Re: [PATCH bpf-next v4 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: incorrect error log messages and ordering in link_create()
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commit f2293d2ca04aeb83f219d17a0904b9b58058c572
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(),
creating and finalizing a 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]
Can calling bpf_prog_attach_check_attach_type() before switching on
prog->type cause link_create() to log the wrong error message or skip
logging on mismatched attach types?

First, in kernel/bpf/syscall.c:bpf_prog_attach_check_attach_type(), a
cgroup skb program checks permissions before validating the attach type:

	case BPF_PROG_TYPE_CGROUP_SKB:
		if (!bpf_token_capable(prog->aux->token, CAP_NET_ADMIN))
			/* cg-skb progs can be loaded by unpriv user.
			 * check permissions at attach time.
			 */
			return -EPERM;

When that capability check fails, link_create() still logs
"Invalid attach_type.\n" even though attach_type may be valid.

Second, for program types that are not supported by link_create() and have
no mapping in attach_type_to_prog_type() (such as socket filter, sched act,
raw tracepoint, or syscall programs), bpf_prog_attach_check_attach_type()
hits its default branch and returns -EINVAL before link_create() reaches
switch (prog->type):

	default:
		ptype = attach_type_to_prog_type(attach_type);
		if (ptype == BPF_PROG_TYPE_UNSPEC || ptype != prog->type)
			return -EINVAL;
		return 0;

Doesn't that cause link_create() to log "Invalid attach_type.\n" and jump
to out, leaving the new default branch that logs "Invalid prog type.\n"
unreachable for almost all unsupported program types?

Third, for tracing and lsm programs, bpf_prog_attach_check_attach_type()
only checks that attach_type maps to prog->type in
attach_type_to_prog_type(), while the check against
prog->expected_attach_type happens later 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;
		}

Should that mismatch also log "Invalid attach_type.\n"?

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Sashiko AI review · https://sashiko.dev/#/patchset/20261024153608.58569-1-leon.hwang@linux.dev?part=1