On Thu, Nov 09, 2023 at 07:55:50AM -0800, Alexei Starovoitov wrote:
> On Wed, Nov 8, 2023 at 11:26 PM Hou Tao <houtao@xxxxxxxxxxxxxxx> wrote:
> >
> > Hi,
> >
> > On 11/9/2023 2:36 PM, Martin KaFai Lau wrote:
> > > On 11/7/23 6:06 AM, Hou Tao wrote:
> > >> From: Hou Tao <houtao1@xxxxxxxxxx>
> > >>
> > >> bpf_map_of_map_fd_get_ptr() will convert the map fd to the pointer
> > >> saved in map-in-map. bpf_map_of_map_fd_put_ptr() will release the
> > >> pointer saved in map-in-map. These two helpers will be used by the
> > >> following patches to fix the use-after-free problems for map-in-map.
> > >>
> > >> Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>
> > >> ---
> > >> kernel/bpf/map_in_map.c | 51 +++++++++++++++++++++++++++++++++++++++++
> > >> kernel/bpf/map_in_map.h | 11 +++++++--
> > >> 2 files changed, 60 insertions(+), 2 deletions(-)
> > >>
> > >>
> > SNIP
> > >> +void bpf_map_of_map_fd_put_ptr(void *ptr, bool need_defer)
> > >> +{
> > >> + struct bpf_inner_map_element *element = ptr;
> > >> +
> > >> + /* Do bpf_map_put() after a RCU grace period and a tasks trace
> > >> + * RCU grace period, so it is certain that the bpf program which is
> > >> + * manipulating the map now has exited when bpf_map_put() is
> > >> called.
> > >> + */
> > >> + if (need_defer)
> > >
> > > "need_defer" should only happen from the syscall cmd? Instead of
> > > adding rcu_head to each element, how about
> > > "synchronize_rcu_mult(call_rcu, call_rcu_tasks)" here?
> >
> > No. I have tried the method before, but it didn't work due to dead-lock
> > (will mention that in commit message in v2). The reason is that bpf
> > syscall program may also do map update through sys_bpf helper. Because
> > bpf syscall program is running with sleep-able context and has
> > rcu_read_lock_trace being held, so call synchronize_rcu_mult(call_rcu,
> > call_rcu_tasks) will lead to dead-lock.
>
> Dead-lock? why?
>
> I think it's legal to do call_rcu_tasks_trace() while inside RCU CS
> or RCU tasks trace CS.
Just confirming that this is the case. If invoking call_rcu_tasks_trace()
within under either rcu_read_lock() or rcu_read_lock_trace() deadlocks,
then there is a bug that needs fixing. ;-)
Thanx, Paul
