Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes: >> > After that, one can pin bpf_link temporarily and re-open it as >> > writable one, provided CAP_DAC_OVERRIDE capability is present. All >> > that works already, because pinned bpf_link is just a file, so one can >> > do fchmod on it and all that will go through normal file access >> > permission check code path. >> >> Ah, I did not know that was possible - I was assuming that bpffs was >> doing something special to prevent that. But if not, great! >> >> > Unfortunately, just re-opening same FD as writable (which would >> > be possible if fcntl(fd, F_SETFL, S_IRUSR >> > S_IWUSR) was supported on Linux) without pinning is not possible. >> > Opening link from /proc/<pid>/fd/<link-fd> doesn't seem to work >> > either, because backing inode is not BPF FS inode. I'm not sure, but >> > maybe we can support the latter eventually. But either way, I think >> > given this is to be used for manual troubleshooting, going through few >> > extra hoops to force-detach bpf_link is actually a good thing. >> >> Hmm, I disagree that deliberately making users jump through hoops is a >> good thing. Smells an awful lot like security through obscurity to me; >> and we all know how well that works anyway... > > Depends on who users are? bpftool can implement this as one of > `bpftool link` sub-commands and allow human operators to force-detach > bpf_link, if necessary. Yeah, I would expect this to be the common way this would be used: built into tools. > I think applications shouldn't do this (programmatically) at all, > which is why I think it's actually good that it's harder and not > obvious, this will make developer think again before implementing > this, hopefully. For me it's about discouraging bad practice. I guess I just don't share your optimism that making people jump through hoops will actually discourage them :) If people know what they are doing it should be enough to document it as discouraged. And if they don't, they are perfectly capable of finding and copy-pasting the sequence of hoop-jumps required to achieve what they want, probably with more bugs added along the way. So in the end I think that all you're really achieving is annoying people who do have a legitimate reason to override the behaviour (which includes yourself as a bpftool developer :)). That's what I meant by the 'security through obscurity' comment. -Toke
