[PATCH 1/2] powerpc64/bpf: Implement JIT support for private stack
adubey
adubey at linux.ibm.com
Tue Feb 24 06:24:01 AEDT 2026
On 2026-02-23 16:40, Christophe Leroy (CS GROUP) wrote:
> Le 16/02/2026 à 16:22, adubey at linux.ibm.com a écrit :
>> From: Abhishek Dubey <adubey at linux.ibm.com>
>>
>> Provision the private stack as a per-CPU allocation during
>> bpf_int_jit_compile(). Align the stack to 16 bytes and place guard
>> regions at both ends to detect runtime stack overflow and underflow.
>>
>> Round the private stack size up to the nearest 16-byte boundary.
>> Make each guard region 16 bytes to preserve the required overall
>> 16-byte alignment. When private stack is set, skip bpf stack size
>> accounting in kernel stack.
>>
>> There is no stack pointer in powerpc. Stack referencing during JIT
>> is done using frame pointer. Frame pointer calculation goes like:
>>
>> BPF frame pointer = Priv stack allocation start address +
>> Overflow guard +
>> Actual stack size defined by verifier
>>
>> Memory layout:
>>
>> High Addr
>> +--------------------------------------------------+
>> |
>> |
>> | 16 bytes Underflow guard (0xEB9F12345678eb9fULL)
>> |
>> |
>> |
>> BPF FP ->
>> +--------------------------------------------------+
>> |
>> |
>> | Private stack - determined by verifier
>> |
>> | 16-bytes aligned
>> |
>> |
>> |
>>
>> +--------------------------------------------------+
>> |
>> |
>> Lower Addr | 16 byte Overflow guard (0xEB9F12345678eb9fULL)
>> |
>> |
>> |
>> Priv stack alloc
>> ->+--------------------------------------------------+
>> start
>>
>> Update BPF_REG_FP to point to the calculated offset within the
>> allocated private stack buffer. Now, BPF stack usage reference
>> in the allocated private stack.
>>
>> The patch is rebase over fixes by Hari:
>> https://lore.kernel.org/bpf/20260216065639.1750181-1-hbathini@linux.ibm.com/T/#mf02cad9096fa4ad1f05610b1f464da1cddf7445a
>>
>> Signed-off-by: Abhishek Dubey <adubey at linux.ibm.com>
>> ---
>> arch/powerpc/net/bpf_jit.h | 5 +++
>> arch/powerpc/net/bpf_jit_comp.c | 74
>> +++++++++++++++++++++++++++++++
>> arch/powerpc/net/bpf_jit_comp64.c | 38 +++++++++++++---
>> 3 files changed, 112 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
>> index 7354e1d72f79..5a115c54e43a 100644
>> --- a/arch/powerpc/net/bpf_jit.h
>> +++ b/arch/powerpc/net/bpf_jit.h
>> @@ -178,8 +178,13 @@ struct codegen_context {
>> bool is_subprog;
>> bool exception_boundary;
>> bool exception_cb;
>> + u64 priv_sp;
>
> Are you sure you want a u64, not a long ?
After bot's warning, I was considering void __percpu *priv_sp in
codegen_context, to keep compiler happy throughout.
If not above, I would prefer long(~= pointer). Thoughts? @Christophe
>
>> };
>> +/* Memory size & magic-value to detect private stack
>> overflow/underflow */
>> +#define PRIV_STACK_GUARD_SZ 16
>> +#define PRIV_STACK_GUARD_VAL 0xEB9F12345678eb9fULL
>> +
>> #define bpf_to_ppc(r) (ctx->b2p[r])
>> #ifdef CONFIG_PPC32
>> diff --git a/arch/powerpc/net/bpf_jit_comp.c
>> b/arch/powerpc/net/bpf_jit_comp.c
>> index 278e09b57560..7a78e03d482f 100644
>> --- a/arch/powerpc/net/bpf_jit_comp.c
>> +++ b/arch/powerpc/net/bpf_jit_comp.c
>> @@ -129,6 +129,39 @@ bool bpf_jit_needs_zext(void)
>> return true;
>> }
>> +static void priv_stack_init_guard(void __percpu *priv_stack_ptr,
>> int alloc_size)
>> +{
>> + int cpu, underflow_idx = (alloc_size - PRIV_STACK_GUARD_SZ) >> 3;
>> + u64 *stack_ptr;
>> +
>> + for_each_possible_cpu(cpu) {
>> + stack_ptr = per_cpu_ptr(priv_stack_ptr, cpu);
>> + stack_ptr[0] = PRIV_STACK_GUARD_VAL;
>> + stack_ptr[1] = PRIV_STACK_GUARD_VAL;
>> + stack_ptr[underflow_idx] = PRIV_STACK_GUARD_VAL;
>> + stack_ptr[underflow_idx + 1] = PRIV_STACK_GUARD_VAL;
>> + }
>> +}
>> +
>> +static void priv_stack_check_guard(void __percpu *priv_stack_ptr, int
>> alloc_size,
>> + struct bpf_prog *fp)
>> +{
>> + int cpu, underflow_idx = (alloc_size - PRIV_STACK_GUARD_SZ) >> 3;
>> + u64 *stack_ptr;
>> +
>> + for_each_possible_cpu(cpu) {
>> + stack_ptr = per_cpu_ptr(priv_stack_ptr, cpu);
>> + if (stack_ptr[0] != PRIV_STACK_GUARD_VAL ||
>> + stack_ptr[1] != PRIV_STACK_GUARD_VAL ||
>> + stack_ptr[underflow_idx] != PRIV_STACK_GUARD_VAL ||
>> + stack_ptr[underflow_idx + 1] != PRIV_STACK_GUARD_VAL) {
>> + pr_err("BPF private stack overflow/underflow detected for prog
>> %sx\n",
>> + bpf_jit_get_prog_name(fp));
>> + break;
>> + }
>> + }
>> +}
>> +
>> struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
>> {
>> u32 proglen;
>> @@ -140,6 +173,8 @@ struct bpf_prog *bpf_int_jit_compile(struct
>> bpf_prog *fp)
>> struct codegen_context cgctx;
>> int pass;
>> int flen;
>> + int priv_stack_alloc_size;
>> + void __percpu *priv_stack_ptr = NULL;
>> struct bpf_binary_header *fhdr = NULL;
>> struct bpf_binary_header *hdr = NULL;
>> struct bpf_prog *org_fp = fp;
>> @@ -173,6 +208,25 @@ struct bpf_prog *bpf_int_jit_compile(struct
>> bpf_prog *fp)
>> fp->aux->jit_data = jit_data;
>> }
>> + if (!priv_stack_ptr && fp->aux->jits_use_priv_stack) {
>> + /*
>> + * Allocate private stack of size equivalent to
>> + * verifier-calculated stack size plus two memory
>> + * guard regions to detect private stack overflow
>> + * and underflow.
>> + */
>> + priv_stack_alloc_size = round_up(fp->aux->stack_depth, 16) +
>> + 2 * PRIV_STACK_GUARD_SZ;
>> + priv_stack_ptr = __alloc_percpu_gfp(priv_stack_alloc_size, 16,
>> GFP_KERNEL);
>> + if (!priv_stack_ptr) {
>> + fp = org_fp;
>> + goto out_priv_stack;
>> + }
>> +
>> + priv_stack_init_guard(priv_stack_ptr, priv_stack_alloc_size);
>> + fp->aux->priv_stack_ptr = priv_stack_ptr;
>> + }
>> +
>> flen = fp->len;
>> addrs = jit_data->addrs;
>> if (addrs) {
>> @@ -209,6 +263,7 @@ struct bpf_prog *bpf_int_jit_compile(struct
>> bpf_prog *fp)
>> cgctx.is_subprog = bpf_is_subprog(fp);
>> cgctx.exception_boundary = fp->aux->exception_boundary;
>> cgctx.exception_cb = fp->aux->exception_cb;
>> + cgctx.priv_sp = priv_stack_ptr ? (u64)priv_stack_ptr : 0;
>
> priv_stack_ptr is a pointer, ie a long.
> priv_sp is a u64, ie a long long.
Ack!
>
> This will mismatch on powerpc/32
long/void* should work fine everywhere.
>
>> /* Scouting faux-generate pass 0 */
>> if (bpf_jit_build_body(fp, NULL, NULL, &cgctx, addrs, 0, false)) {
>> @@ -306,7 +361,12 @@ struct bpf_prog *bpf_int_jit_compile(struct
>> bpf_prog *fp)
>> }
>> bpf_prog_fill_jited_linfo(fp, addrs);
>> out_addrs:
>> + if (!image && priv_stack_ptr) {
>> + free_percpu(priv_stack_ptr);
>> + fp->aux->priv_stack_ptr = NULL;
>
> Shouldn't those two lines be swapped ?
Good catch! This can lead to use-after-free situation.
>
>> + }
>> kfree(addrs);
>> +out_priv_stack:
>> kfree(jit_data);
>> fp->aux->jit_data = NULL;
>> } else {
>> @@ -419,6 +479,8 @@ void bpf_jit_free(struct bpf_prog *fp)
>> if (fp->jited) {
>> struct powerpc_jit_data *jit_data = fp->aux->jit_data;
>> struct bpf_binary_header *hdr;
>> + void __percpu *priv_stack_ptr;
>> + int priv_stack_alloc_size;
>> /*
>> * If we fail the final pass of JIT (from jit_subprogs),
>> @@ -432,6 +494,13 @@ void bpf_jit_free(struct bpf_prog *fp)
>> }
>> hdr = bpf_jit_binary_pack_hdr(fp);
>> bpf_jit_binary_pack_free(hdr, NULL);
>> + priv_stack_ptr = fp->aux->priv_stack_ptr;
>> + if (priv_stack_ptr) {
>> + priv_stack_alloc_size = round_up(fp->aux->stack_depth, 16) +
>> + 2 * PRIV_STACK_GUARD_SZ;
>> + priv_stack_check_guard(priv_stack_ptr, priv_stack_alloc_size, fp);
>> + free_percpu(priv_stack_ptr);
>> + }
>> WARN_ON_ONCE(!bpf_prog_kallsyms_verify_off(fp));
>> }
>> @@ -453,6 +522,11 @@ bool bpf_jit_supports_kfunc_call(void)
>> return true;
>> }
>> +bool bpf_jit_supports_private_stack(void)
>> +{
>> + return IS_ENABLED(CONFIG_PPC64);
>> +}
>> +
>> bool bpf_jit_supports_arena(void)
>> {
>> return IS_ENABLED(CONFIG_PPC64);
>> diff --git a/arch/powerpc/net/bpf_jit_comp64.c
>> b/arch/powerpc/net/bpf_jit_comp64.c
>> index 640b84409687..206ef43b4090 100644
>> --- a/arch/powerpc/net/bpf_jit_comp64.c
>> +++ b/arch/powerpc/net/bpf_jit_comp64.c
>> @@ -183,6 +183,20 @@ void bpf_jit_realloc_regs(struct codegen_context
>> *ctx)
>> {
>> }
>> +static void emit_fp_priv_stack(u32 *image, struct codegen_context
>> *ctx, void __percpu *ptr)
>> +{
>> + /* Load percpu data offset */
>> + EMIT(PPC_RAW_LD(bpf_to_ppc(TMP_REG_1), _R13,
>> + offsetof(struct paca_struct, data_offset)));
>> + PPC_LI64(bpf_to_ppc(BPF_REG_FP), (u64)ptr);
>> + /*
>> + * Set frame pointer with percpu allocated
>> + * buffer for private stack.
>> + */
>> + EMIT(PPC_RAW_ADD(bpf_to_ppc(BPF_REG_FP),
>> + bpf_to_ppc(TMP_REG_1), bpf_to_ppc(BPF_REG_FP)));
>> +}
>> +
>> /*
>> * For exception boundary & exception_cb progs:
>> * return increased size to accommodate additional NVRs.
>> @@ -197,6 +211,12 @@ static int bpf_jit_stack_size(struct
>> codegen_context *ctx)
>> void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
>> {
>> int i;
>> + void __percpu *priv_frame_ptr = NULL;
>> +
>> + if (ctx->priv_sp) {
>> + priv_frame_ptr = (void*) ctx->priv_sp + PRIV_STACK_GUARD_SZ +
>> + round_up(ctx->stack_size, 16);
>> + }
>> /* Instruction for trampoline attach */
>> EMIT(PPC_RAW_NOP());
>> @@ -251,7 +271,7 @@ void bpf_jit_build_prologue(u32 *image, struct
>> codegen_context *ctx)
>> }
>> EMIT(PPC_RAW_STDU(_R1, _R1,
>> - -(bpf_jit_stack_size(ctx) + ctx->stack_size)));
>> + -(bpf_jit_stack_size(ctx) + (ctx->priv_sp ? 0 :
>> ctx->stack_size))));
>> }
>> /*
>> @@ -307,9 +327,16 @@ void bpf_jit_build_prologue(u32 *image, struct
>> codegen_context *ctx)
>> * Exception_cb not restricted from using stack area or arena.
>> * Setup frame pointer to point to the bpf stack area
>> */
>> - if (bpf_is_seen_register(ctx, bpf_to_ppc(BPF_REG_FP)))
>> - EMIT(PPC_RAW_ADDI(bpf_to_ppc(BPF_REG_FP), _R1,
>> - STACK_FRAME_MIN_SIZE + ctx->stack_size));
>> + if (bpf_is_seen_register(ctx, bpf_to_ppc(BPF_REG_FP))) {
>> + if (ctx->priv_sp && priv_frame_ptr) {
>> + /* Set up private stack pointer */
>> + emit_fp_priv_stack(image, ctx, priv_frame_ptr);
>> + } else {
>> + /* Setup frame pointer to point to the bpf stack area */
>> + EMIT(PPC_RAW_ADDI(bpf_to_ppc(BPF_REG_FP), _R1,
>> + STACK_FRAME_MIN_SIZE + ctx->stack_size));
>> + }
>> + }
>> if (ctx->arena_vm_start)
>> PPC_LI64(bpf_to_ppc(ARENA_VM_START), ctx->arena_vm_start);
>> @@ -339,7 +366,8 @@ static void bpf_jit_emit_common_epilogue(u32
>> *image, struct codegen_context *ctx
>> /* Tear down our stack frame */
>> if (bpf_has_stack_frame(ctx)) {
>> - EMIT(PPC_RAW_ADDI(_R1, _R1, bpf_jit_stack_size(ctx) +
>> ctx->stack_size));
>> + EMIT(PPC_RAW_ADDI(_R1, _R1,
>> + bpf_jit_stack_size(ctx) + (ctx->priv_sp ? 0 : ctx->stack_size)));
>> if (ctx->seen & SEEN_FUNC || ctx->exception_cb) {
>> EMIT(PPC_RAW_LD(_R0, _R1, PPC_LR_STKOFF));
-Abhishek
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