linux-kernel - Re: [PATCH 1/3] readfile: implement readfile syscall

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Message-ID: <CAMuHMdWeuzVjZMpR+d20X0mDCpVpNzEU0qpZO4LiMVGCgJBy3A@mail.gmail.com>
Date:   Sat, 4 Jul 2020 20:35:36 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
        Shuah Khan <shuah@...nel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-man@...r.kernel.org,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH 1/3] readfile: implement readfile syscall

Hi Greg,

On Sat, Jul 4, 2020 at 4:05 PM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> 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@...iv.linux.org.uk>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

Thanks for your patch!

> --- a/fs/open.c
> +++ b/fs/open.c

> +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@...ux-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