2022-07-13 16:10:59

by Uros Bizjak

Subject: [PATCH 0/2] ucount: Fix and improve atomic_long_inc_below

The series fixes wrong argument type and improves atomic_long_inc_below
by using atomic_long_try_cmpxchg instead of atomic_long_cmpxchg.

Signed-off-by: Uros Bizjak <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>

Uros Bizjak (2):
ucount: Fix atomic_long_inc_below argument type
ucount: Use atomic_long_try_cmpxchg in atomic_long_inc_below

kernel/ucount.c | 16 +++++++---------
1 file changed, 7 insertions(+), 9 deletions(-)

--
2.35.3

2022-07-13 16:17:01

by Uros Bizjak

Subject: [PATCH 1/2] ucount: Fix atomic_long_inc_below argument type

The type of u argument of atomic_long_inc_below should be long
to avoid unwanted truncation to int.

Fixes: f9c82a4ea89c ("Increase size of ucounts to atomic_long_t")
Signed-off-by: Uros Bizjak <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>
---
kernel/ucount.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/ucount.c b/kernel/ucount.c
index 06ea04d44685..974ac1585009 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -216,7 +216,7 @@ void put_ucounts(struct ucounts *ucounts)
}
}

-static inline bool atomic_long_inc_below(atomic_long_t *v, int u)
+static inline bool atomic_long_inc_below(atomic_long_t *v, long u)
{
long c, old;
c = atomic_long_read(v);
--
2.35.3

2022-07-13 16:42:13

by Uros Bizjak

Subject: [PATCH 2/2] ucount: Use atomic_long_try_cmpxchg in atomic_long_inc_below

Use atomic_long_try_cmpxchg instead of
atomic_long_cmpxchg (*ptr, old, new) == old in atomic_long_inc_below.
x86 CMPXCHG instruction returns success in ZF flag, so this change saves
a compare after cmpxchg (and related move instruction in front of cmpxchg).

Also, atomic_long_try_cmpxchg implicitly assigns old *ptr value to "old"
when cmpxchg fails, enabling further code simplifications.

No functional change intended.

Signed-off-by: Uros Bizjak <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>
---
kernel/ucount.c | 14 ++++++--------
1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/kernel/ucount.c b/kernel/ucount.c
index 974ac1585009..5d980460104e 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -218,16 +218,14 @@ void put_ucounts(struct ucounts *ucounts)

static inline bool atomic_long_inc_below(atomic_long_t *v, long u)
{
- long c, old;
- c = atomic_long_read(v);
- for (;;) {
+ long c = atomic_long_read(v);
+
+ do {
if (unlikely(c >= u))
return false;
- old = atomic_long_cmpxchg(v, c, c+1);
- if (likely(old == c))
- return true;
- c = old;
- }
+ } while (!atomic_long_try_cmpxchg(v, &c, c+1));
+
+ return true;
}

struct ucounts *inc_ucount(struct user_namespace *ns, kuid_t uid,
--
2.35.3

2022-07-15 18:30:56

by Eric W. Biederman

Subject: Re: [PATCH 1/2] ucount: Fix atomic_long_inc_below argument type

Uros Bizjak <[email protected]> writes:

> The type of u argument of atomic_long_inc_below should be long
> to avoid unwanted truncation to int.

Reviewed-by: "Eric W. Biederman" <[email protected]>

>
> Fixes: f9c82a4ea89c ("Increase size of ucounts to atomic_long_t")
> Signed-off-by: Uros Bizjak <[email protected]>
> Cc: "Eric W. Biederman" <[email protected]>
> ---
> kernel/ucount.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/ucount.c b/kernel/ucount.c
> index 06ea04d44685..974ac1585009 100644
> --- a/kernel/ucount.c
> +++ b/kernel/ucount.c
> @@ -216,7 +216,7 @@ void put_ucounts(struct ucounts *ucounts)
> }
> }
>
> -static inline bool atomic_long_inc_below(atomic_long_t *v, int u)
> +static inline bool atomic_long_inc_below(atomic_long_t *v, long u)
> {
> long c, old;
> c = atomic_long_read(v);

2022-07-15 18:31:11

by Eric W. Biederman

Subject: Re: [PATCH 0/2] ucount: Fix and improve atomic_long_inc_below

Uros Bizjak <[email protected]> writes:

> The series fixes wrong argument type and improves atomic_long_inc_below
> by using atomic_long_try_cmpxchg instead of atomic_long_cmpxchg.
>
> Signed-off-by: Uros Bizjak <[email protected]>
> Cc: "Eric W. Biederman" <[email protected]>

The bug fix at least looks correct. I haven't reviewed the
atomic_long_try_cmpxchg yet.

Thanks,
Eric


>
> Uros Bizjak (2):
> ucount: Fix atomic_long_inc_below argument type
> ucount: Use atomic_long_try_cmpxchg in atomic_long_inc_below
>
> kernel/ucount.c | 16 +++++++---------
> 1 file changed, 7 insertions(+), 9 deletions(-)