Re: [PATCH 2/2] [v2] bpf: fix bpf_probe_read_kernel prototype mismatch

[Yonghong Song <yhs@meta.com>]

On 5/24/23 6:28 AM, Arnd Bergmann wrote:
> On Wed, May 24, 2023, at 05:12, Yonghong Song wrote:
>> On 5/23/23 12:43 PM, Arnd Bergmann wrote:
> 
>>> Aside from the warning, this addresses a bug on 32-bit architectures
>>> from incorrect argument passing with the mismatched prototype.
>>
>> Could you explain what is this '32-bit architectures ... incorrect
>> argument passing' thing?
> 
> I've expanded that paragraph now:
> 
> | Aside from the warning, this addresses a bug on 32-bit architectures
> | from incorrect argument passing with the mismatched prototype:
> | BPF_CALL_x() functions use 64-bit arguments that are passed in
> | pairs of register or on the stack on 32-bit architectures, while the
> | normal function uses one register per argument.
> 
> Let me know if you think I should put more details in there.

Please mention the function you try to address for the bug on
32-bit architecture is:

u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void 
*unsafe_ptr)

which will be incompatible with
BPF_CALL_3(bpf_probe_read_kernel, void *, dst, u32, size,
            const void *, unsafe_ptr)
in bpf_trace.c.

So you fixed this bug by using internal function 
bpf_probe_read_kernel_common() instead.

Thanks.

> 
>>> @@ -1635,11 +1636,13 @@ bool bpf_opcode_in_insntable(u8 code)
>>>    }
>>>    
>>>    #ifndef CONFIG_BPF_JIT_ALWAYS_ON
>>> -u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr)
>>> +#ifndef CONFIG_BPF_EVENTS
>>> +int bpf_probe_read_kernel_common(void * dst, u32 size, const void *unsafe_ptr)
>>
>> void * dst => void *dst
>>
> 
> Fixed now.
> 
> Thanks,
> 
>       Arnd