[lxc-devel] [PATCH 1/2 v3] Add clone_update_unexp_ovl_paths() function

[Christian Brauner]

On Mon, Nov 02, 2015 at 03:52:25PM +0100, Christian Brauner wrote:
> On Mon, Nov 02, 2015 at 02:38:16PM +0000, Serge Hallyn wrote:
> > Quoting Christian Brauner (christianvanbrauner at gmail.com):
> > > This functions updates absolute paths for overlay upper- and workdirs so users
> > > can simply clone and start new containers without worrying about absolute paths
> > > in lxc.mount.entry overlay entries.
> > > 
> > > Signed-off-by: Christian Brauner <christianvanbrauner at gmail.com>
> > > ---
> > >  src/lxc/confile.c | 102 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
> > >  src/lxc/confile.h |   3 ++
> > >  2 files changed, 105 insertions(+)
> > > 
> > > diff --git a/src/lxc/confile.c b/src/lxc/confile.c
> > > index f7d6814..47b0b8b 100644
> > > --- a/src/lxc/confile.c
> > > +++ b/src/lxc/confile.c
> > > @@ -2598,6 +2598,108 @@ void clear_unexp_config_line(struct lxc_conf *conf, const char *key, bool rm_sub
> > >  	}
> > >  }
> > >  
> > > +bool clone_update_unexp_ovl_paths(struct lxc_conf *conf, const char *oldpath,
> > > +				  const char *newpath, const char *oldname,
> > > +				  const char *newname, const char *ovldir)
> > > +{
> > > +	const char *key = "lxc.mount.entry";
> > > +	int ret;
> > > +	char *lstart = conf->unexpanded_config;
> > > +	char *lend;
> > > +	char *p;
> > > +	char *q;
> > > +	size_t newdirlen = strlen(ovldir) + strlen(newpath) + strlen(newname) + 2;
> > > +	size_t olddirlen = strlen(ovldir) + strlen(oldpath) + strlen(oldname) + 2;
> > > +	char *olddir = alloca(olddirlen + 1);
> > > +	char *newdir = alloca(newdirlen + 1);
> > > +
> > > +	ret = snprintf(olddir, olddirlen + 1, "%s=%s/%s", ovldir, oldpath, oldname);
> > > +	if (ret < 0 || ret >= olddirlen + 1) {
> > > +		ERROR("Bug in %s", __func__);
> > > +		return false;
> > > +	}
> > > +	ret = snprintf(newdir, newdirlen + 1, "%s=%s/%s", ovldir, newpath, newname);
> > > +	if (ret < 0 || ret >= newdirlen + 1) {
> > > +		ERROR("Bug in %s", __func__);
> > > +		return false;
> > > +	}
> > > +	if (!conf->unexpanded_config)
> > > +		return true;
> > > +	while (*lstart) {
> > > +		lend = strchr(lstart, '\n');
> > > +		if (!lend)
> > > +			lend = lstart + strlen(lstart);
> > > +		else
> > > +			lend++;
> > > +		if (strncmp(lstart, key, strlen(key)) != 0) {
> > > +			lstart = lend;
> > > +			continue;
> > > +		}
> > > +		p = strchr(lstart + strlen(key), '=');
> > > +		if (!p) {
> > > +			lstart = lend;
> > > +			continue;
> > > +		}
> > > +		p++;
> > > +		while (isblank(*p))
> > > +			p++;
> > > +		if (p >= lend)
> > > +                        continue;
> > > +                /* Whenever an lxc.mount.entry entry is found in a line we check
> > > +                *  if the substring " overlay" or the substring " aufs" is
> > > +                *  present before doing any further work. We check for "
> > > +                *  overlay" and " aufs" since both substrings need to have at
> > > +                *  least one space before them in a valid overlay
> > > +                *  lxc.mount.entry (/A B overlay).  When the space before is
> > > +                *  missing it is very likely that these substrings are part of a
> > > +                *  path or something else. */
> > > +		if (!strstr(p, " overlay") && !strstr(p, " aufs")) {
> > > +			lstart = lend;
> > > +			continue;
> > > +		}
> > > +		if (!(q = strstr(p, olddir))) {
> > > +			lstart = lend;
> > > +			continue;
> > > +		}
> > 
> > You're still not doing anything to ensure that 'overlays'/'aufs' and the
> > olddir are actually occurring on this same line.  So they could be showing
> > up in a comment or just later line.

I don't think this is the case. At the beginning of the while() we search for a
newline character and make lend point to the character immediately after it.

		lend = strchr(lstart, '\n');
		if (!lend)
			lend = lstart + strlen(lstart);
		else
			lend++;

Then we check

		if (!strstr(p, " overlay") && !strstr(p, " aufs"))

if this check fails we set lstart to lend which points at the character after
'\n'. The same for the next line etc. etc.

If on a line the check:

		if (!strstr(p, " overlay") && !strstr(p, " aufs"))

is true we then check for the presence of olddir in p. 

		if (!(q = strstr(p, olddir))) {
			lstart = lend;
			continue;
		}

And we do that for every single line. But the algorithm guarantees, as far as I
understand it, that " overlay" or " aufs" and the olddir appear on the same
line. They cannot e.g. appear in successive lines. If you are however worried
about updating comments. We can easily adapt the function to check if a line,
when all whitespace is skipped, begins with a '#' and if so, skip that line.