[RFC,1/2] fcntl: add F_CREATED

Commit Message

Christian Brauner July 24, 2024, 1:15 p.m. UTC

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.

* 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.

* 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.

  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 <[e-mail]>
---
 fs/fcntl.c                 | 10 ++++++++++
 include/uapi/linux/fcntl.h |  3 +++
 2 files changed, 13 insertions(+)

Comments

Jeff Layton July 24, 2024, 1:56 p.m. UTC | #1

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 <[e-mail]>
> ---
>  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 <[e-mail]>

Jan Kara July 24, 2024, 4:10 p.m. UTC | #2

On Wed 24-07-24 15:15:35, 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.
> 
> * 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.
> 
> * 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.
> 
>   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 <[e-mail]>

Looks like a sensible functionality. I agree with Jeff that the behavior
wrt dup(2) / fork(2) needs to be clearly stated but is hardly surprising
given how everything else with file descriptors and descriptions works.
So feel free to add:

Reviewed-by: Jan Kara <[e-mail]>

								Honza

> ---
>  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:
> 
> -- 
> 2.43.0
>

Christian Brauner July 25, 2024, 8:38 a.m. UTC | #3

On Wed, Jul 24, 2024 at 09:56:09AM GMT, Jeff Layton wrote:
> 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.

Yes, but it's the behavior one expects. Dup'ing an fd implies fd1->file
== fd2->file and so it's correct behavior. IOW, someone must confuse
open() with dup() to be surprised by this.

Jeff Layton July 25, 2024, 11:16 a.m. UTC | #4

On Thu, 2024-07-25 at 10:38 +0200, Christian Brauner wrote:
> On Wed, Jul 24, 2024 at 09:56:09AM GMT, Jeff Layton wrote:
> > 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.
> 
> Yes, but it's the behavior one expects. Dup'ing an fd implies fd1->file
> == fd2->file and so it's correct behavior. IOW, someone must confuse
> open() with dup() to be surprised by this.

It's the behavior _we_ generally expect, but a lot of people don't
understand the subtleties of file descriptions vs. file descriptors.
Spelling out this behavior explicitly would probably prevent a few
headaches.

Patch

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: