From c43c348a84460b003fa757dd43d8c89848ae220d Mon Sep 17 00:00:00 2001 From: Yuan Chen Date: Thu, 24 Sep 2026 10:37:36 +0800 Subject: [PATCH] bpf: Keep target extended until its last freplace link detaches The is_extended / prog_array_member_cnt protocol introduced by commit d6083f040d5d ("bpf: Prevent tailcall infinite loop caused by freplace") keeps a prog extended by a freplace program out of prog_array maps, and vice versa: once a tail call re-enters an extended subprogram, its tail_call_cnt resets on every execution and the loop never terminates. But is_extended is a plain boolean, while one target prog can carry several freplace links at the same time, one on its entry and one on a global subprogram. __bpf_trampoline_unlink_prog() cleared is_extended whenever *any* freplace link detached, so detaching one of two links re-armed the unbounded loop through the remaining one. Replace the is_extended boolean with a count of the freplace links attached to each target prog, so the target stays extended until its last link detaches. Also rename bpf_freplace_check_tgt_prog() to bpf_freplace_link_tgt_prog(), as the helper has never been a pure check: it reserves the target prog on success. Fixes: d6083f040d5d ("bpf: Prevent tailcall infinite loop caused by freplace") Signed-off-by: Yuan Chen Acked-by: Jiri Olsa Link: https://lore.kernel.org/bpf/20260924023737.1140521-2-chenyuan_fl@163.com Signed-off-by: Kumar Kartikeya Dwivedi --- include/linux/bpf.h | 4 ++-- kernel/bpf/arraymap.c | 2 +- kernel/bpf/trampoline.c | 8 ++++---- 3 files changed, 7 insertions(+), 7 deletions(-) diff --git a/include/linux/bpf.h b/include/linux/bpf.h index 9525d3ecd923..1e1ce2afe2ed 100644 --- a/include/linux/bpf.h +++ b/include/linux/bpf.h @@ -1833,7 +1833,6 @@ struct bpf_prog_aux { bool xdp_has_frags; bool exception_cb; bool exception_boundary; - bool is_extended; /* true if extended by freplace program */ bool jits_use_priv_stack; bool priv_stack_requested; bool changes_pkt_data; @@ -1845,7 +1844,7 @@ struct bpf_prog_aux { u8 verdict; } sig; u64 prog_array_member_cnt; /* counts how many times as member of prog_array */ - struct mutex ext_mutex; /* mutex for is_extended and prog_array_member_cnt */ + struct mutex ext_mutex; /* mutex for freplace_link_cnt and prog_array_member_cnt */ struct bpf_arena *arena; void (*recursion_detected)(struct bpf_prog *prog); /* callback if recursion is detected */ /* BTF_KIND_FUNC_PROTO for valid attach_btf_id */ @@ -1877,6 +1876,7 @@ struct bpf_prog_aux { char name[BPF_OBJ_NAME_LEN]; u64 (*bpf_exception_cb)(u64 cookie, u64 sp, u64 bp, u64, u64); u16 stack_arg_sp_adjust; + u16 freplace_link_cnt; /* counts freplace links extending this prog */ #ifdef CONFIG_SECURITY void *security; #endif diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c index 0ce26b538075..0fe9afd4a591 100644 --- a/kernel/bpf/arraymap.c +++ b/kernel/bpf/arraymap.c @@ -1035,7 +1035,7 @@ static void *prog_fd_array_get_ptr(struct bpf_map *map, } mutex_lock(&prog->aux->ext_mutex); - is_extended = prog->aux->is_extended; + is_extended = prog->aux->freplace_link_cnt > 0; if (!is_extended) prog->aux->prog_array_member_cnt++; mutex_unlock(&prog->aux->ext_mutex); diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c index 90b70ea0d370..da85bd580ef0 100644 --- a/kernel/bpf/trampoline.c +++ b/kernel/bpf/trampoline.c @@ -843,7 +843,7 @@ static enum bpf_tramp_prog_type bpf_attach_type_to_tramp(struct bpf_prog *prog) } } -static int bpf_freplace_check_tgt_prog(struct bpf_prog *tgt_prog) +static int bpf_freplace_link_tgt_prog(struct bpf_prog *tgt_prog) { struct bpf_prog_aux *aux = tgt_prog->aux; @@ -857,7 +857,7 @@ static int bpf_freplace_check_tgt_prog(struct bpf_prog *tgt_prog) */ return -EBUSY; - aux->is_extended = true; + aux->freplace_link_cnt++; return 0; } @@ -970,7 +970,7 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_node *node, /* Cannot attach extension if fentry/fexit are in use. */ if (cnt) return -EBUSY; - err = bpf_freplace_check_tgt_prog(tgt_prog); + err = bpf_freplace_link_tgt_prog(tgt_prog); if (err) return err; tr->extension_prog = node->link->prog; @@ -1016,7 +1016,7 @@ static int __bpf_trampoline_unlink_prog(struct bpf_tramp_node *node, tr->extension_prog->bpf_func, NULL); tr->extension_prog = NULL; guard(mutex)(&tgt_prog->aux->ext_mutex); - tgt_prog->aux->is_extended = false; + tgt_prog->aux->freplace_link_cnt--; return err; } bpf_trampoline_remove_prog(tr, node);