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

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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



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