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Description of problem:
Bug 1081282 is a kernel netlink bug which can cause interface information not to be reported by netlink under complex circumstances involving a Cisco "enic" device, and virtual functions.
Version-Release number of selected component (if applicable):
All.
How reproducible:
Can be reproduced deterministically, but requires a specialized hardware setup.
Steps to Reproduce:
1. Have a machine with a Cisco "enic" device, with 40 or more SR-IOV virtual functions.
2. Attempt to execute getifaddrs() call (e.g. run "sudo false" or the sample code from the getifaddrs() manpage)
Actual results:
getifaddrs() never returns
Expected results:
getifaddrs() returns expected information
Additional info:
Although this is a kernel bug (and we're tracking a fix for that at https://bugzilla.redhat.com/show_bug.cgi?id=1081282), there's a straightforward workaround we can implement from libc, so we should probably do that as well.
Basically, the problem only arises when a "get all" GETLINK is issued with no IFLA_EXT_MASK attribute specified. If the IFLA_EXT_MASK attribute is specified (even if it's 0), the kernel behaviour will change to correctly calculate packet sizes avoiding the bug.
Created attachment 888683[details]
Draft workaround implementation patch
For easier reference, here is the glibc patch I used to make the scratch build above.
Yes, the kernel has now been fixed in https://errata.devel.redhat.com/advisory/17605, so this workaround isn't particularly urgent.
I think this change should be safe for older kernels. Sufficiently old RHEL6 kernels don't support the IFLA_EXT_MASK option, but I think it will just ignore it.
However, I'm not 100% certain of that.
OK, we are going to drop this patch because of bug 1236733. We added it and tested it solely based on the advice form kernel SEG, but we didn't notice the warnings from netlink. Given that the fix is only theoretical we are going to drop it and respin.