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Wed, 17 Feb 2021 05:52:20 -0800 (PST) MIME-Version: 1.0 References: <20210125153057.3623715-1-balsini@android.com> <20210125153057.3623715-5-balsini@android.com> In-Reply-To: <20210125153057.3623715-5-balsini@android.com> From: Miklos Szeredi Date: Wed, 17 Feb 2021 14:52:09 +0100 Message-ID: Subject: Re: [PATCH RESEND V12 4/8] fuse: Passthrough initialization and release To: Alessio Balsini Cc: Akilesh Kailash , Amir Goldstein , Antonio SJ Musumeci , David Anderson , Giuseppe Scrivano , Jann Horn , Jens Axboe , Martijn Coenen , Palmer Dabbelt , Paul Lawrence , Peng Tao , Stefano Duo , Zimuzo Ezeozue , wuyan , fuse-devel , kernel-team , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 25, 2021 at 4:31 PM Alessio Balsini wrote: > > Implement the FUSE passthrough ioctl that associates the lower > (passthrough) file system file with the fuse_file. > > The file descriptor passed to the ioctl by the FUSE daemon is used to > access the relative file pointer, that will be copied to the fuse_file > data structure to consolidate the link between the FUSE and lower file > system. > > To enable the passthrough mode, user space triggers the > FUSE_DEV_IOC_PASSTHROUGH_OPEN ioctl and, if the call succeeds, receives > back an identifier that will be used at open/create response time in the > fuse_open_out field to associate the FUSE file to the lower file system > file. > The value returned by the ioctl to user space can be: > - > 0: success, the identifier can be used as part of an open/create > reply. > - <= 0: an error occurred. > The value 0 represents an error to preserve backward compatibility: the > fuse_open_out field that is used to pass the passthrough_fh back to the > kernel uses the same bits that were previously as struct padding, and is > commonly zero-initialized (e.g., in the libfuse implementation). > Removing 0 from the correct values fixes the ambiguity between the case > in which 0 corresponds to a real passthrough_fh, a missing > implementation of FUSE passthrough or a request for a normal FUSE file, > simplifying the user space implementation. > > For the passthrough mode to be successfully activated, the lower file > system file must implement both read_iter and write_iter file > operations. This extra check avoids special pseudo files to be targeted > for this feature. > Passthrough comes with another limitation: no further file system > stacking is allowed for those FUSE file systems using passthrough. > > Signed-off-by: Alessio Balsini > --- > fs/fuse/inode.c | 5 +++ > fs/fuse/passthrough.c | 87 ++++++++++++++++++++++++++++++++++++++++++- > 2 files changed, 90 insertions(+), 2 deletions(-) > > diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c > index a1104d5abb70..7ebc398fbacb 100644 > --- a/fs/fuse/inode.c > +++ b/fs/fuse/inode.c > @@ -1133,6 +1133,11 @@ EXPORT_SYMBOL_GPL(fuse_send_init); > > static int free_fuse_passthrough(int id, void *p, void *data) > { > + struct fuse_passthrough *passthrough = (struct fuse_passthrough *)p; > + > + fuse_passthrough_release(passthrough); > + kfree(p); > + > return 0; > } > > diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c > index 594060c654f8..cf993e83803e 100644 > --- a/fs/fuse/passthrough.c > +++ b/fs/fuse/passthrough.c > @@ -3,19 +3,102 @@ > #include "fuse_i.h" > > #include > +#include > > int fuse_passthrough_open(struct fuse_dev *fud, > struct fuse_passthrough_out *pto) > { > - return -EINVAL; > + int res; > + struct file *passthrough_filp; > + struct fuse_conn *fc = fud->fc; > + struct inode *passthrough_inode; > + struct super_block *passthrough_sb; > + struct fuse_passthrough *passthrough; > + > + if (!fc->passthrough) > + return -EPERM; > + > + /* This field is reserved for future implementation */ > + if (pto->len != 0) > + return -EINVAL; > + > + passthrough_filp = fget(pto->fd); > + if (!passthrough_filp) { > + pr_err("FUSE: invalid file descriptor for passthrough.\n"); > + return -EBADF; > + } > + > + if (!passthrough_filp->f_op->read_iter || > + !passthrough_filp->f_op->write_iter) { > + pr_err("FUSE: passthrough file misses file operations.\n"); > + res = -EBADF; > + goto err_free_file; > + } > + > + passthrough_inode = file_inode(passthrough_filp); > + passthrough_sb = passthrough_inode->i_sb; > + if (passthrough_sb->s_stack_depth >= FILESYSTEM_MAX_STACK_DEPTH) { > + pr_err("FUSE: fs stacking depth exceeded for passthrough\n"); No need to print an error to the logs, this can be a perfectly normal occurrence. However I'd try to find a more unique error value than EINVAL so that the fuse server can interpret this as "not your fault, but can't support passthrough on this file". E.g. EOPNOTSUPP. > + res = -EINVAL; > + goto err_free_file; > + } > + > + passthrough = kmalloc(sizeof(struct fuse_passthrough), GFP_KERNEL); > + if (!passthrough) { > + res = -ENOMEM; > + goto err_free_file; > + } > + > + passthrough->filp = passthrough_filp; > + > + idr_preload(GFP_KERNEL); > + spin_lock(&fc->passthrough_req_lock); Should be okay to use fc->lock, since neither adding nor removing the passthrough ID should be a heavily used operation, and querying the mapping is lockless. Thanks, Miklos