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

[Jiayuan Chen <jiayuan.chen@linux.dev>]

On 8/25/26 9:58 PM, Jiri Olsa wrote:
> 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
>
Thanks Jirka

Sound reasonable, I will do it.