On Sat, 2026-03-07 at 10:56 -0800, Andy Lutomirski wrote: > On Sat, Mar 7, 2026 at 6:09 AM Dorjoy Chowdhury <[email protected]> > wrote: > > > > This flag indicates the path should be opened if it's a regular file. > > This is useful to write secure programs that want to avoid being > > tricked into opening device nodes with special semantics while thinking > > they operate on regular files. This is a requested feature from the > > uapi-group[1]. > > > > I think this needs a lot more clarification as to what "regular" > means. If it's literally > > > A corresponding error code EFTYPE has been introduced. For example, if > > openat2 is called on path /dev/null with OPENAT2_REGULAR in the flag > > param, it will return -EFTYPE. EFTYPE is already used in BSD systems > > like FreeBSD, macOS. > > I think this needs more clarification as to what "regular" means, > since S_IFREG may not be sufficient. The UAPI group page says: > > Use-Case: this would be very useful to write secure programs that want > to avoid being tricked into opening device nodes with special > semantics while thinking they operate on regular files. This is > particularly relevant as many device nodes (or even FIFOs) come with > blocking I/O (or even blocking open()!) by default, which is not > expected from regular files backed by “fast” disk I/O. Consider > implementation of a naive web browser which is pointed to > file://dev/zero, not expecting an endless amount of data to read. > > What about procfs? What about sysfs? What about /proc/self/fd/17 > where that fd is a memfd? What about files backed by non-"fast" disk > I/O like something on a flaky USB stick or a network mount or FUSE? > > Are we concerned about blocking open? (open blocks as a matter of > course.) Are we concerned about open having strange side effects? > Are we concerned about write having strange side effects? Are we > concerned about cases where opening the file as root results in > elevated privilege beyond merely gaining the ability to write to that > specific path on an ordinary filesystem? >
Above the use-case, it also says: "O_REGULAR (inspired by the existing O_DIRECTORY flag for open()), which opens a file only if it is of type S_IFREG." Since we allow programs to open a directory under /proc or /sys using O_DIRECTORY, I don't think we should do anything different here. To the VFS, all of the examples you gave above are S_IFREG "regular files", even if they are backed by something quite irregular. -- Jeff Layton <[email protected]>
