Re: Linux 5.12-rc7

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Apr 13, 2021 at 02:45:46PM +0200, Eric Dumazet wrote:
> On Tue, Apr 13, 2021 at 12:43 PM Eric Dumazet <edumazet@google.com> wrote:
> >
> > On Tue, Apr 13, 2021 at 11:24 AM Eric Dumazet <edumazet@google.com> wrote:
> > >
> > > On Mon, Apr 12, 2021 at 10:05 PM Guenter Roeck <linux@roeck-us.net> wrote:
> > > >
> > > > On 4/12/21 10:38 AM, Eric Dumazet wrote:
> > > > [ ... ]
> > > >
> > > > > Yes, I think this is the real issue here. This smells like some memory
> > > > > corruption.
> > > > >
> > > > > In my traces, packet is correctly received in AF_PACKET queue.
> > > > >
> > > > > I have checked the skb is well formed.
> > > > >
> > > > > But the user space seems to never call poll() and recvmsg() on this
> > > > > af_packet socket.
> > > > >
> > > >
> > > > After sprinkling the kernel with debug messages:
> > > >
> > > > 424   00:01:33.674181 sendto(6, "E\0\1H\0\0\0\0@\21y\246\0\0\0\0\377\377\377\377\0D\0C\00148\346\1\1\6\0\246\336\333\v\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0RT\0\
> > > > 424   00:01:33.693873 close(6)          = 0
> > > > 424   00:01:33.694652 fcntl64(5, F_SETFD, FD_CLOEXEC) = 0
> > > > 424   00:01:33.695213 clock_gettime64(CLOCK_MONOTONIC, 0x7be18a18) = -1 EFAULT (Bad address)
> > > > 424   00:01:33.695889 write(2, "udhcpc: clock_gettime(MONOTONIC) failed\n", 40) = -1 EFAULT (Bad address)
> > > > 424   00:01:33.697311 exit_group(1)     = ?
> > > > 424   00:01:33.698346 +++ exited with 1 +++
> > > >
> > > > I only see that after adding debug messages in the kernel, so I guess there must be
> > > > a heisenbug somehere.
> > > >
> > > > Anyway, indeed, I see (another kernel debug message):
> > > >
> > > > __do_sys_clock_gettime: Returning -EFAULT on address 0x7bacc9a8
> > > >
> > > > So udhcpc doesn't even try to read the reply because it crashes after sendto()
> > > > when trying to read the current time. Unless I am missing something, that means
> > > > that the problem happens somewhere on the send side.
> > > >
> > > > To make things even more interesting, it looks like the failing system call
> > > > isn't always clock_gettime().
> > > >
> > > > Guenter
> > >
> > >
> > > I think GRO fast path has never worked on SUPERH. Probably SUPERH has
> > > never used a fast NIC (10Gbit+)
> > >
> > > The following hack fixes the issue.
> > >
> > >
> > > diff --git a/net/core/dev.c b/net/core/dev.c
> > > index af8c1ea040b9364b076e2d72f04dc3de2d7e2f11..91ba89a645ff91d4cd4f3d8dc8a009bcb67da344
> > > 100644
> > > --- a/net/core/dev.c
> > > +++ b/net/core/dev.c
> > > @@ -5916,13 +5916,16 @@ static struct list_head
> > > *gro_list_prepare(struct napi_struct *napi,
> > >
> > >  static void skb_gro_reset_offset(struct sk_buff *skb)
> > >  {
> > > +#if !defined(CONFIG_SUPERH)
> > >         const struct skb_shared_info *pinfo = skb_shinfo(skb);
> > >         const skb_frag_t *frag0 = &pinfo->frags[0];
> > > +#endif
> > >
> > >         NAPI_GRO_CB(skb)->data_offset = 0;
> > >         NAPI_GRO_CB(skb)->frag0 = NULL;
> > >         NAPI_GRO_CB(skb)->frag0_len = 0;
> > >
> > > +#if !defined(CONFIG_SUPERH)
> > >         if (!skb_headlen(skb) && pinfo->nr_frags &&
> > >             !PageHighMem(skb_frag_page(frag0))) {
> > >                 NAPI_GRO_CB(skb)->frag0 = skb_frag_address(frag0);
> > > @@ -5930,6 +5933,7 @@ static void skb_gro_reset_offset(struct sk_buff *skb)
> > >                                                     skb_frag_size(frag0),
> > >                                                     skb->end - skb->tail);
> > >         }
> > > +#endif
> > >  }
> > >
> > >  static void gro_pull_from_frag0(struct sk_buff *skb, int grow)
> >
> > OK ... more sh debugging :
> >
> > diff --git a/arch/sh/mm/alignment.c b/arch/sh/mm/alignment.c
> > index fb517b82a87b1065cf38c06cb3c178ce86587b00..5d18f9f792991105a8aa05cc6231b7d4532d72c9
> > 100644
> > --- a/arch/sh/mm/alignment.c
> > +++ b/arch/sh/mm/alignment.c
> > @@ -27,7 +27,7 @@ static unsigned long se_multi;
> >     valid! */
> >  static int se_usermode = UM_WARN | UM_FIXUP;
> >  /* 0: no warning 1: print a warning message, disabled by default */
> > -static int se_kernmode_warn;
> > +static int se_kernmode_warn = 1;
> >
> >  core_param(alignment, se_usermode, int, 0600);
> >
> > @@ -103,7 +103,7 @@ void unaligned_fixups_notify(struct task_struct
> > *tsk, insn_size_t insn,
> >                           (void *)instruction_pointer(regs), insn);
> >         else if (se_kernmode_warn)
> >                 pr_notice_ratelimited("Fixing up unaligned kernel access "
> > -                         "in \"%s\" pid=%d pc=0x%p ins=0x%04hx\n",
> > +                         "in \"%s\" pid=%d pc=%px ins=0x%04hx\n",
> >                           tsk->comm, task_pid_nr(tsk),
> >                           (void *)instruction_pointer(regs), insn);
> >  }
> >
> > I now see something of interest :
> > Fixing up unaligned kernel access in "udhcpc" pid=91 pc=8c43fc2e ins=0x6236
> > Fixing up unaligned kernel access in "udhcpc" pid=91 pc=8c43fc2e ins=0x6236
> > Fixing up unaligned kernel access in "udhcpc" pid=91 pc=8c43fc30 ins=0x6636
> > Fixing up unaligned kernel access in "udhcpc" pid=91 pc=8c43fc30 ins=0x6636
> > Fixing up unaligned kernel access in "udhcpc" pid=91 pc=8c43fc3a ins=0x6636
> > Fixing up unaligned kernel access in "udhcpc" pid=91 pc=8c43fc3a ins=0x6636
> > Fixing up unaligned kernel access in "udhcpc" pid=91 pc=8c43fc3a ins=0x6636
> > Fixing up unaligned kernel access in "udhcpc" pid=91 pc=8c43fc3a ins=0x6636
> > Fixing up unaligned kernel access in "udhcpc" pid=91 pc=8c43fc3a ins=0x6636
> > Fixing up unaligned kernel access in "udhcpc" pid=91 pc=8c43fc3a ins=0x6636
> >
> > So basically the frag0 idea only works if drivers respect NET_IP_ALIGN
> > (So that IP header is 4-byte aligned)
> >
> > It seems either virtio_net or qemu does not respect the contract.
> >
> > A possible generic fix  would then be :
> >
> >
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index af8c1ea040b9364b076e2d72f04dc3de2d7e2f11..1f79b9aa9a3f2392fddd1401f95ad098b5e03204
> > 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -5924,7 +5924,8 @@ static void skb_gro_reset_offset(struct sk_buff *skb)
> >         NAPI_GRO_CB(skb)->frag0_len = 0;
> >
> >         if (!skb_headlen(skb) && pinfo->nr_frags &&
> > -           !PageHighMem(skb_frag_page(frag0))) {
> > +           !PageHighMem(skb_frag_page(frag0)) &&
> > +           (!NET_IP_ALIGN || !(skb_frag_off(frag0) & 3))) {
> >                 NAPI_GRO_CB(skb)->frag0 = skb_frag_address(frag0);
> >                 NAPI_GRO_CB(skb)->frag0_len = min_t(unsigned int,
> >                                                     skb_frag_size(frag0),
> 
> 
> Official submission :
> 
> https://patchwork.kernel.org/project/netdevbpf/patch/20210413124136.2750358-1-eric.dumazet@gmail.com/

Thanks a lot Eric!

-- 
MST