fs: report f_fsid from s_dev for "simple" filesystems

There are many "simple" filesystems (*) that report null f_fsid in
statfs(2).  Those "simple" filesystems report sb->s_dev as the st_dev
field of the stat syscalls for all inodes of the filesystem (**).

In order to enable fanotify reporting of events with fsid on those
"simple" filesystems, report the sb->s_dev number in f_fsid field of
statfs(2).

(*) For most of the "simple" filesystem refered to in this commit, the
->statfs() operation is simple_statfs(). Some of those fs assign the
simple_statfs() method directly in their ->s_op struct and some assign it
indirectly via a call to simple_fill_super() or to pseudo_fs_fill_super()
with either custom or "simple" s_op.
We also make the same change to efivarfs and hugetlbfs, although they do
not use simple_statfs(), because they use the simple_* inode opreations
(e.g. simple_lookup()).

(**) For most of the "simple" filesystems, the ->getattr() method is not
assigned, so stat() is implemented by generic_fillattr().  A few "simple"
filesystem use the simple_getattr() method which also calls
generic_fillattr() to fill most of the stat struct.

The two exceptions are procfs and 9p. procfs implements several different
->getattr() methods, but they all end up calling generic_fillattr() to
fill the st_dev field from sb->s_dev.

9p has more complicated ->getattr() methods, but they too, end up calling
generic_fillattr() to fill the st_dev field from sb->s_dev.

Note that 9p and kernfs also call simple_statfs() from custom ->statfs()
methods which already fill the f_fsid field, but v9fs_statfs() calls
simple_statfs() only in case f_fsid was not filled and kenrfs_statfs()
overwrites f_fsid after calling simple_statfs().

Link: https://lore.kernel.org/r/20230919094820.g5bwharbmy2dq46w@quack3/
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/r/20231023143049.2944970-1-amir73il@gmail.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/fs/efivarfs/super.c b/fs/efivarfs/super.c
index 996271473609..2933090ad11f 100644
--- a/fs/efivarfs/super.c
+++ b/fs/efivarfs/super.c
@@ -30,6 +30,7 @@ static int efivarfs_statfs(struct dentry *dentry, struct kstatfs *buf)
 			 EFI_VARIABLE_BOOTSERVICE_ACCESS |
 			 EFI_VARIABLE_RUNTIME_ACCESS;
 	u64 storage_space, remaining_space, max_variable_size;
+	u64 id = huge_encode_dev(dentry->d_sb->s_dev);
 	efi_status_t status;
 
 	/* Some UEFI firmware does not implement QueryVariableInfo() */
@@ -53,6 +54,7 @@ static int efivarfs_statfs(struct dentry *dentry, struct kstatfs *buf)
 	buf->f_blocks	= storage_space;
 	buf->f_bfree	= remaining_space;
 	buf->f_type	= dentry->d_sb->s_magic;
+	buf->f_fsid	= u64_to_fsid(id);
 
 	/*
 	 * In f_bavail we declare the free space that the kernel will allow writing