bpf: Use preempt_count() directly in bpf_send_signal_common()

JIRA: https://issues.redhat.com/browse/RHEL-85486

Conflicts: Context change due to missing commit 6280cf7 ("bpf:
Implement bpf_send_signal_task() kfunc").

commit b4a8b5b
Author: Hou Tao <houtao1@huawei.com>
Date:   Thu Feb 20 12:22:59 2025 +0800

    bpf: Use preempt_count() directly in bpf_send_signal_common()

    bpf_send_signal_common() uses preemptible() to check whether or not the
    current context is preemptible. If it is preemptible, it will use
    irq_work to send the signal asynchronously instead of trying to hold a
    spin-lock, because spin-lock is sleepable under PREEMPT_RT.

    However, preemptible() depends on CONFIG_PREEMPT_COUNT. When
    CONFIG_PREEMPT_COUNT is turned off (e.g., CONFIG_PREEMPT_VOLUNTARY=y),
    !preemptible() will be evaluated as 1 and bpf_send_signal_common() will
    use irq_work unconditionally.

    Fix it by unfolding "!preemptible()" and using "preempt_count() != 0 ||
    irqs_disabled()" instead.

    Fixes: 87c5441 ("bpf: Send signals asynchronously if !preemptible")
    Signed-off-by: Hou Tao <houtao1@huawei.com>
    Link: https://lore.kernel.org/r/20250220042259.1583319-1-houtao@huaweicloud.com
    Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>

Author: jeromemarchand

kernel/trace/bpf_trace.c

@@ -832,7 +832,7 @@ static int bpf_send_signal_common(u32 sig, enum pid_type type)
 	if (unlikely(is_global_init(current)))
 		return -EPERM;
 
-	if (!preemptible()) {
+	if (preempt_count() != 0 || irqs_disabled()) {
 		/* Do an early check on signal validity. Otherwise,
 		 * the error is lost in deferred irq_work.
 		 */