[PATCH v1] LoongArch: BPF: Implement arch_bpf_stack_walk()

From: George Guo <[email protected]>

Implement arch_bpf_stack_walk() on top of the ORC unwinder within
the LoongArch BPF JIT backend to provide generic BPF stack walking
capabilities.

This function is required by advanced BPF features, including timed
may_goto timeout tracing and BPF exceptions. It will be invoked in
the BPF core subsystem unwinding paths: bpf_prog_find_from_stack(),
bpf_stream_stage_dump_stack(), and bpf_throw().

Co-developed-by: Tiezhu Yang <[email protected]>
Signed-off-by: Tiezhu Yang <[email protected]>
Signed-off-by: George Guo <[email protected]>
---
This is an independent feature, so split it from
https://lore.kernel.org/loongarch/[email protected]/
Based on the latest loongarch-next, tested on Loongson-3A6000.

 arch/loongarch/net/bpf_jit.c | 38 ++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
index 7533917c6d47..78c20df2a2f3 100644
--- a/arch/loongarch/net/bpf_jit.c
+++ b/arch/loongarch/net/bpf_jit.c
@@ -2402,6 +2402,44 @@ bool bpf_jit_supports_subprog_tailcalls(void)
 	return true;
 }
 
+#if defined(CONFIG_UNWINDER_ORC)
+#include <asm/unwind.h>
+
+static noinline void walk_bpf_stackframe(bool (*consume_fn)(void *cookie, u64 ip, u64 sp, u64 bp),
+					 void *cookie, unsigned long fp)
+{
+	struct pt_regs dummyregs;
+	struct pt_regs *regs = &dummyregs;
+	struct unwind_state state;
+	unsigned long addr;
+
+	regs->regs[3] = (unsigned long)__builtin_frame_address(0);
+	regs->csr_era = (unsigned long)__builtin_return_address(0);
+	regs->regs[1] = 0;
+	regs->regs[22] = fp;
+
+	for (unwind_start(&state, current, regs);
+	     !unwind_done(&state); unwind_next_frame(&state)) {
+		addr = unwind_get_return_address(&state);
+		if (!addr || !consume_fn(cookie, (u64)addr, (u64)state.sp, (u64)state.fp))
+			break;
+	}
+}
+
+void arch_bpf_stack_walk(bool (*consume_fn)(void *cookie, u64 ip, u64 sp, u64 bp), void *cookie)
+{
+	unsigned long fp;
+
+	/*
+	 * Capture the live frame pointer ($r22) at the very front-line before
+	 * any kernel C code clobbers it. This must be a thin wrapper with no
+	 * large stack locals to prevent the compiler from reusing $r22 early.
+	 */
+	asm volatile("move %0, $r22" : "=r"(fp));
+	walk_bpf_stackframe(consume_fn, cookie, fp);
+}
+#endif /* CONFIG_UNWINDER_ORC */
+
 bool bpf_jit_supports_timed_may_goto(void)
 {
 	return true;
-- 
2.42.0