Re: [PATCH bpf-next v5 1/6] bpf: Add syscall common attrs support for link_create

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] bpf: inaccurate attach_type and prog_type error logging in link_create()
--

commit bc490436683f8010bb5c27c3e3a1582ddc24a909
Author: Leon Hwang <leon.hwang@linux.dev>

bpf: Add syscall common attrs support for link_create

This commit adds syscall common attributes support to link_create() in
kernel/bpf/syscall.c, initializing and finalizing a bpf_verifier_log
instance to report errors for invalid prog_fd, attach_type, and prog_type.

Link: https://lore.kernel.org/bpf/20260514004634.3DA09C19425@smtp.kernel.org/

> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 90bc7230afa8a..db870e795d044 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -5862,8 +5862,11 @@ static int bpf_map_do_batch(const union bpf_attr *attr,
>  }
>  
>  #define BPF_LINK_CREATE_LAST_FIELD link_create.uprobe_multi.path_fd
> -static int link_create(union bpf_attr *attr, bpfptr_t uattr)
> +static int link_create(union bpf_attr *attr, bpfptr_t uattr, struct bpf_common_attr *attr_common,
> +		       bpfptr_t uattr_common, u32 size_common)
>  {
> +	struct bpf_verifier_log *log;
> +	struct bpf_log_attr attr_log;
>  	struct bpf_prog *prog;
>  	int ret;
>  
> @@ -5873,14 +5876,24 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr)
>  	if (attr->link_create.attach_type == BPF_STRUCT_OPS)
>  		return bpf_struct_ops_link_create(attr);
>  
> +	log = bpf_log_attr_create_vlog(&attr_log, attr_common, uattr_common, size_common);
> +	if (IS_ERR(log))
> +		return PTR_ERR(log);
> +
>  	prog = bpf_prog_get(attr->link_create.prog_fd);
> -	if (IS_ERR(prog))
> -		return PTR_ERR(prog);
> +	if (IS_ERR(prog)) {
> +		bpf_log(log, "Invalid prog_fd.\n");
> +		ret = PTR_ERR(prog);
> +		prog = NULL;
> +		goto out;
> +	}
>  
>  	ret = bpf_prog_attach_check_attach_type(prog,
>  						attr->link_create.attach_type);
> -	if (ret)
> +	if (ret) {
> +		bpf_log(log, "Invalid attach_type.\n");
>  		goto out;
> +	}
>  
>  	switch (prog->type) {
>  	case BPF_PROG_TYPE_CGROUP_SKB:
> @@ -5959,12 +5972,20 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr)
>  			ret = bpf_uprobe_multi_link_attach(attr, prog);
>  		break;
>  	default:
> +		bpf_log(log, "Invalid prog type.\n");
>  		ret = -EINVAL;
>  	}

[Severity: Medium]
Does calling bpf_prog_attach_check_attach_type() before switch (prog->type)
in kernel/bpf/syscall.c:link_create() cause misleading or missing log
messages in a few cases?

First, for almost all program types that are unsupported by link_create()
(such as BPF_PROG_TYPE_SOCKET_FILTER, BPF_PROG_TYPE_SCHED_ACT,
BPF_PROG_TYPE_RAW_TRACEPOINT, or BPF_PROG_TYPE_SYSCALL),
link_create() -> bpf_prog_attach_check_attach_type() hits the default
branch:

kernel/bpf/syscall.c:bpf_prog_attach_check_attach_type() {
	...
	default:
		ptype = attach_type_to_prog_type(attach_type);
		if (ptype == BPF_PROG_TYPE_UNSPEC || ptype != prog->type)
			return -EINVAL;
		return 0;
}

Because attach_type_to_prog_type() never returns those program types for any
attach_type, bpf_prog_attach_check_attach_type() returns -EINVAL and
link_create() logs "Invalid attach_type.\n" instead of reaching the default
branch of switch (prog->type) to log "Invalid prog type.\n".

Second, for BPF_PROG_TYPE_CGROUP_SKB, bpf_prog_attach_check_attach_type()
also performs a capability check:

	case BPF_PROG_TYPE_CGROUP_SKB:
		if (!bpf_token_capable(prog->aux->token, CAP_NET_ADMIN))
			return -EPERM;

When this check fails with -EPERM, link_create() still logs
"Invalid attach_type.\n" even when attach_type is valid.

Third, for BPF_PROG_TYPE_LSM and BPF_PROG_TYPE_TRACING,
bpf_prog_attach_check_attach_type() only checks that attach_type maps to the
general program type. The stricter expected_attach_type check happens inside
switch (prog->type) in link_create():

	case BPF_PROG_TYPE_LSM:
	case BPF_PROG_TYPE_TRACING:
		if (attr->link_create.attach_type != prog->expected_attach_type) {
			ret = -EINVAL;
			goto out;
		}

If a caller passes an attach_type of the same program type that does not
match prog->expected_attach_type, link_create() jumps to out without logging
"Invalid attach_type.\n".

Should the program type validation and attach_type logging be ordered so
that unsupported program types, -EPERM failures, and expected_attach_type
mismatches report accurate messages?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261026154948.99408-1-leon.hwang@linux.dev?part=1