On Mon, Aug 24, 2026 at 05:28:36PM +0800, Jiayuan Chen wrote:
> The kernel refuses to attach to a nop10 that crosses a page boundary,
> since it can't be atomically rewritten:
>
> /* can_optimize(), arch/x86/kernel/uprobes.c */
> /* We can't do cross page atomic writes yet. */
> return PAGE_SIZE - (vaddr & ~PAGE_MASK) >= OPT_INSN_SIZE;
>
> Whether the nop10 crosses a page is purely up to the binary layout, so
> this does happen in practice. libbpf doesn't check for it and blindly
> shifts the uprobe onto the nop10, and the attach then fails with
> -ENOTSUPP. Just keep the uprobe on the preceding 1-byte nop in that
> case, it works everywhere as a regular int3 uprobe.
>
> Fixes: 41a5c7df4466 ("libbpf: Add support to detect nop,nop5 instructions
> combo for usdt probe")
> Signed-off-by: Jiayuan Chen <[email protected]>
> ---
> tools/lib/bpf/usdt.c | 23 +++++++++++++++++++++--
> 1 file changed, 21 insertions(+), 2 deletions(-)
>
> diff --git a/tools/lib/bpf/usdt.c b/tools/lib/bpf/usdt.c
> index 2e56e3ab5b6c..c266ac93cbdd 100644
> --- a/tools/lib/bpf/usdt.c
> +++ b/tools/lib/bpf/usdt.c
> @@ -614,11 +614,28 @@ static bool has_nop_combo(int fd, long off)
> return false;
> return memcmp(buf, nop_combo, 11) == 0;
> }
> +
> +/*
> + * The kernel refuses to attach to a nop10 that crosses a page boundary,
> + * as it can't be atomically rewritten. Page offset of the probe is the
> + * same in the file and in any mapping, so this can be checked statically.
> + */
> +static bool nop10_within_page(long off)
> +{
> + long page_sz = getpagesize();
> +
> + return off % page_sz + 10 <= page_sz;
could this be another check in has_nop_combo ? so we do not
need to introduce another stub
otherwise lgtm, thanks
jirka
> +}
> #else
> static bool has_nop_combo(int fd, long off)
> {
> return false;
> }
> +
> +static bool nop10_within_page(long off)
> +{
> + return false;
> +}
> #endif
>
> static int collect_usdt_targets(struct usdt_manager *man, struct elf_fd
> *elf_fd, const char *path,
> @@ -827,9 +844,11 @@ static int collect_usdt_targets(struct usdt_manager
> *man, struct elf_fd *elf_fd,
> /*
> * We have uprobe syscall and usdt with nop,nop10 instructions
> combo,
> * so we can place the uprobe directly on nop10 (+1) and get
> this probe
> - * optimized.
> + * optimized. If the nop10 crosses a page boundary, keep the
> uprobe
> + * on the preceding 1-byte nop, which the kernel accepts
> everywhere.
> */
> - if (man->has_uprobe_syscall && has_nop_combo(elf_fd->fd,
> usdt_rel_ip)) {
> + if (man->has_uprobe_syscall && has_nop_combo(elf_fd->fd,
> usdt_rel_ip) &&
> + nop10_within_page(usdt_rel_ip + 1)) {
> usdt_abs_ip++;
> usdt_rel_ip++;
> }
> --
> 2.43.0
>
