Re: [PATCH v2 5/6] powerpc64/bpf: Support exceptions

[Hari Bathini <hbathini@linux.ibm.com>]

On 16/01/26 12:17 pm, adubey wrote:
> On 2026-01-16 12:14, adubey wrote:
>> On 2026-01-16 11:57, Hari Bathini wrote:
>>> On 14/01/26 5:14 pm, adubey@linux.ibm.com wrote:
>>>> From: Abhishek Dubey <adubey@linux.ibm.com>
>>>>
>>>> The modified prologue/epilogue generation code now
>>>> enables exception-callback to use the stack frame of
>>>> the program marked as exception boundary, where callee
>>>> saved registers are stored.
>>>>
>>>> As per ppc64 ABIv2 documentation[1], r14-r31 are callee
>>>> saved registers. BPF programs on ppc64 already saves
>>>> r26-r31 registers. Saving the remaining set of callee
>>>> saved registers(r14-r25) is handled in the next patch.
>>>>
>>>> [1] https://ftp.rtems.org/pub/rtems/people/sebh/ 
>>>> ABI64BitOpenPOWERv1.1_16July2015_pub.pdf
>>>>
>>>> Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
>>>> ---
>>>>   arch/powerpc/net/bpf_jit.h        |  2 ++
>>>>   arch/powerpc/net/bpf_jit_comp.c   |  7 ++++
>>>>   arch/powerpc/net/bpf_jit_comp64.c | 53 ++++++++++++++++++++ 
>>>> +----------
>>>>   3 files changed, 45 insertions(+), 17 deletions(-)
>>>>
>>>> diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
>>>> index 5d735bc5e6bd..fb548ae5d143 100644
>>>> --- a/arch/powerpc/net/bpf_jit.h
>>>> +++ b/arch/powerpc/net/bpf_jit.h
>>>> @@ -179,6 +179,8 @@ struct codegen_context {
>>>>       u64 arena_vm_start;
>>>>       u64 user_vm_start;
>>>>       bool is_subprog;
>>>> +    bool exception_boundary;
>>>> +    bool exception_cb;
>>>>   };
>>>>     #define bpf_to_ppc(r)    (ctx->b2p[r])
>>>> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/ 
>>>> bpf_jit_comp.c
>>>> index e3088cf089d1..26991940d36e 100644
>>>> --- a/arch/powerpc/net/bpf_jit_comp.c
>>>> +++ b/arch/powerpc/net/bpf_jit_comp.c
>>>> @@ -207,6 +207,8 @@ struct bpf_prog *bpf_int_jit_compile(struct 
>>>> bpf_prog *fp)
>>>>       cgctx.arena_vm_start = bpf_arena_get_kern_vm_start(fp->aux- 
>>>> >arena);
>>>>       cgctx.user_vm_start = bpf_arena_get_user_vm_start(fp->aux- 
>>>> >arena);
>>>>       cgctx.is_subprog = bpf_is_subprog(fp);
>>>> +    cgctx.exception_boundary = fp->aux->exception_boundary;
>>>> +    cgctx.exception_cb = fp->aux->exception_cb;
>>>>         /* Scouting faux-generate pass 0 */
>>>>       if (bpf_jit_build_body(fp, NULL, NULL, &cgctx, addrs, 0, 
>>>> false)) {
>>>> @@ -436,6 +438,11 @@ void bpf_jit_free(struct bpf_prog *fp)
>>>>       bpf_prog_unlock_free(fp);
>>>>   }
>>>>   +bool bpf_jit_supports_exceptions(void)
>>>> +{
>>>> +    return IS_ENABLED(CONFIG_PPC64);
>>>> +}
>>>> +
>>>>   bool bpf_jit_supports_subprog_tailcalls(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 ec58395f74f7..a6083dd9786c 100644
>>>> --- a/arch/powerpc/net/bpf_jit_comp64.c
>>>> +++ b/arch/powerpc/net/bpf_jit_comp64.c
>>>> @@ -89,7 +89,9 @@ static inline bool bpf_has_stack_frame(struct 
>>>> codegen_context *ctx)
>>>>        * - the bpf program uses its stack area
>>>>        * The latter condition is deduced from the usage of BPF_REG_FP
>>>>        */
>>>> -    return ctx->seen & SEEN_FUNC || bpf_is_seen_register(ctx, 
>>>> bpf_to_ppc(BPF_REG_FP));
>>>> +    return ctx->seen & SEEN_FUNC ||
>>>> +           bpf_is_seen_register(ctx, bpf_to_ppc(BPF_REG_FP)) ||
>>>> +           ctx->exception_cb;
>>>>   }
>>>>     /*
>>>> @@ -190,23 +192,32 @@ void bpf_jit_build_prologue(u32 *image, struct 
>>>> codegen_context *ctx)
>>>>           EMIT(PPC_RAW_STDU(_R1, _R1, -(BPF_PPC_STACKFRAME + ctx- 
>>>> >stack_size)));
>>>>       }
>>>>   -    /*
>>>> -     * Back up non-volatile regs -- BPF registers 6-10
>>>> -     * If we haven't created our own stack frame, we save these
>>>> -     * in the protected zone below the previous stack frame
>>>> -     */
>>>> -    for (i = BPF_REG_6; i <= BPF_REG_10; i++)
>>>> -        if (bpf_is_seen_register(ctx, bpf_to_ppc(i)))
>>>> -            EMIT(PPC_RAW_STD(bpf_to_ppc(i), _R1, 
>>>> bpf_jit_stack_offsetof(ctx, bpf_to_ppc(i))));
>>>> +    if (!ctx->exception_cb) {
>>>> +        /*
>>>> +         * Back up non-volatile regs -- BPF registers 6-10
>>>> +         * If we haven't created our own stack frame, we save these
>>>> +         * in the protected zone below the previous stack frame
>>>> +         */
>>>> +        for (i = BPF_REG_6; i <= BPF_REG_10; i++)
>>>> +            if (ctx->exception_boundary || 
>>>> bpf_is_seen_register(ctx, bpf_to_ppc(i)))
>>>> +                EMIT(PPC_RAW_STD(bpf_to_ppc(i), _R1,
>>>> +                    bpf_jit_stack_offsetof(ctx, bpf_to_ppc(i))));
>>>>   -    if (ctx->arena_vm_start)
>>>> -        EMIT(PPC_RAW_STD(bpf_to_ppc(ARENA_VM_START), _R1,
>>>> +        if (ctx->exception_boundary || ctx->arena_vm_start)
>>>> +            EMIT(PPC_RAW_STD(bpf_to_ppc(ARENA_VM_START), _R1,
>>>>                    bpf_jit_stack_offsetof(ctx, 
>>>> bpf_to_ppc(ARENA_VM_START))));
>>>>   -    /* 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,
>>>> +        /* 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));
>>>> +    } else {
>>>> +        /*
>>>> +         * Exception callback receives Frame Pointer of main
>>>> +         * program as third arg
>>>> +         */
>>>> +        EMIT(PPC_RAW_MR(_R1, _R5));
>>>> +    }
>>>>         if (ctx->arena_vm_start)
>>>>           PPC_LI64(bpf_to_ppc(ARENA_VM_START), ctx->arena_vm_start);
>>>> @@ -218,17 +229,25 @@ static void bpf_jit_emit_common_epilogue(u32 
>>>> *image, struct codegen_context *ctx
>>>>         /* Restore NVRs */
>>>>       for (i = BPF_REG_6; i <= BPF_REG_10; i++)
>>>> -        if (bpf_is_seen_register(ctx, bpf_to_ppc(i)))
>>>> +        if (ctx->exception_cb || bpf_is_seen_register(ctx, 
>>>> bpf_to_ppc(i)))
>>>>               EMIT(PPC_RAW_LD(bpf_to_ppc(i), _R1, 
>>>> bpf_jit_stack_offsetof(ctx, bpf_to_ppc(i))));
>>>>   -    if (ctx->arena_vm_start)
>>>> +    if (ctx->exception_cb || ctx->arena_vm_start)
>>>>           EMIT(PPC_RAW_LD(bpf_to_ppc(ARENA_VM_START), _R1,
>>>>                   bpf_jit_stack_offsetof(ctx, 
>>>> bpf_to_ppc(ARENA_VM_START))));
>>>>
>>>
>>>> +    if (ctx->exception_cb) {
>>>> +        /*
>>>> +         * LR value from boundary-frame is received as second 
>>>> parameter
>>>> +         * in exception callback.
>>>> +         */
>>>> +        EMIT(PPC_RAW_MTLR(_R4));
>>>> +    }
>>>> +
>>>
>>> No. Both second and third parameter of exception_cb() are stack pointer
>>> (or frame pointer, if you prefer that) and not LR.
>>> The above hunk is wrong. It still worked because of mtlr
>>> instruction below that restored LR from PPC_LR_STKOFF offset
>>> on the stack. Please drop the above hunk.
>> Okay, but from patch 3/6:
> My mistake, its patch 4/6.

Please note the consume_fn() is not exception_cb(). It is
bpf_stack_walker() that helps with the stack walk to be precise.
You can find the exception_cb() call in bpf_throw() kfunc as:

   ctx.aux->bpf_exception_cb(cookie, ctx.sp, ctx.bp, 0, 0);

>>
>> ip = frame[STACK_FRAME_LR_SAVE];
>> ...
>> if (ip && !consume_fn(cookie, ip, fp, fp))
>> we send cookie as first, LR as second parameter and fp as third & 
>> fourth both.
>> Default callback expects 4 parameter, so we copy fp in both.
>>
>>>
>>>>       /* Tear down our stack frame */
>>>>       if (bpf_has_stack_frame(ctx)) {
>>>>           EMIT(PPC_RAW_ADDI(_R1, _R1, BPF_PPC_STACKFRAME + ctx- 
>>>> >stack_size));
>>>> -        if (ctx->seen & SEEN_FUNC) {
>>>> +        if (ctx->seen & SEEN_FUNC || ctx->exception_cb) {
>>>>               EMIT(PPC_RAW_LD(_R0, _R1, PPC_LR_STKOFF));
>>>>               EMIT(PPC_RAW_MTLR(_R0));
>>>>           }
>>>
- Hari