netdev - Re: [PATCH bpf-next 1/9] bpf: introduce bpf_spin

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Message-ID: <20190117001615.pleazxhez7aplc3m@kafai-mbp.dhcp.thefacebook.com>
Date:   Thu, 17 Jan 2019 00:16:18 +0000
From:   Martin Lau <kafai@...com>
To:     Alexei Starovoitov <ast@...nel.org>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "jakub.kicinski@...ronome.com" <jakub.kicinski@...ronome.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 1/9] bpf: introduce bpf_spin_lock

On Tue, Jan 15, 2019 at 09:08:22PM -0800, Alexei Starovoitov wrote:
[ ... ]

> +/* copy everything but bpf_spin_lock */
> +static inline void copy_map_value(struct bpf_map *map, void *dst, void *src)
> +{
> +	if (unlikely(map_value_has_spin_lock(map))) {
> +		u32 off = map->spin_lock_off;
> +
> +		memcpy(dst, src, off);
> +		memcpy(dst + off + sizeof(struct bpf_spin_lock),
> +		       src + off + sizeof(struct bpf_spin_lock),
> +		       map->value_size - off - sizeof(struct bpf_spin_lock));
> +	} else {
> +		memcpy(dst, src, map->value_size);
> +	}
> +}
> +
[ ... ]

> +int btf_find_spin_lock(const struct btf *btf, const struct btf_type *t)
> +{
> +	const struct btf_member *member;
> +	u32 i, off = -ENOENT;
> +
> +	if (BTF_INFO_KIND(t->info) != BTF_KIND_STRUCT)
> +		return -EINVAL;
> +
> +	for_each_member(i, t, member) {
> +		const struct btf_type *member_type = btf_type_by_id(btf,
> +								    member->type);
> +		if (!btf_type_is_struct(member_type))
may be using "BTF_INFO_KIND(t->info) != BTF_KIND_STRUCT" here also.

> +			continue;
> +		if (member_type->size != sizeof(struct bpf_spin_lock))
> +			continue;
> +		if (strcmp(__btf_name_by_offset(btf, member_type->name_off),
> +			   "bpf_spin_lock"))
> +			continue;
> +		if (off != -ENOENT)
> +			/* only one 'struct bpf_spin_lock' is allowed */
> +			return -E2BIG;
> +		off = btf_member_bit_offset(t, member);
> +		if (off % 8)
> +			/* valid C code cannot generate such BTF */
> +			return -EINVAL;
> +		off /= 8;
> +		if (off % __alignof__(struct bpf_spin_lock))
> +			/* valid struct bpf_spin_lock will be 4 byte aligned */
> +			return -EINVAL;
> +	}
> +	return off;
> +}
> +
[ ... ]


> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index b155cd17c1bd..ebf0a673cb83 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
[ ... ]

>  	err = security_bpf_map_alloc(map);
> @@ -740,7 +757,7 @@ static int map_lookup_elem(union bpf_attr *attr)
>  			err = -ENOENT;
>  		} else {
>  			err = 0;
> -			memcpy(value, ptr, value_size);
> +			copy_map_value(map, value, ptr);
copy_map_value() skips the bpf_spin_lock and "value" has not been zero-ed.
"value" is then copied to the "__user *uvalue".
May be init the bpf_spin_lock part of the "uvalue" to 0?

btw, somehow patch 6 and 7 are missing:
https://patchwork.ozlabs.org/cover/1025640/