On 5/2/25 10:16, Allison Karlitskaya wrote: > hi, > > Please excuse me if these are dumb questions. I'm not great at this stuff. :) > > In fuse_backing_open() there's a check with an interesting comment: > > /* TODO: relax CAP_SYS_ADMIN once backing files are visible to lsof */ > res = -EPERM; > if (!fc->passthrough || !capable(CAP_SYS_ADMIN)) > goto out; > > I've done some research into this but I wasn't able to find any > original discussion about what led to this, or about current plans to > "relax" this restriction -- only speculation about it being a > potential mechanism to "hide" open files. > > It would be nice to have an official story about this, on the record. > What's the concrete problem here, and what would it take to solve it? > Are there plans? Is help required? Would it be possible to relax the > check to having CAP_SYS_ADMIN in the userns which owns the mount (ie: > ns_capable(...))? What would it take to do that? It would be > wonderful to be able to use this inside of containers. > > The most obvious guess about direction (based on the comment) is that > we need to do something to make sure that fds that are registered with > backing IDs remain visible in the output of `lsof` even after the > original fd is closed? > > Thanks in advance for any information you can give. Even if the > answer is "no, it's impossible" it would be great to have that on > record. There is a private discussion, Chen and Amir are discussing exactly this topic. <quote> >>Chen >> Additionally, according to previous discussions, backing files are >somewhat >> similar to the fixed files in `io_uring`. >> If it is considered acceptable for the fixed files in `io_uring` to >> have their status visible in `fdinfo`, >> then exposing backing file information via `/sys/fs/fuse/connections` >> also seems like a feasible approach. >Amir > Yes I agree. > That sounds like a good approach to expose the backing files to userspace and allow admin to force close then by aborting the connection. > In fact if you want you can start with that. > I don't think it will be controversial. > I think this will be useful step towards relaxing cap sys admin. </quote> I think it would be good to document all these details somewhere, really hard to follow all of it. Thanks, Bernd > > Cheers > > lis > >
