Hi Greg,

On Sat, Jul 4, 2020 at 4:05 PM Greg Kroah-Hartman
[off-list ref] wrote:

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, this is a "simple" syscall, with the
attempt to make reading "simple" files easier with less syscall
overhead.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Thanks for your patch!

+SYSCALL_DEFINE5(readfile, int, dfd, const char __user *, filename,
+               char __user *, buffer, size_t, bufsize, int, flags)
+{
+       struct open_flags op;
+       struct open_how how;
+       struct file *file;
+       loff_t pos = 0;
+       int retval;
+
+       /* only accept a small subset of O_ flags that make sense */
+       if ((flags & (O_NOFOLLOW | O_NOATIME)) != flags)
+               return -EINVAL;
+
+       /* add some needed flags to be able to open the file properly */
+       flags |= O_RDONLY | O_LARGEFILE;
+
+       how = build_open_how(flags, 0000);
+       retval = build_open_flags(&how, &op);
+       if (retval)
+               return retval;
+
+       file = readfile_open(dfd, filename, &op);
+       if (IS_ERR(file))
+               return PTR_ERR(file);
+
+       retval = vfs_read(file, buffer, bufsize, &pos);

Should there be a way for the user to be informed that the file doesn't
fit in the provided buffer (.e.g. -EFBIG)?

+
+       filp_close(file, NULL);
+
+       return retval;
+}

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds