Re: [dm-devel] [PATCH v2 22/29] multipath: fix leaks in check_path_valid()

[Benjamin Marzinski <bmarzins@redhat.com>]

On Wed, Dec 16, 2020 at 06:34:05PM +0100, Martin Wilck wrote:
> On Fri, 2020-10-16 at 12:44 +0200, mwilck@suse.com wrote:
> > From: Martin Wilck <mwilck@suse.com>
> > 
> > There were two leaks in check_path_valid(): if path status was
> > successfully determined before calling store_pathvec(), free_path()
> > wasn't called. Also, if an error exit occured, neither cleanup
> > function was called.
> > 
> > This patch fixes both, at the cost of using "static" for the pp and
> > pathvec variables.
> 
> Looking at this again after 2 months, I think the on_exit() part of
> this patch is wrong. First, we can't use on_exit() on every platform,
> as e.g. musl libc doesn't have it. To replace this by the more portable
> atexit(), we'd need to declare the two variables "pp" and "pathvec"
> with file scope, which is very ugly. But more importantly, using static
> variables here causes check_path_valid() to be non-reentrant. While it
> doesn't have to be, it's still a coding pattern we haven't been using
> anywhere else, just to avoid a "memory leak" for an irregular exit,
> which isn't a real memory leak, actually.

A while ago, we had a conversation where we talked about not adding too
much complexity to deal with issues on shutdown, that will just go away
when multipathd stops. I still think that we shouldn't worry too much
about making sure we always free everything when it will automatically
get freed by the system anyway.

-Ben

> 
> One day we should remove the exit() calls somewhere deep down in our
> libraries, and deal with the respective errors cleanly.
> 
> @lixiaokeng, I hope this is ok for you, as you brought the issue up
> originally ("[QUESTION] memory leak in main (multipath)").
> 
> Regards
> Martin
> 
> > 
> > Reviewed-by: Benjamin Marzinski <bmarzins@redhat.com>
> > Signed-off-by: Martin Wilck <mwilck@suse.com>
> > ---
> >  multipath/main.c | 55 +++++++++++++++++++++++++++++++++++++---------
> > --
> >  1 file changed, 43 insertions(+), 12 deletions(-)
> > 
> > diff --git a/multipath/main.c b/multipath/main.c
> > index 049a36f..9974993 100644
> > --- a/multipath/main.c
> > +++ b/multipath/main.c
> > @@ -93,7 +93,7 @@ void rcu_register_thread_memb(void) {}
> >  void rcu_unregister_thread_memb(void) {}
> >  
> >  static int
> > -filter_pathvec (vector pathvec, char * refwwid)
> > +filter_pathvec (vector pathvec, const char *refwwid)
> >  {
> >  	int i;
> >  	struct path * pp;
> > @@ -592,12 +592,37 @@ out:
> >  	return r;
> >  }
> >  
> > +static void cleanup_pathvec(__attribute__((unused)) int dummy, void
> > *arg)
> > +{
> > +	vector *ppv = arg;
> > +
> > +	if (ppv && *ppv) {
> > +		free_pathvec(*ppv, FREE_PATHS);
> > +		*ppv = NULL;
> > +	}
> > +}
> > +
> > +static void cleanup_path(__attribute__((unused)) int dummy, void
> > *arg)
> > +{
> > +	struct path **ppp = arg;
> > +
> > +	if (ppp && *ppp) {
> > +		free_path(*ppp);
> > +		*ppp = NULL;
> > +	}
> > +}
> > +
> >  static int
> >  check_path_valid(const char *name, struct config *conf, bool
> > is_uevent)
> >  {
> >  	int fd, r = PATH_IS_ERROR;
> > -	struct path *pp = NULL;
> > -	vector pathvec = NULL;
> > +	static struct path *pp = NULL;
> > +	static vector pathvec = NULL;
> > +	const char *wwid;
> > +
> > +	/* register these as exit handlers in case we exit irregularly
> > */
> > +	on_exit(cleanup_path, &pp);
> > +	on_exit(cleanup_pathvec, &pathvec);
> >  
> >  	pp = alloc_path();
> >  	if (!pp)
> > @@ -667,13 +692,17 @@ check_path_valid(const char *name, struct
> > config *conf, bool is_uevent)
> >  	if (store_path(pathvec, pp) != 0) {
> >  		free_path(pp);
> >  		goto fail;
> > +	} else {
> > +		/* make sure path isn't freed twice */
> > +		wwid = pp->wwid;
> > +		pp = NULL;
> >  	}
> >  
> >  	/* For find_multipaths = SMART, if there is more than one path
> >  	 * matching the refwwid, then the path is valid */
> >  	if (path_discovery(pathvec, DI_SYSFS | DI_WWID) < 0)
> >  		goto fail;
> > -	filter_pathvec(pathvec, pp->wwid);
> > +	filter_pathvec(pathvec, wwid);
> >  	if (VECTOR_SIZE(pathvec) > 1)
> >  		r = PATH_IS_VALID;
> >  	else
> > @@ -681,21 +710,23 @@ check_path_valid(const char *name, struct
> > config *conf, bool is_uevent)
> >  
> >  out:
> >  	r = print_cmd_valid(r, pathvec, conf);
> > -	free_pathvec(pathvec, FREE_PATHS);
> >  	/*
> >  	 * multipath -u must exit with status 0, otherwise udev won't
> >  	 * import its output.
> >  	 */
> >  	if (!is_uevent && r == PATH_IS_NOT_VALID)
> > -		return RTVL_FAIL;
> > -	return RTVL_OK;
> > +		r = RTVL_FAIL;
> > +	else
> > +		r = RTVL_OK;
> > +	goto cleanup;
> >  
> >  fail:
> > -	if (pathvec)
> > -		free_pathvec(pathvec, FREE_PATHS);
> > -	else
> > -		free_path(pp);
> > -	return RTVL_FAIL;
> > +	r = RTVL_FAIL;
> > +
> > +cleanup:
> > +	cleanup_path(0, &pp);
> > +	cleanup_pathvec(0, &pathvec);
> > +	return r;
> >  }
> >  
> >  static int
> 

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