Re: [PATCH RFC v2 08/15] bpf, x86: Maintain Tasks RCU trampoline nesting in the BPF trampoline

Emit an increment of current->rcu_tramp_nesting once the trampoline's
frame is set up and a decrement before the final register restore, so
that a task preempted while running fentry/fexit/fmod_ret/LSM programs
or the __bpf_tramp_enter()/__bpf_tramp_exit() glue is not treated as
Tasks-RCU quiescent.  Drop the count around the call to the original
function: that may run arbitrarily long without sleeping and must not pin
a Tasks RCU grace period, and the trampoline frame above it is held by
im->pcref rather than by Tasks RCU (see bpf_tramp_image_put()).  The
fmod_ret early-exit branch and the ip_after_call -> ip_epilogue poke both
skip the decrement/increment pair around the original call, so the count
stays balanced on every path.

The sequence is "mov r11, gs:[current_task]; inc/dec dword [r11 + off]";
r11 is scratch at every emission point and (u32)&current_task is a valid
sign-extended %gs-absolute with the current per-CPU layout, the same form
the JIT already uses for this_cpu_off.  The image is dynamically
allocated text, so the instructions outside the bracketed region are
covered by the irq-exit IP check.

Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 arch/x86/net/bpf_jit_comp.c | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 2853e87797a7..a375c1b7bd50 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c

@@ -722,6 +722,31 @@ static void emit_indirect_jump(u8 **pprog, int bpf_reg, u8 *ip)
 	*pprog = prog;
 }
 
+/*
+ * Tasks RCU trampoline nesting, see rcu_tasks_trampoline_enter().
+ *
+ *   mov r11, QWORD PTR gs:[current_task]
+ *   inc/dec DWORD PTR [r11 + offsetof(struct task_struct, rcu_tramp_nesting)]
+ *
+ * r11 (AUX_REG) is scratch in the trampoline at every point this is emitted.
+ */
+static void emit_rcu_tasks_tramp_nesting(u8 **pprog, bool enter)
+{
+#ifdef CONFIG_TASKS_RCU
+	u8 *prog = *pprog;
+
+	/* mov r11, gs:[abs32] */
+	EMIT2(0x65, 0x4C);
+	EMIT3(0x8B, 0x1C, 0x25);
+	EMIT((u32)(unsigned long)&current_task, 4);
+	/* inc/dec dword ptr [r11 + disp32] */
+	EMIT3(0x41, 0xFF, enter ? 0x83 : 0x8B);
+	EMIT(offsetof(struct task_struct, rcu_tramp_nesting), 4);
+
+	*pprog = prog;
+#endif