On Fri, Feb 19, 2021 at 09:40:21AM +0100, Miklos Szeredi wrote: > On Fri, Feb 19, 2021 at 8:05 AM Peng Tao <bergwolf@xxxxxxxxx> wrote: > > > > On Wed, Feb 17, 2021 at 9:41 PM Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > > > > What I think would be useful is to have an explicit > > > FUSE_DEV_IOC_PASSTHROUGH_CLOSE ioctl, that would need to be called > > > once the fuse server no longer needs this ID. If this turns out to > > > be a performance problem, we could still add the auto-close behavior > > > with an explicit FOPEN_PASSTHROUGH_AUTOCLOSE flag later. > > Hi Miklos, > > > > W/o auto closing, what happens if user space daemon forgets to call > > FUSE_DEV_IOC_PASSTHROUGH_CLOSE? Do we keep the ID alive somewhere? > > Kernel would keep the ID open until explicit close or fuse connection > is released. > > There should be some limit on the max open files referenced through > ID's, though. E.g. inherit RLIMIT_NOFILE from mounting task. > > Thanks, > Miklos I like the idea of FUSE_DEV_IOC_PASSTHROUGH_CLOSE to revoke the passthrough access, that is something I was already working on. What I had in mind was simply to break that 1:1 connection between fuse_file and lower filp setting a specific fuse_file::passthrough::filp to NULL, but this is slightly different from what you mentioned. AFAIU you are suggesting to allocate one ID for each lower fs file opened with passthrough within a connection, and maybe using idr_find at every read/write/mmap operation to check if passthrough is enabled on that file. Something similar to fuse2_map_get(). This way the fuse server can pass the same ID to one or more fuse_file(s). FUSE_DEV_IOC_PASSTHROUGH_CLOSE would idr_remove the ID, so idr_find would fail, preventing the use of passthrough on that ID. CMIIW. After FUSE_DEV_IOC_PASSTHROUGH_CLOSE(ID) it may happen that if some fuse_file(s) storing that ID are still open and the same ID is reclaimed in a new idr_alloc, this would lead to mismatching lower fs filp being used by our fuse_file(s). So also the ID stored in the fuse_file(s) must be invalidated to prevent future uses of deallocated IDs. Would it make sense to have a list of fuse_files using the same ID, that must be traversed at FUSE_DEV_IOC_PASSTHROUGH_CLOSE time? Negative values (maybe -ENOENT) might be used to mark IDs as invalid, and tested before idr_find at read/write/mmap to avoid the idr_find complexity in case passthrough is disabled for that file. What do you think? I agree with all the above comments to this patch, i.e., add FOPEN_PASSTHROUGH, drop fuse_passthrough_out, header version+changelog, that will be fixed in V13. Thanks, Alessio
