Re: [PATCH] [BZ #21812] getifaddrs() Don't return ifa entries with NULL names

[Daniel Alvarez Sanchez <dalvarez@redhat.com>]

Thanks a lot for reviewing. It'd be nice to get this in soon as it can
happen easily on systems with lots of interfaces.

Here it's a reproducer:

$ sudo unshare -n
# cat create-links.sh
#!/bin/bash
for i in {1..512}; do ifname="dum$i"; ip li add $ifname type dummy; ip
li set $ifname up; done
# ./create-links.sh
# cat loop-add-del-link.sh
#!/bin/bash
while :; do ifname="test$RANDOM"; ip li add $ifname type dummy; ip li
set $ifname up; sleep 0.1; ip li del $ifname; done
# ./loop-add-del-link.sh &
[1] 6260
# ./loop-add-del-link.sh &
[2] 6314
# ./loop-add-del-link.sh &
[3] 6374
# ./loop-add-del-link.sh &
[4] 6476
# ./loop-add-del-link.sh &
[5] 6628
# cat dalvarez-tester.c
#include <sys/types.h>
#include <ifaddrs.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(void)
{
        struct ifaddrs *ifaddr, *ifa;

        for (;;)
        {
                if (getifaddrs(&ifaddr) == -1) {
                        perror("getifaddrs");
                        return EXIT_FAILURE;
                }


                /* Walk through linked list, maintaining head pointer so we
                   can free list later */

                for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) {
                        if (ifa->ifa_name == NULL) {
                                printf("Gotcha!!!!\n");
                                return EXIT_FAILURE;
                        }
                }

                freeifaddrs(ifaddr);
                usleep(10);
        }

        return EXIT_SUCCESS;
}
On Fri, Jun 15, 2018 at 9:13 PM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 15/06/2018 10:29, Daniel Alvarez wrote:
> > Due to bug 21812, a lookup operation in map_newlink() turns out
> > into an insert because of holes in the interface part of the map.
> > This leads to incorrectly set the name of the interface to NULL when
> > the interface is not present for the address being processed (most
> > likely because the interface was added between the RTM_GETLINK and
> > RTM_GETADDR calls to the kernel). When such changes are detected
> > by the kernel, it'll mark the dump as "inconsistent" by setting
> > NLM_F_DUMP_INTR flag on the next netlink message.
> >
> > This patch checks this condition and retries the whole operation.
> > Hopes are that next time the interface corresponding to the address
> > entry is present in the list and correct name is returned.
> >
> > Signed-off-by: Daniel Alvarez <dalvarez@redhat.com>
> > Signed-off-by: Jakub Sitnicki <jkbs@redhat.com>
>
> We don't use DCO, instead we use copyright assignments. However the
> change below seems to fit as a trivial change.
>
> Patch looks good and I can't think in a easy way to add testcase which
> is guarantee to be reproducible (maybe by using a network namespace and
> force inject an interface so kernel will set NLM_F_DUMP_INTR?).
>
> > ---
> >  sysdeps/unix/sysv/linux/ifaddrs.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/sysdeps/unix/sysv/linux/ifaddrs.c b/sysdeps/unix/sysv/linux/ifaddrs.c
> > index 32381f54e4..5659c9605b 100644
> > --- a/sysdeps/unix/sysv/linux/ifaddrs.c
> > +++ b/sysdeps/unix/sysv/linux/ifaddrs.c
> > @@ -370,6 +370,14 @@ getifaddrs_internal (struct ifaddrs **ifap)
> >         if ((pid_t) nlh->nlmsg_pid != nh.pid || nlh->nlmsg_seq != nlp->seq)
> >           continue;
> >
> > +      /* If the dump got interrupted, we can't relay on the results so
> > +         try again. */
> > +      if (nlh->nlmsg_flags & NLM_F_DUMP_INTR)
> > +        {
> > +          result = -EAGAIN;
> > +          goto exit_free;
> > +        }
> > +
> >         if (nlh->nlmsg_type == NLMSG_DONE)
> >           break;              /* ok */
> >
> >