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SubjectRe: [RFC PATCH v1 1/4] Increase size of ucounts to atomic_long_t
On Mon, Nov 02, 2020 at 07:03:01PM +0100, Christian Brauner wrote:
> On Mon, Nov 02, 2020 at 05:50:30PM +0100, Alexey Gladkov wrote:
> > In order to be able to use ucounts for rlimits, the size must be increased.
> > For example user_struct.mq_bytes (RLIMIT_MSGQUEUE) is unsigned long.
>
> I don't have any issues with this change I just wonder what the exact
> reason is. It's not immediately obvious to me.

Right now user_struct.mq_bytes that is currently used for checking
RLIMIT_MSGQUEUE is unsigned log, but ucounts is signed int. The rlimit is
also unsigned long. If I migrate RLIMIT_MSGQUEUE to ucounts I will
decrease counter and possibly break backward compatibility. Technically,
it can be violated anyway.

linux/ipc/mqueue.c:376:

mq_bytes += mq_treesize;
spin_lock(&mq_lock);
if (u->mq_bytes + mq_bytes < u->mq_bytes ||
u->mq_bytes + mq_bytes > rlimit(RLIMIT_MSGQUEUE)) {
spin_unlock(&mq_lock);
/* mqueue_evict_inode() releases info->messages */
ret = -EMFILE;
goto out_inode;
}
u->mq_bytes += mq_bytes;
spin_unlock(&mq_lock);

> >
> > Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
> > ---
> > include/linux/user_namespace.h | 4 ++--
> > kernel/ucount.c | 14 +++++++-------
> > 2 files changed, 9 insertions(+), 9 deletions(-)
> >
> > diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> > index 6ef1c7109fc4..fc75af812d73 100644
> > --- a/include/linux/user_namespace.h
> > +++ b/include/linux/user_namespace.h
> > @@ -86,7 +86,7 @@ struct user_namespace {
> > struct ctl_table_header *sysctls;
> > #endif
> > struct ucounts *ucounts;
> > - int ucount_max[UCOUNT_COUNTS];
> > + long ucount_max[UCOUNT_COUNTS];
> > } __randomize_layout;
> >
> > struct ucounts {
> > @@ -94,7 +94,7 @@ struct ucounts {
> > struct user_namespace *ns;
> > kuid_t uid;
> > int count;
> > - atomic_t ucount[UCOUNT_COUNTS];
> > + atomic_long_t ucount[UCOUNT_COUNTS];
> > };
> >
> > extern struct user_namespace init_user_ns;
> > diff --git a/kernel/ucount.c b/kernel/ucount.c
> > index 11b1596e2542..7b2bca8582ef 100644
> > --- a/kernel/ucount.c
> > +++ b/kernel/ucount.c
> > @@ -175,14 +175,14 @@ static void put_ucounts(struct ucounts *ucounts)
> > kfree(ucounts);
> > }
> >
> > -static inline bool atomic_inc_below(atomic_t *v, int u)
> > +static inline bool atomic_long_inc_below(atomic_long_t *v, int u)
> > {
> > - int c, old;
> > - c = atomic_read(v);
> > + long c, old;
> > + c = atomic_long_read(v);
> > for (;;) {
> > if (unlikely(c >= u))
> > return false;
> > - old = atomic_cmpxchg(v, c, c+1);
> > + old = atomic_long_cmpxchg(v, c, c+1);
> > if (likely(old == c))
> > return true;
> > c = old;
> > @@ -199,14 +199,14 @@ struct ucounts *inc_ucount(struct user_namespace *ns, kuid_t uid,
> > int max;
> > tns = iter->ns;
> > max = READ_ONCE(tns->ucount_max[type]);
> > - if (!atomic_inc_below(&iter->ucount[type], max))
> > + if (!atomic_long_inc_below(&iter->ucount[type], max))
> > goto fail;
> > }
> > return ucounts;
> > fail:
> > bad = iter;
> > for (iter = ucounts; iter != bad; iter = iter->ns->ucounts)
> > - atomic_dec(&iter->ucount[type]);
> > + atomic_long_dec(&iter->ucount[type]);
> >
> > put_ucounts(ucounts);
> > return NULL;
> > @@ -216,7 +216,7 @@ void dec_ucount(struct ucounts *ucounts, enum ucount_type type)
> > {
> > struct ucounts *iter;
> > for (iter = ucounts; iter; iter = iter->ns->ucounts) {
> > - int dec = atomic_dec_if_positive(&iter->ucount[type]);
> > + int dec = atomic_long_dec_if_positive(&iter->ucount[type]);
> > WARN_ON_ONCE(dec < 0);
> > }
> > put_ucounts(ucounts);
> > --
> > 2.25.4
> >
>

--
Rgrds, legion

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