Subject: Re: [PATCH 2/3] bpf: Add d_path helper

From: Jiri Olsa <[email-masked]>
Date: Fri, 3 Apr 2020 11:01:58 +0200

On Thu, Apr 02, 2020 at 04:02:55PM +0200, Florent Revest wrote:
> On Wed, 2020-04-01 at 13:09 +0200, Jiri Olsa wrote:
> > + * int bpf_d_path(struct path *path, char *buf, u32 sz)
> > + *	Description
> > + *		Return full path for given 'struct path' object, which
> > + *		needs to be the kernel BTF 'path' object. The path is
> > + *		returned in buffer provided 'buf' of size 'sz'.
> > + *
> > + *	Return
> > + *		length of returned string on success, or a negative
> > + *		error in case of failure
> > + *
> 
> You might want to add that d_path is ambiguous since it can add
> " (deleted)" at the end of your path and you don't know whether this is
> actually part of the file path or not. :) 

right

> 
> > +BPF_CALL_3(bpf_d_path, struct path *, path, char *, buf, u32, sz)
> > +{
> > +	char *p = d_path(path, buf, sz - 1);
> 
> I am curious why you'd use sz - 1 here? In my experience, d_path's
> output is 0 limited so you shouldn't need to keep an extra byte for
> that (if that was the intention here).
> 
> > +	int len;
> > +
> > +	if (IS_ERR(p)) {
> > +		len = PTR_ERR(p);
> > +	} else {
> > +		len = strlen(p);
> > +		if (len && p != buf) {
> > +			memmove(buf, p, len);
> 
> Have you considered returning the offset within buf instead and let the
> BPF program do pointer arithmetics to find the beginning of the string?

we could do that.. I was following some other user of d_path,
which I can't find at the moment ;-) I'll check

> 
> > +			buf[len] = 0;
> 
> If my previous comment about sz - 1 is true, then this wouldn't be
> necessary, you could just use memmove with len + 1.

hum, you might be right, I'll check on this

thanks,
jirka

> > +		}
> > +	}
> > +
> > +	return len;
> > +}