Re: [PATCH 4/9] powerpc/bpf: Handle large branch ranges with BPF_EXIT

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 04/10/2021 à 20:24, Naveen N. Rao a écrit :
> Christophe Leroy wrote:
>>
>>
>> Le 01/10/2021 à 23:14, Naveen N. Rao a écrit :
>>> In some scenarios, it is possible that the program epilogue is outside
>>> the branch range for a BPF_EXIT instruction. Instead of rejecting such
>>> programs, emit an indirect branch. We track the size of the bpf program
>>> emitted after the initial run and do a second pass since BPF_EXIT can
>>> end up emitting different number of instructions depending on the
>>> program size.
>>>
>>> Suggested-by: Jordan Niethe <jniethe5@gmail.com>
>>> Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
>>> ---
>>>   arch/powerpc/net/bpf_jit.h        |  3 +++
>>>   arch/powerpc/net/bpf_jit_comp.c   | 22 +++++++++++++++++++++-
>>>   arch/powerpc/net/bpf_jit_comp32.c |  2 +-
>>>   arch/powerpc/net/bpf_jit_comp64.c |  2 +-
>>>   4 files changed, 26 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
>>> index 89bd744c2bffd4..4023de1698b9f5 100644
>>> --- a/arch/powerpc/net/bpf_jit.h
>>> +++ b/arch/powerpc/net/bpf_jit.h
>>> @@ -126,6 +126,7 @@
>>>   #define SEEN_FUNC    0x20000000 /* might call external helpers */
>>>   #define SEEN_TAILCALL    0x40000000 /* uses tail calls */
>>> +#define SEEN_BIG_PROG    0x80000000 /* large prog, >32MB */
>>>   #define SEEN_VREG_MASK    0x1ff80000 /* Volatile registers r3-r12 */
>>>   #define SEEN_NVREG_MASK    0x0003ffff /* Non volatile registers 
>>> r14-r31 */
>>> @@ -179,6 +180,8 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 
>>> *image, struct codegen_context *
>>>   void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx);
>>>   void bpf_jit_build_epilogue(u32 *image, struct codegen_context *ctx);
>>>   void bpf_jit_realloc_regs(struct codegen_context *ctx);
>>> +int bpf_jit_emit_exit_insn(u32 *image, struct codegen_context *ctx,
>>> +                    int tmp_reg, unsigned long exit_addr);
>>>   #endif
>>> diff --git a/arch/powerpc/net/bpf_jit_comp.c 
>>> b/arch/powerpc/net/bpf_jit_comp.c
>>> index fcbf7a917c566e..3204872fbf2738 100644
>>> --- a/arch/powerpc/net/bpf_jit_comp.c
>>> +++ b/arch/powerpc/net/bpf_jit_comp.c
>>> @@ -72,6 +72,21 @@ static int bpf_jit_fixup_subprog_calls(struct 
>>> bpf_prog *fp, u32 *image,
>>>       return 0;
>>>   }
>>> +int bpf_jit_emit_exit_insn(u32 *image, struct codegen_context *ctx,
>>> +                    int tmp_reg, unsigned long exit_addr)
>>> +{
>>> +    if (!(ctx->seen & SEEN_BIG_PROG) && 
>>> is_offset_in_branch_range(exit_addr)) {
>>> +        PPC_JMP(exit_addr);
>>> +    } else {
>>> +        ctx->seen |= SEEN_BIG_PROG;
>>> +        PPC_FUNC_ADDR(tmp_reg, (unsigned long)image + exit_addr);
>>> +        EMIT(PPC_RAW_MTCTR(tmp_reg));
>>> +        EMIT(PPC_RAW_BCTR());
>>> +    }
>>> +
>>> +    return 0;
>>> +}
>>> +
>>>   struct powerpc64_jit_data {
>>>       struct bpf_binary_header *header;
>>>       u32 *addrs;
>>> @@ -155,12 +170,17 @@ struct bpf_prog *bpf_int_jit_compile(struct 
>>> bpf_prog *fp)
>>>           goto out_addrs;
>>>       }
>>> +    if (!is_offset_in_branch_range((long)cgctx.idx * 4))
>>> +        cgctx.seen |= SEEN_BIG_PROG;
>>> +
>>>       /*
>>>        * If we have seen a tail call, we need a second pass.
>>>        * This is because bpf_jit_emit_common_epilogue() is called
>>>        * from bpf_jit_emit_tail_call() with a not yet stable ctx->seen.
>>> +     * We also need a second pass if we ended up with too large
>>> +     * a program so as to fix branches.
>>>        */
>>> -    if (cgctx.seen & SEEN_TAILCALL) {
>>> +    if (cgctx.seen & (SEEN_TAILCALL | SEEN_BIG_PROG)) {
>>>           cgctx.idx = 0;
>>>           if (bpf_jit_build_body(fp, 0, &cgctx, addrs, false)) {
>>>               fp = org_fp;
>>> diff --git a/arch/powerpc/net/bpf_jit_comp32.c 
>>> b/arch/powerpc/net/bpf_jit_comp32.c
>>> index a74d52204f8da2..d2a67574a23066 100644
>>> --- a/arch/powerpc/net/bpf_jit_comp32.c
>>> +++ b/arch/powerpc/net/bpf_jit_comp32.c
>>> @@ -852,7 +852,7 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 
>>> *image, struct codegen_context *
>>>                * we'll just fall through to the epilogue.
>>>                */
>>>               if (i != flen - 1)
>>> -                PPC_JMP(exit_addr);
>>> +                bpf_jit_emit_exit_insn(image, ctx, tmp_reg, exit_addr);
>>
>> On ppc32, if you use tmp_reg you must flag it. But I think you could 
>> use r0 instead.
> 
> Indeed. Can we drop tracking of the temp registers and using them while
> remapping registers? Are you seeing significant benefits with re-use of 
> those temp registers?
> 

I'm not sure to follow you.

On ppc32, all volatile registers are used for function arguments, so 
temp registers are necessarily non-volatile so we track them as all 
non-volatile registers we use.

I think saving on stack only the non-volatile registers we use provides 
real benefit, otherwise you wouldn't have implemented it would you ?

Anyway here you should use _R0 instead of tmp_reg.

Christophe