On Tue, Aug 25, 2026 at 7:29 AM Jiayuan Chen <[email protected]> wrote: > > > 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
I'd go a step further and collapse three checks, including man->has_uprobe_syscall, into a single "can we do uprobe nop optimization" function? > > > > otherwise lgtm, thanks > > > > jirka > > > Thanks Jirka > > Sound reasonable, I will do it. >
