Re: [PATCH] [v4] bpf: fix bpf_probe_read_kernel prototype mismatch

[Yonghong Song]

On 7/25/23 1:41 PM, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> bpf_probe_read_kernel() has a __weak definition in core.c and another
> definition with an incompatible prototype in kernel/trace/bpf_trace.c,
> when CONFIG_BPF_EVENTS is enabled.
>
> Since the two are incompatible, there cannot be a shared declaration in
> a header file, but the lack of a prototype causes a W=1 warning:
>
> kernel/bpf/core.c:1638:12: error: no previous prototype for 'bpf_probe_read_kernel' [-Werror=missing-prototypes]
>
> On 32-bit architectures, the local prototype
>
> u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr)
>
> passes arguments in other registers as the one in bpf_trace.c
>
> BPF_CALL_3(bpf_probe_read_kernel, void *, dst, u32, size,
>              const void *, unsafe_ptr)
>
> which uses 64-bit arguments in pairs of registers.
>
> As both versions of the function are fairly simple and only really
> differ in one line, just move them into a header file as an inline
> function that does not add any overhead for the bpf_trace.c callers
> and actually avoids a function call for the other one.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Link: https://lore.kernel.org/all/ac25cb0f-b804-1649-3afb-1dc6138c2716@xxxxxxxxxxxxx/
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

LGTM but there are some additional changes in kernel so you need to
rebase on top of master branch with the following additional change:

@@ -2082,7 +2076,7 @@ static u64 ___bpf_prog_run(u64 *regs, const struct 
bpf_insn *insn)
                DST = *(SIZE *)(unsigned long) (SRC + insn->off);       \
                CONT;                                                   \
        LDX_PROBE_MEMSX_##SIZEOP:                                       \
-               bpf_probe_read_kernel(&DST, sizeof(SIZE),               \
+               bpf_probe_read_kernel_common(&DST, sizeof(SIZE), 
       \
                                      (const void *)(long) (SRC + 
insn->off));  \
                DST = *((SIZE *)&DST);                                  \
                CONT;

Thanks!

> --
> v4: rewrite again to use a shared inline helper
> v3: clarify changelog text further.
> v2: rewrite completely to fix the mismatch.
> ---
>   include/linux/bpf.h      | 12 ++++++++++++
>   kernel/bpf/core.c        | 10 ++--------
>   kernel/trace/bpf_trace.c | 11 -----------
>   3 files changed, 14 insertions(+), 19 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index ceaa8c23287fc..abe75063630b8 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -2661,6 +2661,18 @@ static inline void bpf_dynptr_set_rdonly(struct bpf_dynptr_kern *ptr)
>   }
>   #endif /* CONFIG_BPF_SYSCALL */
>   
> +static __always_inline int
> +bpf_probe_read_kernel_common(void *dst, u32 size, const void *unsafe_ptr)
> +{
> +	int ret = -EFAULT;
> +
> +	if (IS_ENABLED(CONFIG_BPF_EVENTS))
> +		ret = copy_from_kernel_nofault(dst, unsafe_ptr, size);
> +	if (unlikely(ret < 0))
> +		memset(dst, 0, size);
> +	return ret;
> +}
> +
>   void __bpf_free_used_btfs(struct bpf_prog_aux *aux,
>   			  struct btf_mod_pair *used_btfs, u32 len);
>   
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index dd70c58c9d3a3..9cdf53bfb8bd3 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -1634,12 +1634,6 @@ 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)
> -{
> -	memset(dst, 0, size);
> -	return -EFAULT;
> -}
> -
>   /**
>    *	___bpf_prog_run - run eBPF program on a given context
>    *	@regs: is the array of MAX_BPF_EXT_REG eBPF pseudo-registers
> @@ -1930,8 +1924,8 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
>   		DST = *(SIZE *)(unsigned long) (SRC + insn->off);	\
>   		CONT;							\
>   	LDX_PROBE_MEM_##SIZEOP:						\
> -		bpf_probe_read_kernel(&DST, sizeof(SIZE),		\
> -				      (const void *)(long) (SRC + insn->off));	\
> +		bpf_probe_read_kernel_common(&DST, sizeof(SIZE),	\
> +			      (const void *)(long) (SRC + insn->off));	\
>   		DST = *((SIZE *)&DST);					\
>   		CONT;
>   
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index c92eb8c6ff08d..83bde2475ae54 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -223,17 +223,6 @@ const struct bpf_func_proto bpf_probe_read_user_str_proto = {
>   	.arg3_type	= ARG_ANYTHING,
>   };
>   
> -static __always_inline int
> -bpf_probe_read_kernel_common(void *dst, u32 size, const void *unsafe_ptr)
> -{
> -	int ret;
> -
> -	ret = copy_from_kernel_nofault(dst, unsafe_ptr, size);
> -	if (unlikely(ret < 0))
> -		memset(dst, 0, size);
> -	return ret;
> -}
> -
>   BPF_CALL_3(bpf_probe_read_kernel, void *, dst, u32, size,
>   	   const void *, unsafe_ptr)
>   {