Re: [PATCH bpf] selftests/bpf: fix endianness issues in test_sysctl

[Ilya Leoshkevich <iii@linux.ibm.com>]

> Am 16.08.2019 um 02:05 schrieb Yonghong Song <yhs@fb.com>:
> 
>> +# define __bpf_constant_be64_to_cpu(x)	___constant_swab64(x)
> 
> bpf_endian.h is used for both bpf program and native applications.
> Could you make sure it works for bpf programs? It should be, but want to 
> double check.

Yes:

#include <linux/compiler_attributes.h>
#include "bpf_endian.h"
u64 answer() { return __bpf_constant_be64_to_cpu(42); }

compiles to

        r0 = 3026418949592973312 ll
        exit

on x86.

> The __constant_swab64 looks like a little bit expensive
> for bpf programs compared to __builtin_bswap64. But
> __builtin_bswap64 may not be available for all architectures, esp.
> 32bit system. So macro __bpf__ is required to use it.

Isn't ___constant_swab64 supposed to be 100% compile-time?

Also, I think __builtin_bswap64 should be available everywhere for
userspace. At least the following test does not indicate any problems:

for cc in "x86_64-linux-gnu-gcc -m32" \
          "x86_64-linux-gnu-gcc -m64" \
          "aarch64-linux-gnu-gcc" \
          "arm-linux-gnueabihf-gcc" \
          "mips64el-linux-gnuabi64-gcc" \
          "powerpc64le-linux-gnu-gcc -m32" \
          "s390x-linux-gnu-gcc -m31" \
          "s390x-linux-gnu-gcc -m64" \
          "sparc64-linux-gnu-gcc -m32" \
          "sparc64-linux-gnu-gcc -m64" \
          "clang -target bpf -m32" \
          "clang -target bpf -m64"; do
	echo "*** $cc ***"
	echo "long long f(long long x) { return __builtin_bswap64(x); }" | \
	$cc -x c -S - -O3 -o -;
done

Only sparc64 doesn't support it directly, but then it just calls
libgcc's __bswapdi2. This might not be ok only for kernel native code
(though even there we have e.g. arch/arm/lib/bswapsdi2.S), but I don't
think this header is used in such context anyway.

>> 
>>  			BPF_MOV64_REG(BPF_REG_1, BPF_REG_7),
>> @@ -1344,20 +1379,26 @@ static size_t probe_prog_length(const struct bpf_insn *fp)
>>  static int fixup_sysctl_value(const char *buf, size_t buf_len,
>>  			      struct bpf_insn *prog, size_t insn_num)
>>  {
>> -	uint32_t value_num = 0;
>> +	uint64_t value_num = 0;
>>  	uint8_t c, i;
>> 
>>  	if (buf_len > sizeof(value_num)) {
>>  		log_err("Value is too big (%zd) to use in fixup", buf_len);
>>  		return -1;
>>  	}
>> +	if (prog[insn_num].code != (BPF_LD | BPF_DW | BPF_IMM)) {
>> +		log_err("Can fixup only BPF_LD_IMM64 insns");
>> +		return -1;
>> +	}
>> 
>>  	for (i = 0; i < buf_len; ++i) {
>>  		c = buf[i];
>>  		value_num |= (c << i * 8);
>>  	}
>> +	value_num = __bpf_le64_to_cpu(value_num);
> 
> Can we avoid to use __bpf_le64_to_cpu?
> Look like we already having the value in buf, can we just cast it
> to get value_num. Note that bpf program and host always have
> the same endianness. This way, no endianness conversion
> is needed.

I think this might be dangerous in case buf is smaller than 8 bytes.