Re: WARNING in sk_stream_kill_queues (5)

Marco Elver
2021-02-03T10:27:30Z

On Mon, 14 Dec 2020 at 11:09, Marco Elver <elver@google.com> wrote:
> On Thu, 10 Dec 2020 at 20:01, Marco Elver <elver@google.com> wrote:
> > On Thu, 10 Dec 2020 at 18:14, Eric Dumazet <edumazet@google.com> wrote:
> > > On Thu, Dec 10, 2020 at 5:51 PM Marco Elver <elver@google.com> wrote:
> > [...]
> > > > So I started putting gdb to work, and whenever I see an allocation
> > > > exactly like the above that goes through tso_fragment() a warning
> > > > immediately follows.
> > > >
> > > > Long story short, I somehow synthesized this patch that appears to fix
> > > > things, but I can't explain why exactly:
> > > >
> > > > | --- a/net/core/skbuff.c
> > > > | +++ b/net/core/skbuff.c
> > > > | @@ -1679,13 +1679,6 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
> > > > |
> > > > | skb_metadata_clear(skb);
> > > > |
> > > > | - /* It is not generally safe to change skb->truesize.
> > > > | - * For the moment, we really care of rx path, or
> > > > | - * when skb is orphaned (not attached to a socket).
> > > > | - */
> > > > | - if (!skb->sk || skb->destructor == sock_edemux)
> > > > | - skb->truesize += size - osize;
> > > > | -
> > > > | return 0;
> > > > |
> > > > | nofrags:
> > > >
> > > > Now, here are the breadcrumbs I followed:
> > > >
> > > >
> > > > 1. Breakpoint on kfence_ksize() -- first allocation that matches the above:
> > > >
> > > > | #0 __kfence_ksize (s=18446612700164612096) at mm/kfence/core.c:726
> > > > | #1 0xffffffff816fbf30 in kfence_ksize (addr=0xffff888436856000) at mm/kfence/core.c:737
> > > > | #2 0xffffffff816217cf in ksize (objp=0xffff888436856000) at mm/slab_common.c:1178
> > > > | #3 0xffffffff84896911 in __alloc_skb (size=914710528, gfp_mask=2592, flags=0, node=-1) at net/core/skbuff.c:217
> > > > | #4 0xffffffff84d0ba73 in alloc_skb_fclone (priority=<optimized out>, size=<optimized out>) at ./include/linux/skbuff.h:1144
> > > > | #5 sk_stream_alloc_skb (sk=0xffff8881176cc000, size=0, gfp=2592, force_schedule=232) at net/ipv4/tcp.c:888
> > > > | #6 0xffffffff84d41c36 in tso_fragment (gfp=<optimized out>, mss_now=<optimized out>, len=<optimized out>,
> > > > | skb=<optimized out>, sk=<optimized out>) at net/ipv4/tcp_output.c:2124
> > > > | #7 tcp_write_xmit (sk=0xffff8881176cc000, mss_now=21950, nonagle=3096, push_one=-1996874776, gfp=0)
> > > > | at net/ipv4/tcp_output.c:2674
> > > > | #8 0xffffffff84d43e48 in __tcp_push_pending_frames (sk=0xffff8881176cc000, cur_mss=337, nonagle=0)
> > > > | at ./include/net/sock.h:918
> > > > | #9 0xffffffff84d3259c in tcp_push_pending_frames (sk=<optimized out>) at ./include/net/tcp.h:1864
> > > > | #10 tcp_data_snd_check (sk=<optimized out>) at net/ipv4/tcp_input.c:5374
> > > > | #11 tcp_rcv_established (sk=0xffff8881176cc000, skb=0x0 <fixed_percpu_data>) at net/ipv4/tcp_input.c:5869
> > > > | #12 0xffffffff84d56731 in tcp_v4_do_rcv (sk=0xffff8881176cc000, skb=0xffff888117f52ea0) at net/ipv4/tcp_ipv4.c:1668
> > > > | [...]
> > > >
> > > > Set watchpoint on skb->truesize:
> > > >
> > > > | (gdb) frame 3
> > > > | #3 0xffffffff84896911 in __alloc_skb (size=914710528, gfp_mask=2592, flags=0, node=-1) at net/core/skbuff.c:217
> > > > | 217 size = SKB_WITH_OVERHEAD(ksize(data));
> > > > | (gdb) p &skb->truesize
> > > > | $5 = (unsigned int *) 0xffff888117f55f90
> > > > | (gdb) awatch *0xffff888117f55f90
> > > > | Hardware access (read/write) watchpoint 6: *0xffff888117f55f90
> > > >
> > > > 2. Some time later, we see that the skb with kfence-allocated data
> > > > is cloned:
> > > >
> > > > | Thread 7 hit Hardware access (read/write) watchpoint 6: *0xffff888117f55f90
> > > > |
> > > > | Value = 1570
> > > > | 0xffffffff84886947 in __skb_clone (n=0xffff888117f55fa0, skb=0xffff888117f55ec0) at net/core/skbuff.c:1002
> > > > | 1002 C(truesize);
> > > > | (gdb) bt
> > > > | #0 0xffffffff84886947 in __skb_clone (n=0xffff888117f55fa0, skb=0xffff888117f55ec0) at net/core/skbuff.c:1002
> > > > | #1 0xffffffff8488bfb9 in skb_clone (skb=0xffff888117f55ec0, gfp_mask=2592) at net/core/skbuff.c:1454
> > > > | #2 0xffffffff84d3cd1c in __tcp_transmit_skb (sk=0xffff8881176cc000, skb=0xffff888117f55ec0, clone_it=0, gfp_mask=2592,
> > > > | rcv_nxt=0) at net/ipv4/tcp_output.c:1267
> > > > | #3 0xffffffff84d4125b in tcp_transmit_skb (gfp_mask=<optimized out>, clone_it=<optimized out>, skb=<optimized out>,
> > > > | sk=<optimized out>) at ./include/linux/tcp.h:439
> > > > | #4 tcp_write_xmit (sk=0xffff8881176cc000, mss_now=392485600, nonagle=1326, push_one=-1996875104, gfp=0)
> > > > | at net/ipv4/tcp_output.c:2688
> > > > | #5 0xffffffff84d43e48 in __tcp_push_pending_frames (sk=0xffff8881176cc000, cur_mss=337, nonagle=0)
> > > > | at ./include/net/sock.h:918
> > > > | #6 0xffffffff84d3259c in tcp_push_pending_frames (sk=<optimized out>) at ./include/net/tcp.h:1864
> > > > | #7 tcp_data_snd_check (sk=<optimized out>) at net/ipv4/tcp_input.c:5374
> > > > | #8 tcp_rcv_established (sk=0xffff8881176cc000, skb=0x0 <fixed_percpu_data>) at net/ipv4/tcp_input.c:5869
> > > > | #9 0xffffffff84d56731 in tcp_v4_do_rcv (sk=0xffff8881176cc000, skb=0xffff888117f57820) at net/ipv4/tcp_ipv4.c:1668
> > > > | #10 0xffffffff8487bf67 in sk_backlog_rcv (skb=<optimized out>, sk=<optimized out>) at ./include/net/sock.h:1010
> > > > [...]
> > > >
> > > >
> > > > 3. The original skb (that was cloned) has its truesize adjusted
> > > > after a pskb_expand_head():
> > > >
> > > > | Thread 2 hit Hardware access (read/write) watchpoint 6: *0xffff888117f55f90
> > > > |
> > > > | Old value = 1570
> > > > | New value = 1954
> > > >
> > > > ^^ the difference between the old and the new value is exactly
> > > > 384, which is also the final underflow of the sk_wmem_queued
> > > > that triggers the warning. Presumably if the original allocation
> > > > had been through kmalloc-1k and not KFENCE, the difference here
> > > > would have been 0, since ksize() of the original allocation in
> > > > step (1) would have been 1024, and not 640 (difference of 384).
> > > >
> > > > | 0xffffffff8488d84b in pskb_expand_head (skb=0xffff888117f55ec0, nhead=401956752, ntail=1954, gfp_mask=2298092192)
> > > > | at net/core/skbuff.c:1687
> > > > | 1687 skb->truesize += size - osize;
> > > > | (gdb) bt
> > > > | #0 0xffffffff8488d84b in pskb_expand_head (skb=0xffff888117f55ec0, nhead=401956752, ntail=1954, gfp_mask=2298092192)
> > > > | at net/core/skbuff.c:1687
> > > > | #1 0xffffffff8488de01 in skb_prepare_for_shift (skb=<optimized out>) at ./arch/x86/include/asm/atomic.h:29
> > > > | #2 skb_prepare_for_shift (skb=0xffff888117f55ec0) at net/core/skbuff.c:3276
> > > > | #3 0xffffffff848936b1 in skb_shift (tgt=0xffff888117f549c0, skb=0xffff888117f55ec0, shiftlen=674) at net/core/skbuff.c:3351
> > > > | #4 0xffffffff84d264de in tcp_skb_shift (shiftlen=<optimized out>, pcount=<optimized out>, from=<optimized out>,
> > > > | to=<optimized out>) at net/ipv4/tcp_input.c:1497
> > > > | #5 tcp_shift_skb_data (dup_sack=<optimized out>, end_seq=<optimized out>, start_seq=<optimized out>, state=<optimized out>,
> > > > | skb=<optimized out>, sk=<optimized out>) at net/ipv4/tcp_input.c:1605
> > > > | #6 tcp_sacktag_walk (skb=0xffff888117f55ec0, sk=0xffff8881176cc000, next_dup=0x894,
> > > > | state=0xffffffff88fa1aa0 <watchpoints+192>, start_seq=0, end_seq=401956752, dup_sack_in=false)
> > > > | at net/ipv4/tcp_input.c:1670
> > > > | #7 0xffffffff84d276de in tcp_sacktag_write_queue (sk=0xffff888117f55f90, ack_skb=0x1888117f55f90, prior_snd_una=2196,
> > > > | state=0xffffffff88fa1aa0 <watchpoints+192>) at net/ipv4/tcp_input.c:1931
> > > > | #8 0xffffffff84d2ca1d in tcp_ack (sk=0xffff8881176cc000, skb=0x1888117f55f90, flag=16643) at net/ipv4/tcp_input.c:3758
> > > > | #9 0xffffffff84d32387 in tcp_rcv_established (sk=0xffff8881176cc000, skb=0xffff888117f54020) at net/ipv4/tcp_input.c:5858
> > > > | #10 0xffffffff84d56731 in tcp_v4_do_rcv (sk=0xffff8881176cc000, skb=0xffff888117f54020) at net/ipv4/tcp_ipv4.c:1668
> > > > [...]
> > > >
> > > >
> > > > Any of this make sense?
> > >
> > > Very nice debugging !
> > >
> > > I guess we could fix this in skb_prepare_for_shift(), eventually
> > > caring for the truesize manipulation
> > > (or reverting the change done in pskb_expand_head(), since only kfence
> > > is having this issue.
> >
> > Phew, good to hear I finally got lucky. :-)
> >
> > Either option is fine, as long as it avoids this problem in future.
> > Hopefully it can be fixed for 5.11.
> >
> > > (All TCP skbs in output path have the same allocation size for skb->head)
> > >
> > > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > > index e578544b2cc7110ec2f6bcf4c29d93e4b4b1ad14..798b51eeeaa4fbed65d41d9eab207dbbf438dab3
> > > 100644
> > > --- a/net/core/skbuff.c
> > > +++ b/net/core/skbuff.c
> > > @@ -3270,7 +3270,14 @@ EXPORT_SYMBOL(skb_split);
> > > */
> > > static int skb_prepare_for_shift(struct sk_buff *skb)
> > > {
> > > - return skb_cloned(skb) && pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
> > > + unsigned int ret = 0, save;
> > > +
> > > + if (skb_cloned(skb)) {
> > > + save = skb->truesize;
> > > + ret = pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
> > > + skb->truesize = save;
> > > + }
> > > + return ret;
> > > }
> >
> > FWIW,
> >
> > Tested-by: Marco Elver <elver@google.com>
>
> Has this patch, or similar, already been sent?

This is now in net-next:
https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=097b9146c0e2

Thanks,
-- Marco