Re: [PATCH bpf 1/2] libbpf: Fix usdt attach failure when nop10 crosses page boundary

[Jiri Olsa <olsajiri@gmail.com>]

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 <jiayuan.chen@linux.dev>
> ---
>  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
>