
> > > +#define WATCHPOINT_ADDR_MASK GENMASK(BITS_PER_LONG-2 - WATCHPOINT_SIZE_BITS, 0)
> > > +static_assert(WATCHPOINT_ADDR_MASK == (1UL << WATCHPOINT_ADDR_BITS) - 1);
> >
> > Nit:
> >
> > Since you use the static_assert(), why not define WATCHPOINT_ADDR_MASK
> > as:
> >
> > #define WATCHPOINT_ADDR_MASK (BIT(WATCHPOINT_SIZE_BITS) - 1)
>
> This is incorrect, as the static_assert()s would have indicated. It
> should probably be (BIT(WATCHPOINT_ADDR_BITS) - 1)?
>
> As an aside, I explicitly did *not* want to use additional arithmetic
> to generate the masks but purely rely on BIT(), and GENMASK(), as it
> would be inconsistent otherwise. The static_assert()s then sanity
> check everything without BIT+GENMASK (because I've grown slightly
> paranoid about off-by-1s here). So I'd rather not start bikeshedding
> about which way around things should go.
>
> In general, GENMASK() is safer, because subtracting 1 to get the mask
> doesn't always work, specifically e.g. (BIT(BITS_PER_LONG) - 1) does
> not work.
>
> > Besides, WATCHPOINT_SIZE_MASK can also be defined as:
>
> No, sorry it cannot.
>
> > #define WATCHPOINT_SIZE_MASK GENMASK(BITS_PER_LONG - 2, WATCHPOINT_SIZE_BITS)
>
> GENMASK(BITS_PER_LONG - 2, WATCHPOINT_SIZE_BITS)
>
> is not equivalent to the current
>
> GENMASK(BITS_PER_LONG-2, BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
>
> Did you mean GENMASK(BITS_PER_LONG-2, WATCHPOINT_ADDR_BITS)? I can
You're right! Guess I should check first about what vim completes for me
;-) And I agree with you on the preference to GENMASK()
> send a v2 for this one.
Let me add an ack for that one, thanks!
Regards,
Boqun
>
> Thanks,
> -- Marco
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH v2] kcsan: Fix encoding masks and regain address bit
2020-11-06 9:34 ` [PATCH v2] " Marco Elver
@ 2020-11-06 10:19 ` Boqun Feng
0 siblings, 0 replies; 10+ messages in thread
From: Boqun Feng @ 2020-11-06 10:19 UTC (permalink / raw)
To: Marco Elver
Cc: paulmck, linux-kernel, kasan-dev, kernel-team, mingo, andreyknvl,
glider, dvyukov, cai
On Fri, Nov 06, 2020 at 10:34:56AM +0100, Marco Elver wrote:
> The watchpoint encoding masks for size and address were off-by-one bit
> each, with the size mask using 1 unnecessary bit and the address mask
> missing 1 bit. However, due to the way the size is shifted into the
> encoded watchpoint, we were effectively wasting and never using the
> extra bit.
>
> For example, on x86 with PAGE_SIZE==4K, we have 1 bit for the is-write
> bit, 14 bits for the size bits, and then 49 bits left for the address.
> Prior to this fix we would end up with this usage:
>
> [ write<1> | size<14> | wasted<1> | address<48> ]
>
> Fix it by subtracting 1 bit from the GENMASK() end and start ranges of
> size and address respectively. The added static_assert()s verify that
> the masks are as expected. With the fixed version, we get the expected
> usage:
>
> [ write<1> | size<14> | address<49> ]
>
> Functionally no change is expected, since that extra address bit is
> insignificant for enabled architectures.
>
> Signed-off-by: Marco Elver <elver@google.com>
Acked-by: Boqun Feng <boqun.feng@gmail.com>
Regards,
Boqun
> ---
> v2:
> * Use WATCHPOINT_ADDR_BITS to avoid duplicating "BITS_PER_LONG-1 -
> WATCHPOINT_SIZE_BITS" per Boqun's suggestion.
> ---
> kernel/kcsan/encoding.h | 14 ++++++--------
> 1 file changed, 6 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/kcsan/encoding.h b/kernel/kcsan/encoding.h
> index 4f73db6d1407..7ee405524904 100644
> --- a/kernel/kcsan/encoding.h
> +++ b/kernel/kcsan/encoding.h
> @@ -37,14 +37,12 @@
> */
> #define WATCHPOINT_ADDR_BITS (BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
>
> -/*
> - * Masks to set/retrieve the encoded data.
> - */
> -#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1)
> -#define WATCHPOINT_SIZE_MASK \
> - GENMASK(BITS_PER_LONG-2, BITS_PER_LONG-2 - WATCHPOINT_SIZE_BITS)
> -#define WATCHPOINT_ADDR_MASK \
> - GENMASK(BITS_PER_LONG-3 - WATCHPOINT_SIZE_BITS, 0)
> +/* Bitmasks for the encoded watchpoint access information. */
> +#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1)
> +#define WATCHPOINT_SIZE_MASK GENMASK(BITS_PER_LONG-2, WATCHPOINT_ADDR_BITS)
> +#define WATCHPOINT_ADDR_MASK GENMASK(WATCHPOINT_ADDR_BITS-1, 0)
> +static_assert(WATCHPOINT_ADDR_MASK == (1UL << WATCHPOINT_ADDR_BITS) - 1);
> +static_assert((WATCHPOINT_WRITE_MASK ^ WATCHPOINT_SIZE_MASK ^ WATCHPOINT_ADDR_MASK) == ~0UL);
>
> static inline bool check_encodable(unsigned long addr, size_t size)
> {
> --
> 2.29.2.222.g5d2a92d10f8-goog
>
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH kcsan 3/3] kcsan: Fix encoding masks and regain address bit
2020-11-06 10:18 ` Boqun Feng
@ 2020-11-06 10:47 ` Marco Elver
0 siblings, 0 replies; 10+ messages in thread
From: Marco Elver @ 2020-11-06 10:47 UTC (permalink / raw)
To: Boqun Feng
Cc: Paul E. McKenney, LKML, kasan-dev, kernel-team, Ingo Molnar,
Andrey Konovalov, Alexander Potapenko, Dmitry Vyukov, Qian Cai
On Fri, 6 Nov 2020 at 11:19, Boqun Feng <boqun.feng@gmail.com> wrote:
> > send a v2 for this one.
>
> Let me add an ack for that one, thanks!
Thank you!
-- Marco
^ permalink raw reply [flat|nested] 10+ messages in thread
end of thread, other threads:[~2020-11-06 10:47 UTC | newest]
Thread overview: 10+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-11-05 22:03 [PATCH kcsan 0/3] KCSAN updates for 5.11 Paul E. McKenney
2020-11-05 22:03 ` [PATCH kcsan 1/3] kcsan: selftest: Ensure that address is at least PAGE_SIZE paulmck
2020-11-05 22:03 ` [PATCH kcsan 2/3] kcsan: Never set up watchpoints on NULL pointers paulmck
2020-11-05 22:03 ` [PATCH kcsan 3/3] kcsan: Fix encoding masks and regain address bit paulmck
2020-11-06 1:23 ` Boqun Feng
2020-11-06 9:03 ` Marco Elver
2020-11-06 10:18 ` Boqun Feng
2020-11-06 10:47 ` Marco Elver
2020-11-06 9:34 ` [PATCH v2] " Marco Elver
2020-11-06 10:19 ` Boqun Feng
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