netdev - Re: [PATCH bpf-next v10 1/2] bpf: add new helper get_file_path for mapping a file descriptor to a pathname

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Date:   Sat, 23 Nov 2019 04:43:54 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Wenbo Zhang <ethercflow@...il.com>, bpf@...r.kernel.org,
        ast@...nel.org.com, daniel@...earbox.net, yhs@...com,
        andrii.nakryiko@...il.com, netdev@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH bpf-next v10 1/2] bpf: add new helper get_file_path for
 mapping a file descriptor to a pathname

On Fri, Nov 22, 2019 at 07:18:28PM -0800, Alexei Starovoitov wrote:
> > +	/* 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);
> > +	memmove(dst, p, ret);
> > +	dst[ret++] = '\0';
> > +	fput(f);
> > +	return ret;
> > +
> > +error:
> > +	memset(dst, '0', size);
> > +	return ret;
> > +}
> 
> Al,
> 
> could you please review about code whether it's doing enough checks to be
> called safely from preempt_disabled region?

Depends.  Which context is it running in?  In particular, which
locks might be already held?