On Mon, Mar 9, 2026 at 1:58 AM Christian Brauner <[email protected]> wrote: > > On Sun, Mar 08, 2026 at 10:10:05AM -0700, Andy Lutomirski wrote: > > On Sun, Mar 8, 2026 at 4:40 AM Jeff Layton <[email protected]> wrote: > > > > > > 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? > > I think this is opening up a barrage of question that I'm not sure are > all that useful. The ability to only open regular file isn't intended to > defend against hung FUSE or NFS servers or other random Linux > special-sauce murder-suicide file descriptor traps. For a lot of those > we have O_PATH which can easily function with the new extension. A lot > of the other special-sauce files (most anonymous inode fds) cannot even > be reopened via e.g., /proc.
On the flip side, /proc itself can certainly be opened. Should O_REGULAR be able to open the more magical /proc and /sys files? Are there any that are problematic? --Andy
