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authorChristian Brauner <brauner@kernel.org>2024-07-24 15:15:35 +0200
committerChristian Brauner <brauner@kernel.org>2024-08-19 13:44:34 +0200
commit820a185896b77814557302b981b092a9e7b36814 (patch)
treea4f380d1bc77068c3a54ea39939103a6f9d7940f
parent47ac09b91befbb6a235ab620c32af719f8208399 (diff)
fcntl: add F_CREATED_QUERY
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_QUERY 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] Link: https://lore.kernel.org/r/20240724-work-fcntl-v1-1-e8153a2f1991@kernel.org Reviewed-by: Jeff Layton <jlayton@kernel.org> Reviewed-by: Jan Kara <jack@suse.cz> Signed-off-by: Christian Brauner <brauner@kernel.org>
-rw-r--r--fs/fcntl.c10
-rw-r--r--include/uapi/linux/fcntl.h3
2 files changed, 13 insertions, 0 deletions
diff --git a/fs/fcntl.c b/fs/fcntl.c
index 300e5d9ad913..22ec683ad8f8 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_query(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_QUERY:
+ err = f_created_query(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_QUERY:
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..e55a3314bcb0 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_QUERY (F_LINUX_SPECIFIC_BASE + 4)
+
/*
* Cancel a blocking posix lock; internal use only until we expose an
* asynchronous lock api to userspace: