linux-kernel - Re: [PATCH -next] libbpf: Add additional null-pointer checking in make_parent

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Date:   Fri, 22 Apr 2022 10:45:08 +0800
From:   cuigaosheng <cuigaosheng1@...wei.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        <gongruiqi1@...wei.com>, <wangweiyang2@...wei.com>
Subject: Re: [PATCH -next] libbpf: Add additional null-pointer checking in
 make_parent_dir

I don't understand why we don't check path for NULL, bpf_link__pin is an external
interface, It will be called by external functions and provide input parameters,
for example in samples/bpf/hbm.c:
> 201 link = bpf_program__attach_cgroup(bpf_prog, cg1); 202 if 
> (libbpf_get_error(link)) { 203 fprintf(stderr, "ERROR: 
> bpf_program__attach_cgroup failed\n"); 204 goto err; 205 } 206 207 
> sprintf(cg_pin_path, "/sys/fs/bpf/hbm%d", cg_id); 208 rc = 
> bpf_link__pin(link, cg_pin_path); 209 if (rc < 0) { 210 printf("ERROR: 
> bpf_link__pin failed: %d\n", rc); 211 goto err; 212 }
if cg_pin_path is NULL, strdup(NULL) will trigger a segmentation fault in
make_parent_dir, I think we should avoid this and add null-pointer checking
for path, just like check_path:
> 7673 static int check_path(const char *path) 7674 { 7675 char *cp, 
> errmsg[STRERR_BUFSIZE]; 7676 struct statfs st_fs; 7677 char *dname, 
> *dir; 7678 int err = 0; 7679 7680 if (path == NULL) 7681 return 
> -EINVAL; 7682 7683 dname = strdup(path); 7684 if (dname == NULL) 7685 
> return -ENOMEM; 7686 7687 dir = dirname(dname); 7688 if (statfs(dir, 
> &st_fs)) { 7689 cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg)); 
> 7690 pr_warn("failed to statfs %s: %s\n", dir, cp); 7691 err = -errno; 
> 7692 } 7693 free(dname); 7694 7695 if (!err && st_fs.f_type != 
> BPF_FS_MAGIC) { 7696 pr_warn("specified path %s is not on BPF FS\n", 
> path); 7697 err = -EINVAL; 7698 } 7699 7700 return err; 7701 }

Thanks.

在 2022/4/22 0:55, Andrii Nakryiko 写道:
> On Thu, Apr 21, 2022 at 6:01 AM Gaosheng Cui <cuigaosheng1@...wei.com> wrote:
>> The make_parent_dir is called without null-pointer checking for path,
>> such as bpf_link__pin. To ensure there is no null-pointer dereference
>> in make_parent_dir, so make_parent_dir requires additional null-pointer
>> checking for path.
>>
>> Signed-off-by: Gaosheng Cui <cuigaosheng1@...wei.com>
>> ---
>>   tools/lib/bpf/libbpf.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index b53e51884f9e..5786e6184bf5 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -7634,6 +7634,9 @@ static int make_parent_dir(const char *path)
>>          char *dname, *dir;
>>          int err = 0;
>>
>> +       if (path == NULL)
>> +               return -EINVAL;
>> +
> API contract is that path shouldn't be NULL. Just like we don't check
> link, obj, prog for NULL in every single API, I don't think we need to
> do it here, unless I'm missing something?
>
>>          dname = strdup(path);
>>          if (dname == NULL)
>>                  return -ENOMEM;
>> --
>> 2.25.1
>>
> .