On Tue, Mar 3, 2020 at 3:10 PM Greg Kroah-Hartman
[off-list ref] wrote:

On Tue, Mar 03, 2020 at 02:43:16PM +0100, Greg Kroah-Hartman wrote:

quoted

On Tue, Mar 03, 2020 at 02:34:42PM +0100, Miklos Szeredi wrote:

quoted

On Tue, Mar 3, 2020 at 2:14 PM Greg Kroah-Hartman
[off-list ref] wrote:

quoted

quoted

Unlimited beers for a 21-line kernel patch?  Sign me up!

Totally untested, barely compiled patch below.

Ok, that didn't even build, let me try this for real now...

Some comments on the interface:

Ok, hey, let's do this proper :)

Alright, how about this patch.

Actually tested with some simple sysfs files.

If people don't strongly object, I'll add "real" tests to it, hook it up
to all arches, write a manpage, and all the fun fluff a new syscall
deserves and submit it "for real".

Just FYI, io_uring is moving towards the same kind of thing... IIRC
you can already use it to batch a bunch of open() calls, then batch a
bunch of read() calls on all the new fds and close them at the same
time. And I think they're planning to add support for doing
open()+read()+close() all in one go, too, except that it's a bit
complicated because passing forward the file descriptor in a generic
way is a bit complicated.

It feels like I'm doing something wrong in that the actuall syscall
logic is just so small.  Maybe I'll benchmark this thing to see if it
makes any real difference...

thanks,

greg k-h

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: [PATCH] readfile: implement readfile syscall

It's a tiny syscall, meant to allow a user to do a single "open this
file, read into this buffer, and close the file" all in a single shot.

Should be good for reading "tiny" files like sysfs, procfs, and other
"small" files.

There is no restarting the syscall, am trying to keep it simple.  At
least for now.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

[...]

+SYSCALL_DEFINE5(readfile, int, dfd, const char __user *, filename,
+               char __user *, buffer, size_t, bufsize, int, flags)
+{
+       int retval;
+       int fd;
+
+       /* Mask off all O_ flags as we only want to read from the file */
+       flags &= ~(VALID_OPEN_FLAGS);
+       flags |= O_RDONLY | O_LARGEFILE;
+
+       fd = do_sys_open(dfd, filename, flags, 0000);
+       if (fd <= 0)
+               return fd;
+
+       retval = ksys_read(fd, buffer, bufsize);
+
+       __close_fd(current->files, fd);
+
+       return retval;
+}

If you're gonna do something like that, wouldn't you want to also
elide the use of the file descriptor table completely? do_sys_open()
will have to do atomic operations in the fd table and stuff, which is
probably moderately bad in terms of cacheline bouncing if this is used
in a multithreaded context; and as a side effect, the fd would be
inherited by anyone who calls fork() concurrently. You'll probably
want to use APIs like do_filp_open() and filp_close(), or something
like that, instead.