Re: [PATCH RFC 1/2] fcntl: add F_CREATED

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2024-07-24 at 15:15 +0200, Christian Brauner wrote:
> Systemd has a helper called openat_report_new() that returns whether a
> file was created anew or it already existed before for cases where
> O_CREAT has to be used without O_EXCL (cf. [1]). That apparently isn't
> something that's specific to systemd but it's where I noticed it.
> 
> The current logic is that it first attempts to open the file without
> O_CREAT | O_EXCL and if it gets ENOENT the helper tries again with both
> flags. If that succeeds all is well. If it now reports EEXIST it
> retries.
> 
> That works fairly well but some corner cases make this more involved. If
> this operates on a dangling symlink the first openat() without O_CREAT |
> O_EXCL will return ENOENT but the second openat() with O_CREAT | O_EXCL
> will fail with EEXIST. The reason is that openat() without O_CREAT |
> O_EXCL follows the symlink while O_CREAT | O_EXCL doesn't for security
> reasons. So it's not something we can really change unless we add an
> explicit opt-in via O_FOLLOW which seems really ugly.
> 
> The caller could try and use fanotify() to register to listen for
> creation events in the directory before calling openat(). The caller
> could then compare the returned tid to its own tid to ensure that even
> in threaded environments it actually created the file. That might work
> but is a lot of work for something that should be fairly simple and I'm
> uncertain about it's reliability.
> 
> The caller could use a bpf lsm hook to hook into security_file_open() to
> figure out whether they created the file. That also seems a bit wild.
> 
> So let's add F_CREATED which allows the caller to check whether they
> actually did create the file. That has caveats of course but I don't
> think they are problematic:
> 
> * In multi-threaded environments a thread can only be sure that it did
>   create the file if it calls openat() with O_CREAT. In other words,
>   it's obviously not enough to just go through it's fdtable and check
>   these fds because another thread could've created the file.
> 

Not exactly. FMODE_CREATED is set in the file description. In principle
a userland program should know which thread actually did the the open()
that results in each fd. This new interface tells us which fd's open
actually resulted in the file being created (which is good).

In any case, I don't see this as a problem. The interface does what it
says on the tin.

> * If there's any codepaths where an openat() with O_CREAT would yield
>   the same struct file as that of another thread it would obviously
>   cause wrong results. I'm not aware of any such codepaths from openat()
>   itself. Imho, that would be a bug.
> 

Definitely a bug. That said, this will have interesting interactions
with dup that may need to be documented. IOW, if you dup a file with
FMODE_CREATED, then the new fd will also report that F_CREATED is true.

> * Related to the previous point, calling the new fcntl() on files created
>   and opened via special-purpose system calls or ioctl()s would cause
>   wrong results only if the affected subsystem a) raises FMODE_CREATED
>   and b) may return the same struct file for two different calls. I'm
>   not seeing anything outside of regular VFS code that raises
>   FMODE_CREATED.
> 

Me neither. This interface is really about "traditional" filesystems.
If you're dealing with some pseudo-fs (proc, sys, debugfs, etc.), I
don't think you can expect to get sane results from this.

>   There is code for b) in e.g., the drm layer where the same struct file
>   is resurfaced but again FMODE_CREATED isn't used and it would be very
>   misleading if it did.
> 
> Link: https://github.com/systemd/systemd/blob/11d5e2b5fbf9f6bfa5763fd45b56829ad4f0777f/src/basic/fs-util.c#L1078 [1]
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  fs/fcntl.c                 | 10 ++++++++++
>  include/uapi/linux/fcntl.h |  3 +++
>  2 files changed, 13 insertions(+)
> 
> diff --git a/fs/fcntl.c b/fs/fcntl.c
> index 300e5d9ad913..55a66ad9b432 100644
> --- a/fs/fcntl.c
> +++ b/fs/fcntl.c
> @@ -343,6 +343,12 @@ static long f_dupfd_query(int fd, struct file *filp)
>  	return f.file == filp;
>  }
>  
> +/* Let the caller figure out whether a given file was just created. */
> +static long f_created(const struct file *filp)
> +{
> +	return !!(filp->f_mode & FMODE_CREATED);
> +}
> +
>  static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
>  		struct file *filp)
>  {
> @@ -352,6 +358,9 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
>  	long err = -EINVAL;
>  
>  	switch (cmd) {
> +	case F_CREATED:
> +		err = f_created(filp);
> +		break;
>  	case F_DUPFD:
>  		err = f_dupfd(argi, filp, 0);
>  		break;
> @@ -463,6 +472,7 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
>  static int check_fcntl_cmd(unsigned cmd)
>  {
>  	switch (cmd) {
> +	case F_CREATED:
>  	case F_DUPFD:
>  	case F_DUPFD_CLOEXEC:
>  	case F_DUPFD_QUERY:
> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> index c0bcc185fa48..d78a6c237688 100644
> --- a/include/uapi/linux/fcntl.h
> +++ b/include/uapi/linux/fcntl.h
> @@ -16,6 +16,9 @@
>  
>  #define F_DUPFD_QUERY	(F_LINUX_SPECIFIC_BASE + 3)
>  
> +/* Was the file just created? */
> +#define F_CREATED	(F_LINUX_SPECIFIC_BASE + 4)
> +
>  /*
>   * Cancel a blocking posix lock; internal use only until we expose an
>   * asynchronous lock api to userspace:
> 

This looks sane to me.

Reviewed-by: Jeff Layton <jlayton@kernel.org>