[PATCH bpf-next v2 1/2] bpf: Introduce bpf_in_interrupt kfunc

[Leon Hwang <leon.hwang@linux.dev>]

Filtering pid_tgid is meaningless when the current task is preempted by
an interrupt.

To address this, introduce the bpf_in_interrupt kfunc, which allows BPF
programs to determine whether they are executing in interrupt context.

This enables programs to avoid applying pid_tgid filtering when running
in such contexts.

Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 kernel/bpf/helpers.c  |  9 +++++++++
 kernel/bpf/verifier.c | 11 +++++++++++
 2 files changed, 20 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 401b4932cc49f..38991b7b4a9e9 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -3711,6 +3711,14 @@ __bpf_kfunc int bpf_strstr(const char *s1__ign, const char *s2__ign)
 	return bpf_strnstr(s1__ign, s2__ign, XATTR_SIZE_MAX);
 }
 
+/**
+ * bpf_in_interrupt - Check whether it's in interrupt context
+ */
+__bpf_kfunc int bpf_in_interrupt(void)
+{
+	return in_interrupt();
+}
+
 __bpf_kfunc_end_defs();
 
 BTF_KFUNCS_START(generic_btf_ids)
@@ -3751,6 +3759,7 @@ BTF_ID_FLAGS(func, bpf_throw)
 #ifdef CONFIG_BPF_EVENTS
 BTF_ID_FLAGS(func, bpf_send_signal_task, KF_TRUSTED_ARGS)
 #endif
+BTF_ID_FLAGS(func, bpf_in_interrupt, KF_FASTCALL)
 BTF_KFUNCS_END(generic_btf_ids)
 
 static const struct btf_kfunc_id_set generic_kfunc_set = {
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5c9dd16b2c56b..e30ecbfc29dad 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -12259,6 +12259,7 @@ enum special_kfunc_type {
 	KF_bpf_res_spin_lock_irqsave,
 	KF_bpf_res_spin_unlock_irqrestore,
 	KF___bpf_trap,
+	KF_bpf_in_interrupt,
 };
 
 BTF_ID_LIST(special_kfunc_list)
@@ -12327,6 +12328,7 @@ BTF_ID(func, bpf_res_spin_unlock)
 BTF_ID(func, bpf_res_spin_lock_irqsave)
 BTF_ID(func, bpf_res_spin_unlock_irqrestore)
 BTF_ID(func, __bpf_trap)
+BTF_ID(func, bpf_in_interrupt)
 
 static bool is_kfunc_ret_null(struct bpf_kfunc_call_arg_meta *meta)
 {
@@ -21977,6 +21979,15 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		   desc->func_id == special_kfunc_list[KF_bpf_rdonly_cast]) {
 		insn_buf[0] = BPF_MOV64_REG(BPF_REG_0, BPF_REG_1);
 		*cnt = 1;
+	} else if (desc->func_id == special_kfunc_list[KF_bpf_in_interrupt]) {
+#if defined(CONFIG_X86_64) && !defined(CONFIG_UML)
+		insn_buf[0] = BPF_MOV64_IMM(BPF_REG_0, (u32)(unsigned long)&__preempt_count);
+		insn_buf[1] = BPF_MOV64_PERCPU_REG(BPF_REG_0, BPF_REG_0);
+		insn_buf[2] = BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, 0);
+		insn_buf[3] = BPF_ALU32_IMM(BPF_AND, BPF_REG_0, NMI_MASK | HARDIRQ_MASK |
+					    (IS_ENABLED(CONFIG_PREEMPT_RT) ? 0 : SOFTIRQ_MASK));
+		*cnt = 4;
+#endif
 	}
 
 	if (env->insn_aux_data[insn_idx].arg_prog) {
-- 
2.50.1