Messages in this thread | | | From | Kuniyuki Iwashima <> | Subject | Re: [PATCH v1 net 11/16] net: Fix a data-race around sysctl_mem. | Date | Wed, 6 Jul 2022 09:27:53 -0700 |
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From: Steven Rostedt <rostedt@goodmis.org> Date: Wed, 6 Jul 2022 09:27:11 -0400 > On Wed, 6 Jul 2022 09:17:07 -0400 > Steven Rostedt <rostedt@goodmis.org> wrote: > > > On Tue, 5 Jul 2022 22:21:25 -0700 > > Kuniyuki Iwashima <kuniyu@amazon.com> wrote: > > > > > --- a/include/trace/events/sock.h > > > +++ b/include/trace/events/sock.h > > > @@ -122,9 +122,9 @@ TRACE_EVENT(sock_exceed_buf_limit, > > > > > > TP_printk("proto:%s sysctl_mem=%ld,%ld,%ld allocated=%ld sysctl_rmem=%d rmem_alloc=%d sysctl_wmem=%d wmem_alloc=%d wmem_queued=%d kind=%s", > > > __entry->name, > > > - __entry->sysctl_mem[0], > > > - __entry->sysctl_mem[1], > > > - __entry->sysctl_mem[2], > > > + READ_ONCE(__entry->sysctl_mem[0]), > > > + READ_ONCE(__entry->sysctl_mem[1]), > > > + READ_ONCE(__entry->sysctl_mem[2]), > > > > This is not reading anything to do with sysctl. It's reading the content of > > what was recorded in the ring buffer. > > > > That is, the READ_ONCE() here is not necessary, and if anything will break > > user space parsing, as this is exported to user space to tell it how to > > read the binary format in the ring buffer. > > I take that back. Looking at the actual trace event, it is pointing to > sysctl memory, which is a major bug. > > TRACE_EVENT(sock_exceed_buf_limit, > > TP_PROTO(struct sock *sk, struct proto *prot, long allocated, int kind), > > TP_ARGS(sk, prot, allocated, kind), > > TP_STRUCT__entry( > __array(char, name, 32) > __field(long *, sysctl_mem) > > sysctl_mem is a pointer. > > __field(long, allocated) > __field(int, sysctl_rmem) > __field(int, rmem_alloc) > __field(int, sysctl_wmem) > __field(int, wmem_alloc) > __field(int, wmem_queued) > __field(int, kind) > ), > > TP_fast_assign( > strncpy(__entry->name, prot->name, 32); > > __entry->sysctl_mem = prot->sysctl_mem; > > > They save the pointer **IN THE RING BUFFER**!!! > > __entry->allocated = allocated; > __entry->sysctl_rmem = sk_get_rmem0(sk, prot); > __entry->rmem_alloc = atomic_read(&sk->sk_rmem_alloc); > __entry->sysctl_wmem = sk_get_wmem0(sk, prot); > __entry->wmem_alloc = refcount_read(&sk->sk_wmem_alloc); > __entry->wmem_queued = READ_ONCE(sk->sk_wmem_queued); > __entry->kind = kind; > ), > > TP_printk("proto:%s sysctl_mem=%ld,%ld,%ld allocated=%ld sysctl_rmem=%d rmem_alloc=%d sysctl_wmem=%d wmem_alloc=%d wmem_queued=%d kind=%s", > __entry->name, > __entry->sysctl_mem[0], > __entry->sysctl_mem[1], > __entry->sysctl_mem[2], > > They are now reading a stale pointer, which can be read at any time. That > is, you get the information of what is in sysctl_mem at the time the ring > buffer is read (which is useless from user space), and not at the time of > the event. > > Thanks for pointing this out. This needs to be fixed.
For the record, Steve fixed this properly here, so I'll drop the tracing part in v2. https://lore.kernel.org/netdev/20220706105040.54fc03b0@gandalf.local.home/
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