Re: [bpf-next v2 1/2] bpf: Add bpf_copy_from_user_str kfunc

[Kui-Feng Lee <sinquersw@gmail.com>]

On 8/11/24 16:54, Jordan Rome wrote:
> This adds a kfunc wrapper around strncpy_from_user,
> which can be called from sleepable BPF programs.
> 
> This matches the non-sleepable 'bpf_probe_read_user_str'
> helper.
> 
> Signed-off-by: Jordan Rome <linux@jordanrome.com>
> ---
>   kernel/bpf/helpers.c | 32 ++++++++++++++++++++++++++++++++
>   1 file changed, 32 insertions(+)
> 
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index d02ae323996b..5eeb7c2ca622 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2939,6 +2939,37 @@ __bpf_kfunc void bpf_iter_bits_destroy(struct bpf_iter_bits *it)
>   	bpf_mem_free(&bpf_global_ma, kit->bits);
>   }
> 
> +/**
> + * bpf_copy_from_user_str() - Copy a string from an unsafe user address
> + * @dst:             Destination address, in kernel space.  This buffer must be at
> + *                   least @dst__szk bytes long.
> + * @dst__szk:        Maximum number of bytes to copy, including the trailing NUL.
> + * @unsafe_ptr__ign: Source address, in user space.
> + *
> + * Copies a NUL-terminated string from userspace to BPF space. If user string is
> + * too long this will still ensure zero termination in the dst buffer unless
> + * buffer size is 0.
> + */
> +__bpf_kfunc int bpf_copy_from_user_str(void *dst, u32 dst__szk, const void __user *unsafe_ptr__ign)
> +{
> +	int ret;
> +
> +	if (unlikely(!dst__szk))
> +		return 0;
> +
> +	ret = strncpy_from_user(dst, unsafe_ptr__ign, dst__szk);
> +	if (unlikely(ret < 0)) {
> +		memset(dst, 0, dst__szk);
> +	} else if (ret >= dst__szk) {
> +		ret = dst__szk;
> +		((char *)dst)[ret - 1] = '\0';
> +	} else if (ret > 0) {
> +		ret++;

I prefer to keep consistent with strncpy_from_user().
Considering ret >= dst__szk, it is not actually copying dst__szk bytes.
The last byte is generated by this function, not copying from
the source buffer.

Copying at most dst__szk - 1 bytes is more concise.
The code could be simpler with this concept.

   ret = strncpy_from_user(dst, unsafe_ptr__ign, dst_szk - 1);
   ((char *)dst)[max(ret, 0)] = 0;

WDYT?

> +	}
> +
> +	return ret;
> +}
> +
>   __bpf_kfunc_end_defs();
> 
>   BTF_KFUNCS_START(generic_btf_ids)
> @@ -3024,6 +3055,7 @@ BTF_ID_FLAGS(func, bpf_preempt_enable)
>   BTF_ID_FLAGS(func, bpf_iter_bits_new, KF_ITER_NEW)
>   BTF_ID_FLAGS(func, bpf_iter_bits_next, KF_ITER_NEXT | KF_RET_NULL)
>   BTF_ID_FLAGS(func, bpf_iter_bits_destroy, KF_ITER_DESTROY)
> +BTF_ID_FLAGS(func, bpf_copy_from_user_str, KF_SLEEPABLE)
>   BTF_KFUNCS_END(common_btf_ids)
> 
>   static const struct btf_kfunc_id_set common_kfunc_set = {
> --
> 2.44.1
>