On Tue, 17 Dec 2024 at 19:25, Song Liu <songliubraving@xxxxxxxx> wrote: > > Hi Alexei, > > Thanks for the review! > > > On Dec 17, 2024, at 8:50 AM, Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > > > > 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. > > I was thinking about something in similar direction. > > If we do this, shall we somehow hide the _locked version of the > kfuncs, so that the user cannot use it? If so, what's the best > way to do it? Just don't add BTF_ID_FLAGS entries for them. You'd also need to make an extra call to add_kfunc_call to add its details before you can do the fixup. That allows find_kfunc_desc to work. I did something similar in earlier versions of resilient locks. In add_kfunc_call's end (instead of directly returning): func_id = get_shadow_kfunc_id(func_id, offset); if (!func_id) return err; return add_kfunc_call(env, func_id, offset); Then check in fixup_kfunc_call to find shadow kfunc id and substitute imm. Can use some other naming instead of "shadow". Probably need to take a prog pointer to make a decision to find the underlying kfunc id in your case. > > Thanks, > Song >
