Re: [PATCH v4 bpf-next 4/6] bpf: fs/xattr: Add BPF kfuncs to set and remove xattrs

[Alexei Starovoitov]

On Mon, Dec 16, 2024 at 10:38 PM Song Liu <song@xxxxxxxxxx> wrote:
>
> Add the following kfuncs to set and remove xattrs from BPF programs:
>
>   bpf_set_dentry_xattr
>   bpf_remove_dentry_xattr
>   bpf_set_dentry_xattr_locked
>   bpf_remove_dentry_xattr_locked
>
> The _locked version of these kfuncs are called from hooks where
> dentry->d_inode is already locked.

...

> + *
> + * Setting and removing xattr requires exclusive lock on dentry->d_inode.
> + * Some hooks already locked d_inode, while some hooks have not locked
> + * d_inode. Therefore, we need different kfuncs for different hooks.
> + * Specifically, hooks in the following list (d_inode_locked_hooks)
> + * should call bpf_[set|remove]_dentry_xattr_locked; while other hooks
> + * should call bpf_[set|remove]_dentry_xattr.
> + */

the inode locking rules might change, so let's hide this
implementation detail from the bpf progs by making kfunc polymorphic.

To struct bpf_prog_aux add:
bool use_locked_kfunc:1;
and set it in bpf_check_attach_target() if it's attaching
to one of d_inode_locked_hooks

Then in fixup_kfunc_call() call some helper that
if (prog->aux->use_locked_kfunc &&
    insn->imm == special_kfunc_list[KF_bpf_remove_dentry_xattr])
     insn->imm = special_kfunc_list[KF_bpf_remove_dentry_xattr_locked];

The progs will be simpler and will suffer less churn
when the kernel side changes.