[PATCH 2/3] LoongArch: BPF: Add fsession support for trampolines

Hengqi Chen
2026-02-26T07:00:25Z

Implement BPF_TRACE_FSESSION support in LoongArch BPF JIT.
The logic here is almost identical to what has been done in
RISC-V JIT.

The key changes are:
- Allocate stack space for function meta and session cookies
- Introduce invoke_bpf() as a wrapper around invoke_bpf_prog()
that populates session cookies before each invocation
- Implement bpf_jit_supports_fsession() callback

Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
---
arch/loongarch/net/bpf_jit.c | 78 +++++++++++++++++++++++++++++++-----
1 file changed, 67 insertions(+), 11 deletions(-)

diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
index e3deb0da6a50..16da53f80266 100644
--- a/arch/loongarch/net/bpf_jit.c
+++ b/arch/loongarch/net/bpf_jit.c
@@ -1549,6 +1549,31 @@ static int invoke_bpf_prog(struct jit_ctx *ctx, struct bpf_tramp_link *l,
return ret;
}

+static int invoke_bpf(struct jit_ctx *ctx, struct bpf_tramp_links *tl,
+ int args_off, int retval_off, int run_ctx_off,
+ int func_meta_off, bool save_ret, u64 func_meta,
+ int cookie_off)
+{
+ int i, cur_cookie = (cookie_off - args_off) / 8;
+
+ for (i = 0; i < tl->nr_links; i++) {
+ int err;
+
+ if (bpf_prog_calls_session_cookie(tl->links[i])) {
+ u64 meta = func_meta | ((u64)cur_cookie << BPF_TRAMP_COOKIE_INDEX_SHIFT);
+
+ emit_store_stack_imm64(ctx, LOONGARCH_GPR_T1, -func_meta_off, meta);
+ cur_cookie--;
+ }
+ err = invoke_bpf_prog(ctx, tl->links[i], args_off, retval_off,
+ run_ctx_off, save_ret);
+ if (err)
+ return err;
+ }
+
+ return 0;
+}
+
static void invoke_bpf_mod_ret(struct jit_ctx *ctx, struct bpf_tramp_links *tl,
int args_off, int retval_off, int run_ctx_off, u32 **branches)
{
@@ -1610,13 +1635,15 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
{
int i, ret, save_ret;
int stack_size, nargs;
- int retval_off, args_off, nargs_off, ip_off, run_ctx_off, sreg_off, tcc_ptr_off;
+ int retval_off, args_off, func_meta_off, ip_off, run_ctx_off, sreg_off, tcc_ptr_off;
+ int cookie_off, cookie_cnt;
bool is_struct_ops = flags & BPF_TRAMP_F_INDIRECT;
void *orig_call = func_addr;
struct bpf_tramp_links *fentry = &tlinks[BPF_TRAMP_FENTRY];
struct bpf_tramp_links *fexit = &tlinks[BPF_TRAMP_FEXIT];
struct bpf_tramp_links *fmod_ret = &tlinks[BPF_TRAMP_MODIFY_RETURN];
u32 **branches = NULL;
+ u64 func_meta;

/*
* FP + 8 [ RA to parent func ] return address to parent
@@ -1634,10 +1661,14 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
* [ ... ]
* FP - args_off [ arg1 ]
*
- * FP - nargs_off [ regs count ]
+ * FP - func_meta_off [ regs count, etc ]
*
* FP - ip_off [ traced func ] BPF_TRAMP_F_IP_ARG
*
+ * [ stack cookie N ]
+ * [ ... ]
+ * FP - cookie_off [ stack cookie 1 ]
+ *
* FP - run_ctx_off [ bpf_tramp_run_ctx ]
*
* FP - sreg_off [ callee saved reg ]
@@ -1671,9 +1702,9 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
stack_size += nargs * 8;
args_off = stack_size;

- /* Room of trampoline frame to store args number */
+ /* function metadata, such as regs count */
stack_size += 8;
- nargs_off = stack_size;
+ func_meta_off = stack_size;

/* Room of trampoline frame to store ip address */
if (flags & BPF_TRAMP_F_IP_ARG) {
@@ -1681,6 +1712,11 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
ip_off = stack_size;
}

+ cookie_cnt = bpf_fsession_cookie_cnt(tlinks);
+ /* room for session cookies */
+ stack_size += cookie_cnt * 8;
+ cookie_off = stack_size;
+
/* Room of trampoline frame to store struct bpf_tramp_run_ctx */
stack_size += round_up(sizeof(struct bpf_tramp_run_ctx), 8);
run_ctx_off = stack_size;
@@ -1734,11 +1770,19 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
if (flags & BPF_TRAMP_F_IP_ARG)
emit_store_stack_imm64(ctx, LOONGARCH_GPR_T1, -ip_off, (u64)func_addr);

- /* store nargs number */
- emit_store_stack_imm64(ctx, LOONGARCH_GPR_T1, -nargs_off, nargs);
+ func_meta = nargs;
+ emit_store_stack_imm64(ctx, LOONGARCH_GPR_T1, -func_meta_off, func_meta);

store_args(ctx, nargs, args_off);

+ if (bpf_fsession_cnt(tlinks)) {
+ /* clear all session cookies' value */
+ for (i = 0; i < cookie_cnt; i++)
+ emit_insn(ctx, std, LOONGARCH_GPR_ZERO, LOONGARCH_GPR_FP, -cookie_off + 8 * i);
+ /* clear return value to make sure fentry always get 0 */
+ emit_insn(ctx, std, LOONGARCH_GPR_ZERO, LOONGARCH_GPR_FP, -retval_off);
+ }
+
/* To traced function */
/* Ftrace jump skips 2 NOP instructions */
if (is_kernel_text((unsigned long)orig_call) ||
@@ -1755,9 +1799,10 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
return ret;
}

- for (i = 0; i < fentry->nr_links; i++) {
- ret = invoke_bpf_prog(ctx, fentry->links[i], args_off, retval_off,
- run_ctx_off, flags & BPF_TRAMP_F_RET_FENTRY_RET);
+ if (fentry->nr_links) {
+ ret = invoke_bpf(ctx, fentry, args_off, retval_off, run_ctx_off,
+ func_meta_off, flags & BPF_TRAMP_F_RET_FENTRY_RET,
+ func_meta, cookie_off);
if (ret)
return ret;
}
@@ -1791,8 +1836,14 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
*branches[i] = larch_insn_gen_bne(LOONGARCH_GPR_T1, LOONGARCH_GPR_ZERO, offset);
}

- for (i = 0; i < fexit->nr_links; i++) {
- ret = invoke_bpf_prog(ctx, fexit->links[i], args_off, retval_off, run_ctx_off, false);
+ /* set "is_return" flag for fsession */
+ func_meta |= (1ULL << BPF_TRAMP_IS_RETURN_SHIFT);
+ if (bpf_fsession_cnt(tlinks))
+ emit_store_stack_imm64(ctx, LOONGARCH_GPR_T1, -func_meta_off, func_meta);
+
+ if (fexit->nr_links) {
+ ret = invoke_bpf(ctx, fexit, args_off, retval_off, run_ctx_off,
+ func_meta_off, false, func_meta, cookie_off);
if (ret)
goto out;
}
@@ -2132,3 +2183,8 @@ bool bpf_jit_supports_subprog_tailcalls(void)
{
return true;
}
+
+bool bpf_jit_supports_fsession(void)
+{
+ return true;
+}
--
2.43.5