bpf: Reject sleepable kprobe_multi programs at attach time

bpf: Reject sleepable kprobe_multi programs at attach time

kprobe.multi programs run in atomic/RCU context and cannot sleep.
However, bpf_kprobe_multi_link_attach() did not validate whether the
program being attached had the sleepable flag set, allowing sleepable
helpers such as bpf_copy_from_user() to be invoked from a non-sleepable
context.

This causes a "sleeping function called from invalid context" splat:

  BUG: sleeping function called from invalid context at ./include/linux/uaccess.h:169
  in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 1787, name: sudo
  preempt_count: 1, expected: 0
  RCU nest depth: 2, expected: 0

Fix this by rejecting sleepable programs early in
bpf_kprobe_multi_link_attach(), before any further processing.

Fixes: 0dcac2725406 ("bpf: Add multi kprobe link")
Signed-off-by: Varun R Mallya <varunrmallya@gmail.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Acked-by: Leon Hwang <leon.hwang@linux.dev>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20260401191126.440683-1-varunrmallya@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>