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 <[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
Thanks Jirka Sound reasonable, I will do it.
