Re: [PATCH 03/13] powerpc/bpf: Update ldimm64 instructions during extra pass

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

Christophe Leroy wrote:
> 
> 
> Le 06/01/2022 à 12:45, Naveen N. Rao a écrit :
>> These instructions are updated after the initial JIT, so redo codegen
>> during the extra pass. Rename bpf_jit_fixup_subprog_calls() to clarify
>> that this is more than just subprog calls.
>> 
>> Fixes: 69c087ba6225b5 ("bpf: Add bpf_for_each_map_elem() helper")
>> Cc: stable@vger.kernel.org # v5.15
>> Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
>> ---
>>   arch/powerpc/net/bpf_jit_comp.c   | 29 +++++++++++++++++++++++------
>>   arch/powerpc/net/bpf_jit_comp32.c |  6 ++++++
>>   arch/powerpc/net/bpf_jit_comp64.c |  7 ++++++-
>>   3 files changed, 35 insertions(+), 7 deletions(-)
>> 
>> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
>> index d6ffdd0f2309d0..56dd1f4e3e4447 100644
>> --- a/arch/powerpc/net/bpf_jit_comp.c
>> +++ b/arch/powerpc/net/bpf_jit_comp.c
>> @@ -23,15 +23,15 @@ static void bpf_jit_fill_ill_insns(void *area, unsigned int size)
>>   	memset32(area, BREAKPOINT_INSTRUCTION, size / 4);
>>   }
>>   
>> -/* Fix the branch target addresses for subprog calls */
>> -static int bpf_jit_fixup_subprog_calls(struct bpf_prog *fp, u32 *image,
>> -				       struct codegen_context *ctx, u32 *addrs)
>> +/* Fix updated addresses (for subprog calls, ldimm64, et al) during extra pass */
>> +static int bpf_jit_fixup_addresses(struct bpf_prog *fp, u32 *image,
>> +				   struct codegen_context *ctx, u32 *addrs)
>>   {
>>   	const struct bpf_insn *insn = fp->insnsi;
>>   	bool func_addr_fixed;
>>   	u64 func_addr;
>>   	u32 tmp_idx;
>> -	int i, ret;
>> +	int i, j, ret;
>>   
>>   	for (i = 0; i < fp->len; i++) {
>>   		/*
>> @@ -66,6 +66,23 @@ static int bpf_jit_fixup_subprog_calls(struct bpf_prog *fp, u32 *image,
>>   			 * of the JITed sequence remains unchanged.
>>   			 */
>>   			ctx->idx = tmp_idx;
>> +		} else if (insn[i].code == (BPF_LD | BPF_IMM | BPF_DW)) {
>> +			tmp_idx = ctx->idx;
>> +			ctx->idx = addrs[i] / 4;
>> +#ifdef CONFIG_PPC32
>> +			PPC_LI32(ctx->b2p[insn[i].dst_reg] - 1, (u32)insn[i + 1].imm);
>> +			PPC_LI32(ctx->b2p[insn[i].dst_reg], (u32)insn[i].imm);
>> +			for (j = ctx->idx - addrs[i] / 4; j < 4; j++)
>> +				EMIT(PPC_RAW_NOP());
>> +#else
>> +			func_addr = ((u64)(u32)insn[i].imm) | (((u64)(u32)insn[i + 1].imm) << 32);
>> +			PPC_LI64(b2p[insn[i].dst_reg], func_addr);
>> +			/* overwrite rest with nops */
>> +			for (j = ctx->idx - addrs[i] / 4; j < 5; j++)
>> +				EMIT(PPC_RAW_NOP());
>> +#endif
> 
> #ifdefs should be avoided as much as possible.
> 
> Here it seems we could easily do an
> 
> 	if (IS_ENABLED(CONFIG_PPC32)) {
> 	} else {
> 	}
> 
> And it looks like the CONFIG_PPC64 alternative would in fact also work 
> on PPC32, wouldn't it ?

We never implemented PPC_LI64() for ppc32:
  /linux/arch/powerpc/net/bpf_jit_comp.c: In function 
  'bpf_jit_fixup_addresses':
  /linux/arch/powerpc/net/bpf_jit_comp.c:81:5: error: this decimal constant is unsigned only in ISO C90 [-Werror]
     81 |     PPC_LI64(b2p[insn[i].dst_reg], func_addr);
	|     ^~~~~~~~
  /linux/arch/powerpc/net/bpf_jit_comp.c:81:5: error: this decimal constant is unsigned only in ISO C90 [-Werror]
  In file included from /linux/arch/powerpc/net/bpf_jit_comp.c:19:
  /linux/arch/powerpc/net/bpf_jit.h:78:40: error: right shift count >= width of type [-Werror=shift-count-overflow]
     78 |     EMIT(PPC_RAW_LI(d, ((uintptr_t)(i) >> 32) &   \
	|                                        ^~


We should move that out from bpf_jit.h


- Naveen