[PATCH v2] bpf: have bpf_real_data_inode() take a struct file

[Christian Brauner]

bpf_real_data_inode() must be usable from the bprm_check_security,
mmap_file and file_mprotect hooks for systemd's RestrictFilesystemAccess
BPF LSM program, so have it take a struct file instead of a dentry.

Amir Goldstein <amir73il@xxxxxxxxx> suggests:

  While doing so, rename it from bpf_real_inode() to
  bpf_real_data_inode(). For a regular file on a union/overlay
  filesystem it resolves to the underlying inode that hosts the data,
  but for a non-regular file it returns the overlay inode. The new name
  makes the "inode hosting the data" intent explicit and avoids the
  ambiguity of "the real inode backing a file". Document the
  non-regular-file behavior in the kfunc too.

Both the signature change and the rename are safe because the kfunc
landed this cycle and has no released users.

Fixes: 9af8c8a54f6e ("bpf: add bpf_real_inode() kfunc")
Signed-off-by: Christian Brauner (Amutable) <brauner@xxxxxxxxxx>
---
Changes in v2:
- Rename bpf_real_inode() -> bpf_real_data_inode() (Amir); the new name
  makes the "inode hosting the data" intent explicit.
- Document that the kfunc returns the overlay inode for non-regular files.
- Link to v1: https://patch.msgid.link/20260622-work-bpf-real_inode-v1-1-4014eb4cbefd@xxxxxxxxxx
---
 fs/bpf_fs_kfuncs.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/fs/bpf_fs_kfuncs.c b/fs/bpf_fs_kfuncs.c
index 768aca2dc0f0..f1863a891db6 100644
--- a/fs/bpf_fs_kfuncs.c
+++ b/fs/bpf_fs_kfuncs.c
@@ -360,18 +360,23 @@ __bpf_kfunc int bpf_cgroup_read_xattr(struct cgroup *cgroup, const char *name__s
 #endif /* CONFIG_CGROUPS */
 
 /**
- * bpf_real_inode - get the real inode backing a dentry
- * @dentry: dentry to resolve
+ * bpf_real_data_inode - get the real inode hosting a file's data
+ * @file: file to resolve
  *
- * If the dentry is on a union/overlay filesystem, return the underlying, real
- * inode that hosts the data.  Otherwise return the inode attached to the
- * dentry itself.
+ * Resolve @file to the inode that hosts its data. For a regular file on a
+ * union/overlay filesystem this is the underlying (upper or lower) inode that
+ * stores the data, not the overlay inode.
  *
- * Return: The real inode backing the dentry, or NULL for a negative dentry.
+ * Data resolution only applies to regular files. For a non-regular file (e.g.
+ * a device node, fifo or socket) on a union/overlay filesystem the overlay
+ * inode itself is returned; for any file on a non-union filesystem the inode
+ * attached to @file is returned.
+ *
+ * Return: The inode hosting @file's data, or NULL.
  */
-__bpf_kfunc struct inode *bpf_real_inode(struct dentry *dentry)
+__bpf_kfunc struct inode *bpf_real_data_inode(struct file *file)
 {
-	return d_real_inode(dentry);
+	return d_real_inode(file_dentry(file));
 }
 
 __bpf_kfunc_end_defs();
@@ -384,7 +389,7 @@ BTF_ID_FLAGS(func, bpf_get_dentry_xattr, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_get_file_xattr, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_set_dentry_xattr, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_remove_dentry_xattr, KF_SLEEPABLE)
-BTF_ID_FLAGS(func, bpf_real_inode, KF_SLEEPABLE | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_real_data_inode, KF_SLEEPABLE | KF_RET_NULL)
 BTF_KFUNCS_END(bpf_fs_kfunc_set_ids)
 
 static int bpf_fs_kfuncs_filter(const struct bpf_prog *prog, u32 kfunc_id)

---
base-commit: 5b33fc6492a7b7a62359157db0f92f5b6e9af690
change-id: 20260622-work-bpf-real_inode-c3202dbaf0ab