Re: [PATCH 2/2] [v3] bpf: fix bpf_probe_read_kernel prototype mismatch

[Daniel Borkmann]

On 6/2/23 3:50 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.
>
> Change the core.c file to only reference the inner
> bpf_probe_read_kernel_common() helper and provide a prototype for that,
> to ensure this is compatible with both definitions.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

Sorry for the delay. I just took in patch 1/2, thanks!

Small comment below:

> --
> v3: clarify changelog text further.
> v2: rewrite completely to fix the mismatch.
> ---
>   include/linux/bpf.h      | 2 ++
>   kernel/bpf/core.c        | 9 ++++++---
>   kernel/trace/bpf_trace.c | 3 +--
>   3 files changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index f58895830adae..55826398acfba 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -2619,6 +2619,8 @@ static inline void bpf_dynptr_set_rdonly(struct bpf_dynptr_kern *ptr)
>   }
>   #endif /* CONFIG_BPF_SYSCALL */
>   
> +int bpf_probe_read_kernel_common(void *dst, u32 size, const void *unsafe_ptr);
> +
>   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 0926714641eb5..565ef6950c7a8 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -35,6 +35,7 @@
>   #include <linux/bpf_verifier.h>
>   #include <linux/nodemask.h>
>   #include <linux/nospec.h>
> +#include <linux/bpf.h>
>   #include <linux/bpf_mem_alloc.h>
>   #include <linux/memcontrol.h>
>   
> @@ -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)
>   {
>   	memset(dst, 0, size);
>   	return -EFAULT;
>   }
> +#endif
>   
>   /**
>    *	___bpf_prog_run - run eBPF program on a given context
> @@ -1931,8 +1934,8 @@ 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 2bc41e6ac9fe0..290fdce2ce535 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -223,8 +223,7 @@ 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 bpf_probe_read_kernel_common(void *dst, u32 size, const void *unsafe_ptr)
>   {
>   	int ret;

Given this is critical fast-path, please find a solution where we don't need
to penalize bpf_probe_read_kernel_common.

Thanks,
Daniel