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bpf: add bpf_real_inode() kfunc

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Add a sleepable BPF kfunc that resolves the real inode backing a dentry
via d_real_inode(). On overlay/union filesystems the inode attached to
the dentry is the overlay inode which does not carry the underlying
device information. d_real_inode() resolves through the overlay and
returns the inode from the lower, real filesystem.

This is used in the RestrictFilesytemAccess bpf program that has been
merged into systemd a little while ago.

Link: https://github.com/systemd/systemd/pull/41340 [1]
Link: https://patch.msgid.link/20260526-work-bpf-verity-v2-1-cd0b1850d31b@kernel.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
This commit is contained in:
Christian Brauner
2026-05-26 14:02:22 +02:00
parent 1a571226cb
commit 9af8c8a54f
1 changed files with 16 additions and 0 deletions
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16
fs/bpf_fs_kfuncs.c
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@@ -353,6 +353,21 @@ __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
*
* 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.
*
* Return: The real inode backing the dentry, or NULL for a negative dentry.
*/
__bpf_kfunc struct inode *bpf_real_inode(struct dentry *dentry)
{
return d_real_inode(dentry);
}
__bpf_kfunc_end_defs();
BTF_KFUNCS_START(bpf_fs_kfunc_set_ids)
@@ -363,6 +378,7 @@ 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_KFUNCS_END(bpf_fs_kfunc_set_ids)
static int bpf_fs_kfuncs_filter(const struct bpf_prog *prog, u32 kfunc_id)