On Thu, Apr 23, 2020 at 06:24:14AM +0200, Miklos Szeredi wrote: > On Thu, Apr 23, 2020 at 2:48 AM Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote: > > On Wed, Apr 22, 2020 at 09:55:56AM +0200, Miklos Szeredi wrote: > > > On Wed, Apr 22, 2020 at 8:06 AM Michael Kerrisk (man-pages) > > > <mtk.manpages@xxxxxxxxx> wrote: > > > > > > > > [CC += linux-api] > > > > > > > > On Wed, 22 Apr 2020 at 07:20, Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote: > > > > > > > > > > Inspired by the X protocol's handling of XIDs, allow userspace to select > > > > > the file descriptor opened by openat2, so that it can use the resulting > > > > > file descriptor in subsequent system calls without waiting for the > > > > > response to openat2. > > > > > > > > > > In io_uring, this allows sequences like openat2/read/close without > > > > > waiting for the openat2 to complete. Multiple such sequences can > > > > > overlap, as long as each uses a distinct file descriptor. > > > > > > If this is primarily an io_uring feature, then why burden the normal > > > openat2 API with this? > > > > This feature was inspired by io_uring; it isn't exclusively of value > > with io_uring. (And io_uring doesn't normally change the semantics of > > syscalls.) > > What's the use case of O_SPECIFIC_FD beyond io_uring? Avoiding a call to dup2 and close, if you need something as a specific file descriptor, such as when setting up to exec something, or when debugging a program. I don't expect it to be as widely used as with io_uring, but I also don't want io_uring versions of syscalls to diverge from the underlying syscalls, and this would be a heavy divergence. > > > This would also allow Implementing a private fd table for io_uring. > > > I.e. add a flag interpreted by file ops (IORING_PRIVATE_FD), including > > > openat2 and freely use the private fd space without having to worry > > > about interactions with other parts of the system. > > > > I definitely don't want to add a special kind of file descriptor that > > doesn't work in normal syscalls taking file descriptors. A file > > descriptor allocated via O_SPECIFIC_FD is an entirely normal file > > descriptor, and works anywhere a file descriptor normally works. > > What's the use case of allocating a file descriptor within io_uring > and using it outside of io_uring? Calling a syscall not provided via io_uring. Calling a library that doesn't use io_uring. Passing the file descriptor via UNIX socket to another program. Passing the file descriptor via exec to another program. Userspace is modular, and file descriptors are widely used.
