re: bpf: Implement minimal BPF perf link

[Colin Ian King]

Hi,

Static analysis with Coverity on linux-next has detected a potential
issue with the following commit:

commit b89fbfbb854c9afc3047e8273cc3a694650b802e
Author: Andrii Nakryiko <andrii@xxxxxxxxxx>
Date:   Sun Aug 15 00:05:57 2021 -0700

    bpf: Implement minimal BPF perf link

The analysis is as follows:

2936 static int bpf_perf_link_attach(const union bpf_attr *attr, struct
bpf_prog *prog)
2937 {

    1. var_decl: Declaring variable link_primer without initializer.

2938        struct bpf_link_primer link_primer;
2939        struct bpf_perf_link *link;
2940        struct perf_event *event;
2941        struct file *perf_file;
2942        int err;
2943

    2. Condition attr->link_create.flags, taking false branch.

2944        if (attr->link_create.flags)
2945                return -EINVAL;
2946
2947        perf_file = perf_event_get(attr->link_create.target_fd);

    3. Condition IS_ERR(perf_file), taking false branch.

2948        if (IS_ERR(perf_file))
2949                return PTR_ERR(perf_file);
2950
2951        link = kzalloc(sizeof(*link), GFP_USER);

    4. Condition !link, taking false branch.

2952        if (!link) {
2953                err = -ENOMEM;
2954                goto out_put_file;
2955        }
2956        bpf_link_init(&link->link, BPF_LINK_TYPE_PERF_EVENT,
&bpf_perf_link_lops, prog);
2957        link->perf_file = perf_file;
2958
2959        err = bpf_link_prime(&link->link, &link_primer);

    5. Condition err, taking false branch.

2960        if (err) {
2961                kfree(link);
2962                goto out_put_file;
2963        }
2964
2965        event = perf_file->private_data;
2966        err = perf_event_set_bpf_prog(event, prog,
attr->link_create.perf_event.bpf_cookie);

    6. Condition err, taking true branch.
2967        if (err) {
    7. uninit_use_in_call: Using uninitialized value link_primer.fd when
calling bpf_link_cleanup.
    8. uninit_use_in_call: Using uninitialized value link_primer.file
when calling bpf_link_cleanup.
    9. uninit_use_in_call: Using uninitialized value link_primer.id when
calling bpf_link_cleanup.

   Uninitialized pointer read (UNINIT)
   10. uninit_use_in_call: Using uninitialized value link_primer.link
when calling bpf_link_cleanup.

2968                bpf_link_cleanup(&link_primer);
2969                goto out_put_file;
2970        }
2971        /* perf_event_set_bpf_prog() doesn't take its own refcnt on
prog */
2972        bpf_prog_inc(prog);

I'm not 100% sure if these are false-positives, but I thought I should
report the issues as potentially there is a pointer access on an
uninitialized pointer on line 2968.

Colin