On Wed, 8 Apr 2026 11:02:37 +0900 Masami Hiramatsu (Google) <[email protected]> wrote:
> Hmm, > > I also found another problem when probing module with fprobe. > > /sys/kernel/tracing # insmod xt_LOG.ko > /sys/kernel/tracing # echo 'f:test log_tg*' > dynamic_events > /sys/kernel/tracing # echo 1 > events/fprobes/test/enable > /sys/kernel/tracing # cat enabled_functions > log_tg [xt_LOG] (1) tramp: 0xffffffffa0004000 > (fprobe_ftrace_entry+0x0/0x490) ->fprobe_ftrace_entry+0x0/0x490 > log_tg_check [xt_LOG] (1) tramp: 0xffffffffa0004000 > (fprobe_ftrace_entry+0x0/0x490) ->fprobe_ftrace_entry+0x0/0x490 > log_tg_destroy [xt_LOG] (1) tramp: 0xffffffffa0004000 > (fprobe_ftrace_entry+0x0/0x490) ->fprobe_ftrace_entry+0x0/0x490 > /sys/kernel/tracing # wc -l enabled_functions > 3 enabled_functions > /sys/kernel/tracing # rmmod xt_LOG > /sys/kernel/tracing # wc -l enabled_functions > 34085 enabled_functions > > It seems to reverse the selected functions if the hash is empty... So I think there are several ways to fix this issue. - Introduce a new flag for ftrace_ops, which is "empty means empty". - Improve ftrace to automatically remove module function entries from registered ftrace_ops. (currently it is done for the ftrace_ops in trace_array list, but not for ftrace_ops list.) - While walking over the fprobe_ip_tables, try to find existing enabled fprobe node. If it does not exist, unregister ftrace_ops. - Count the ftrace_ops hash table size and if it is expected to be 0, unregister the ftrace_ops. The first 2 are changing ftrace (but cleaner). The latter 2 only changes fprobes, but relatively ugly. I think the 2nd one will be the best because we already have done for ftrace_ops of ring-buffer instances. Thanks, > > Thanks, > > On Tue, 7 Apr 2026 19:24:12 +0900 > "Masami Hiramatsu (Google)" <[email protected]> wrote: > > > From: Masami Hiramatsu (Google) <[email protected]> > > > > Commit 2c67dc457bc6 ("tracing: fprobe: optimization for entry only case") > > introduced a different ftrace_ops for entry-only fprobes. > > > > However, when unregistering an fprobe, the kernel only checks if another > > fprobe exists at the same address, without checking which type of fprobe > > it is. > > If different fprobes are registered at the same address, the same address > > will be registered in both fgraph_ops and ftrace_ops, but only one of > > them will be deleted when unregistering. (the one removed first will not > > be deleted from the ops). > > > > This results in junk entries remaining in either fgraph_ops or ftrace_ops. > > For example: > > ======= > > cd /sys/kernel/tracing > > > > # 'Add entry and exit events on the same place' > > echo 'f:event1 vfs_read' >> dynamic_events > > echo 'f:event2 vfs_read%return' >> dynamic_events > > > > # 'Enable both of them' > > echo 1 > events/fprobes/enable > > cat enabled_functions > > vfs_read (2) ->arch_ftrace_ops_list_func+0x0/0x210 > > > > # 'Disable and remove exit event' > > echo 0 > events/fprobes/event2/enable > > echo -:event2 >> dynamic_events > > > > # 'Disable and remove all events' > > echo 0 > events/fprobes/enable > > echo > dynamic_events > > > > # 'Add another event' > > echo 'f:event3 vfs_open%return' > dynamic_events > > cat dynamic_events > > f:fprobes/event3 vfs_open%return > > > > echo 1 > events/fprobes/enable > > cat enabled_functions > > vfs_open (1) tramp: 0xffffffffa0001000 > > (ftrace_graph_func+0x0/0x60) ->ftrace_graph_func+0x0/0x60 subops: > > {ent:fprobe_fgraph_entry+0x0/0x620 ret:fprobe_return+0x0/0x150} > > vfs_read (1) tramp: 0xffffffffa0001000 > > (ftrace_graph_func+0x0/0x60) ->ftrace_graph_func+0x0/0x60 subops: > > {ent:fprobe_fgraph_entry+0x0/0x620 ret:fprobe_return+0x0/0x150} > > ======= > > > > As you can see, an entry for the vfs_read remains. > > > > To fix this issue, when unregistering, the kernel should also check if > > there is the same type of fprobes still exist at the same address, and > > if not, delete its entry from either fgraph_ops or ftrace_ops. > > > > Fixes: 2c67dc457bc6 ("tracing: fprobe: optimization for entry only case") > > Cc: [email protected] > > Signed-off-by: Masami Hiramatsu (Google) <[email protected]> > > --- > > kernel/trace/fprobe.c | 77 > > +++++++++++++++++++++++++++++++++++++++---------- > > 1 file changed, 62 insertions(+), 15 deletions(-) > > > > diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c > > index dcadf1d23b8a..7f75e6e4462c 100644 > > --- a/kernel/trace/fprobe.c > > +++ b/kernel/trace/fprobe.c > > @@ -85,11 +85,9 @@ static int insert_fprobe_node(struct fprobe_hlist_node > > *node) > > return rhltable_insert(&fprobe_ip_table, &node->hlist, > > fprobe_rht_params); > > } > > > > -/* Return true if there are synonims */ > > -static bool delete_fprobe_node(struct fprobe_hlist_node *node) > > +static void delete_fprobe_node(struct fprobe_hlist_node *node) > > { > > lockdep_assert_held(&fprobe_mutex); > > - bool ret; > > > > /* Avoid double deleting */ > > if (READ_ONCE(node->fp) != NULL) { > > @@ -97,13 +95,6 @@ static bool delete_fprobe_node(struct fprobe_hlist_node > > *node) > > rhltable_remove(&fprobe_ip_table, &node->hlist, > > fprobe_rht_params); > > } > > - > > - rcu_read_lock(); > > - ret = !!rhltable_lookup(&fprobe_ip_table, &node->addr, > > - fprobe_rht_params); > > - rcu_read_unlock(); > > - > > - return ret; > > } > > > > /* Check existence of the fprobe */ > > @@ -337,6 +328,32 @@ static bool fprobe_is_ftrace(struct fprobe *fp) > > return !fp->exit_handler; > > } > > > > +static bool fprobe_exists_on_hash(unsigned long ip, bool ftrace) > > +{ > > + struct rhlist_head *head, *pos; > > + struct fprobe_hlist_node *node; > > + struct fprobe *fp; > > + > > + guard(rcu)(); > > + head = rhltable_lookup(&fprobe_ip_table, &ip, > > + fprobe_rht_params); > > + if (!head) > > + return false; > > + /* We have to check the same type on the list. */ > > + rhl_for_each_entry_rcu(node, pos, head, hlist) { > > + if (node->addr != ip) > > + break; > > + fp = READ_ONCE(node->fp); > > + if (likely(fp)) { > > + if ((!ftrace && fp->exit_handler) || > > + (ftrace && !fp->exit_handler)) > > + return true; > > + } > > + } > > + > > + return false; > > +} > > + > > #ifdef CONFIG_MODULES > > static void fprobe_set_ips(unsigned long *ips, unsigned int cnt, int > > remove, > > int reset) > > @@ -360,6 +377,29 @@ static bool fprobe_is_ftrace(struct fprobe *fp) > > return false; > > } > > > > +static bool fprobe_exists_on_hash(unsigned long ip, bool ftrace > > __maybe_unused) > > +{ > > + struct rhlist_head *head, *pos; > > + struct fprobe_hlist_node *node; > > + struct fprobe *fp; > > + > > + guard(rcu)(); > > + head = rhltable_lookup(&fprobe_ip_table, &ip, > > + fprobe_rht_params); > > + if (!head) > > + return false; > > + /* We only need to check fp is there. */ > > + rhl_for_each_entry_rcu(node, pos, head, hlist) { > > + if (node->addr != ip) > > + break; > > + fp = READ_ONCE(node->fp); > > + if (likely(fp)) > > + return true; > > + } > > + > > + return false; > > +} > > + > > #ifdef CONFIG_MODULES > > static void fprobe_set_ips(unsigned long *ips, unsigned int cnt, int > > remove, > > int reset) > > @@ -574,15 +614,20 @@ static int fprobe_addr_list_add(struct > > fprobe_addr_list *alist, unsigned long ad > > static void fprobe_remove_node_in_module(struct module *mod, struct > > fprobe_hlist_node *node, > > struct fprobe_addr_list *alist) > > { > > + lockdep_assert_in_rcu_read_lock(); > > + > > if (!within_module(node->addr, mod)) > > return; > > - if (delete_fprobe_node(node)) > > - return; > > + > > + delete_fprobe_node(node); > > /* > > - * If failed to update alist, just continue to update hlist. > > + * Ignore failure of updating alist, but continue to update hlist. > > * Therefore, at list user handler will not hit anymore. > > + * And don't care the type here, because all fprobes on the same > > + * address must be removed eventually. > > */ > > - fprobe_addr_list_add(alist, node->addr); > > + if (!rhltable_lookup(&fprobe_ip_table, &node->addr, fprobe_rht_params)) > > + fprobe_addr_list_add(alist, node->addr); > > } > > > > /* Handle module unloading to manage fprobe_ip_table. */ > > @@ -943,7 +988,9 @@ int unregister_fprobe(struct fprobe *fp) > > /* Remove non-synonim ips from table and hash */ > > count = 0; > > for (i = 0; i < hlist_array->size; i++) { > > - if (!delete_fprobe_node(&hlist_array->array[i])) > > + delete_fprobe_node(&hlist_array->array[i]); > > + if (!fprobe_exists_on_hash(hlist_array->array[i].addr, > > + fprobe_is_ftrace(fp))) > > addrs[count++] = hlist_array->array[i].addr; > > } > > del_fprobe_hash(fp); > > > > > -- > Masami Hiramatsu (Google) <[email protected]> -- Masami Hiramatsu (Google) <[email protected]>
