On Wed, Jun 24, 2026 at 07:18:36PM +0800, Sun Jian wrote:
> BPF_OBJ_GET_INFO_BY_FD for bpf_link copies struct bpf_link_info back to
> userspace only when ->fill_link_info() succeeds. Some link info providers,
> however, can return -ENOSPC after computing valid metadata when a nested
> userspace output buffer is too small.
we return the count/size/cnt value when the related user space pointer
is not specified.. we return -ENOSPC when user space pointer is
specified but thits space is not big enough
jirka
>
> For example, perf event tracepoint link info can determine the required
> tp_name length before copying the name fails with -ENOSPC. The current
> top-level error handling returns immediately in that case, so userspace
> observes -ENOSPC but loses the metadata needed to retry with a sufficiently
> large buffer.
>
> Allow bpf_link_get_info_by_fd() to copy the top-level bpf_link_info back
> on -ENOSPC, while still returning -ENOSPC to userspace. Also let perf
> event kprobe, uprobe, and tracepoint link info fill their metadata before
> returning -ENOSPC from nested name buffer copying.
>
> Fixes: f2e10bff16a0 ("bpf: Add support for BPF_OBJ_GET_INFO_BY_FD for
> bpf_link")
> Fixes: 1b715e1b0ec5 ("bpf: Support ->fill_link_info for perf_event")
> Signed-off-by: Sun Jian <[email protected]>
> ---
> kernel/bpf/syscall.c | 18 +++++++++---------
> 1 file changed, 9 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 6db306d23b47..80ab02b1c813 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -4060,7 +4060,7 @@ static int bpf_perf_link_fill_kprobe(const struct
> perf_event *event,
> ulen = info->perf_event.kprobe.name_len;
> err = bpf_perf_link_fill_common(event, uname, &ulen, &offset, &addr,
> &type, &missed);
> - if (err)
> + if (err && err != -ENOSPC)
> return err;
> if (type == BPF_FD_TYPE_KRETPROBE)
> info->perf_event.type = BPF_PERF_EVENT_KRETPROBE;
> @@ -4073,7 +4073,7 @@ static int bpf_perf_link_fill_kprobe(const struct
> perf_event *event,
> addr = 0;
> info->perf_event.kprobe.addr = addr;
> info->perf_event.kprobe.cookie = event->bpf_cookie;
> - return 0;
> + return err;
> }
>
> static void bpf_perf_link_fdinfo_kprobe(const struct perf_event *event,
> @@ -4116,7 +4116,7 @@ static int bpf_perf_link_fill_uprobe(const struct
> perf_event *event,
> ulen = info->perf_event.uprobe.name_len;
> err = bpf_perf_link_fill_common(event, uname, &ulen, &offset,
> &ref_ctr_offset,
> &type, NULL);
> - if (err)
> + if (err && err != -ENOSPC)
> return err;
>
> if (type == BPF_FD_TYPE_URETPROBE)
> @@ -4127,7 +4127,7 @@ static int bpf_perf_link_fill_uprobe(const struct
> perf_event *event,
> info->perf_event.uprobe.offset = offset;
> info->perf_event.uprobe.cookie = event->bpf_cookie;
> info->perf_event.uprobe.ref_ctr_offset = ref_ctr_offset;
> - return 0;
> + return err;
> }
>
> static void bpf_perf_link_fdinfo_uprobe(const struct perf_event *event,
> @@ -4180,13 +4180,13 @@ static int bpf_perf_link_fill_tracepoint(const struct
> perf_event *event,
> uname = u64_to_user_ptr(info->perf_event.tracepoint.tp_name);
> ulen = info->perf_event.tracepoint.name_len;
> err = bpf_perf_link_fill_common(event, uname, &ulen, NULL, NULL, NULL,
> NULL);
> - if (err)
> + if (err && err != -ENOSPC)
> return err;
>
> info->perf_event.type = BPF_PERF_EVENT_TRACEPOINT;
> info->perf_event.tracepoint.name_len = ulen;
> info->perf_event.tracepoint.cookie = event->bpf_cookie;
> - return 0;
> + return err;
> }
>
> static int bpf_perf_link_fill_perf_event(const struct perf_event *event,
> @@ -5536,7 +5536,7 @@ static int bpf_link_get_info_by_fd(struct file *file,
> struct bpf_link_info __user *uinfo = u64_to_user_ptr(attr->info.info);
> struct bpf_link_info info;
> u32 info_len = attr->info.info_len;
> - int err;
> + int err = 0;
>
> err = bpf_check_uarg_tail_zero(USER_BPFPTR(uinfo), sizeof(info),
> info_len);
> if (err)
> @@ -5554,7 +5554,7 @@ static int bpf_link_get_info_by_fd(struct file *file,
>
> if (link->ops->fill_link_info) {
> err = link->ops->fill_link_info(link, &info);
> - if (err)
> + if (err && err != -ENOSPC)
> return err;
> }
>
> @@ -5562,7 +5562,7 @@ static int bpf_link_get_info_by_fd(struct file *file,
> put_user(info_len, &uattr->info.info_len))
> return -EFAULT;
>
> - return 0;
> + return err;
> }
>
>
> --
> 2.43.0
>
