libbpf: Fix usdt attach failure when nop10 crosses page boundary

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: ee2862439e5c ("libbpf: Change has_nop_combo to work on top of nop10")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260825150444.31603-1-jiayuan.chen@linux.dev

src/usdt.c

@@ -608,8 +608,12 @@ static bool has_nop_combo(int fd, long off)
 	unsigned char nop_combo[11] = {
 		0x90, 0x66, 0x2e, 0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00,
 	};
+	long page_sz = getpagesize();
 	unsigned char buf[11];
 
+	/* the kernel can't attach to a nop10 that crosses a page boundary */
+	if ((off + 1) % page_sz + 10 > page_sz)
+		return false;
 	if (pread(fd, buf, 11, off) != 11)
 		return false;
 	return memcmp(buf, nop_combo, 11) == 0;