Re: [PATCH bpf-next v13 1/2] bpf: add new helper get_fd_path for mapping a file descriptor to a pathname

From: Wenbo Zhang <ethercflow@gmail.com>
To: Daniel Borkmann <daniel@iogearbox.net>
Cc: Yonghong Song <yhs@fb.com>,
	"bpf@vger.kernel.org" <bpf@vger.kernel.org>,
	"ast@kernel.org" <ast@kernel.org>,
	"bgregg@netflix.com" <bgregg@netflix.com>,
	"andrii.nakryiko@gmail.com" <andrii.nakryiko@gmail.com>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>
Subject: Re: [PATCH bpf-next v13 1/2] bpf: add new helper get_fd_path for mapping a file descriptor to a pathname
Date: Wed, 18 Dec 2019 08:11:34 +0800	[thread overview]
Message-ID: <CABtjQmZW1AEFZcq1=EF61d9TSmSpBD0-0rzGUWS-azg=1m1cEw@mail.gmail.com> (raw)
In-Reply-To: <a8a763e2-65d2-7c71-e99d-ffae1523f0f0@iogearbox.net>

> [...]
>>> + *          On failure, it is filled with zeroes.
> [...]
> You fill it with 0x30's ...

So sorry about this, I'll submit another revision to fix this. Thanks
again for your
preciseness and patience.

Daniel Borkmann <daniel@iogearbox.net> 于2019年12月18日周三 上午3:39写道：
>
> On 12/17/19 5:29 PM, Yonghong Song wrote:
> > On 12/17/19 1:47 AM, Wenbo Zhang wrote:
> [...]
> >> + *          On failure, it is filled with zeroes.
> [...]
> >>     */
> >>    #define __BPF_FUNC_MAPPER(FN)             \
> >>      FN(unspec),                     \
> >> @@ -2938,7 +2964,8 @@ union bpf_attr {
> >>      FN(probe_read_user),            \
> >>      FN(probe_read_kernel),          \
> >>      FN(probe_read_user_str),        \
> >> -    FN(probe_read_kernel_str),
> >> +    FN(probe_read_kernel_str),      \
> >> +    FN(get_fd_path),
> >>
> >>    /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> >>     * function eBPF program intends to call
> >> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> >> index e5ef4ae9edb5..43a6aa6ad967 100644
> >> --- a/kernel/trace/bpf_trace.c
> >> +++ b/kernel/trace/bpf_trace.c
> >> @@ -762,6 +762,71 @@ static const struct bpf_func_proto bpf_send_signal_proto = {
> >>      .arg1_type      = ARG_ANYTHING,
> >>    };
> >>
> >> +BPF_CALL_3(bpf_get_fd_path, char *, dst, u32, size, int, fd)
> >> +{
> >> +    int ret = -EBADF;
> >> +    struct file *f;
> >> +    char *p;
> >> +
> >> +    /* Ensure we're in user context which is safe for the helper to
> >> +     * run. This helper has no business in a kthread.
> >> +     */
> >> +    if (unlikely(in_interrupt() ||
> >> +                 current->flags & (PF_KTHREAD | PF_EXITING))) {
> >> +            ret = -EPERM;
> >> +            goto error;
> >> +    }
> >> +
> >> +    /* Use fget_raw instead of fget to support O_PATH, and it doesn't
> >> +     * have any sleepable code, so it's ok to be here.
> >> +     */
> >> +    f = fget_raw(fd);
> >> +    if (!f)
> >> +            goto error;
> >> +
> >> +    /* For unmountable pseudo filesystem, it seems to have no meaning
> >> +     * to get their fake paths as they don't have path, and to be no
> >> +     * way to validate this function pointer can be always safe to call
> >> +     * in the current context.
> >> +     */
> >> +    if (f->f_path.dentry->d_op && f->f_path.dentry->d_op->d_dname) {
> >> +            ret = -EINVAL;
> >> +            fput(f);
> >> +            goto error;
> >> +    }
> >> +
> >> +    /* After filter unmountable pseudo filesytem, d_path won't call
> >> +     * dentry->d_op->d_name(), the normally path doesn't have any
> >> +     * sleepable code, and despite it uses the current macro to get
> >> +     * fs_struct (current->fs), we've already ensured we're in user
> >> +     * context, so it's ok to be here.
> >> +     */
> >> +    p = d_path(&f->f_path, dst, size);
> >> +    if (IS_ERR(p)) {
> >> +            ret = PTR_ERR(p);
> >> +            fput(f);
> >> +            goto error;
> >> +    }
> >> +
> >> +    ret = strlen(p) + 1;
> >> +    memmove(dst, p, ret);
> >> +    fput(f);
> >> +    return ret;
> >> +
> >> +error:
> >> +    memset(dst, '0', size);
>
> You fill it with 0x30's ...
>
> >> +    return ret;
> >> +}