Re: [dm-devel] [PATCH 04/21] libmultipath: never allocate an alias that's already taken

[Benjamin Marzinski <bmarzins@redhat.com>]

On Fri, Sep 01, 2023 at 08:02:17PM +0200, mwilck@suse.com wrote:
> From: Martin Wilck <mwilck@suse.com>
> 
> If the bindings file is changed in a way that multipathd can't handle
> (e.g. by swapping the aliases of two maps), multipathd must not try
> to re-use an alias that is already used by another map. Creating
> or renaming a map with such an alias will fail. We already avoid
> this for some cases, but not for all. Fix it.
> 
> Signed-off-by: Martin Wilck <mwilck@suse.com>
> Cc: David Bond <dbond@suse.com>
> ---
>  libmultipath/alias.c | 36 +++++++++++++++++++++++++++++++-----
>  tests/alias.c        |  2 +-
>  2 files changed, 32 insertions(+), 6 deletions(-)
> 
> diff --git a/libmultipath/alias.c b/libmultipath/alias.c
> index 9b9b789..f7834d1 100644
> --- a/libmultipath/alias.c
> +++ b/libmultipath/alias.c
> @@ -120,7 +120,7 @@ static bool alias_already_taken(const char *alias, const char *map_wwid)
>  		if (dm_get_uuid(alias, wwid, sizeof(wwid)) == 0 &&
>  		    strncmp(map_wwid, wwid, sizeof(wwid)) == 0)
>  			return false;
> -		condlog(3, "%s: alias '%s' already taken, but not in bindings file. reselecting alias",
> +		condlog(3, "%s: alias '%s' already taken, reselecting alias",
>  			map_wwid, alias);
>  		return true;
>  	}
> @@ -363,28 +363,54 @@ char *get_user_friendly_alias(const char *wwid, const char *file, const char *al
>  		 * allocated correctly
>  		 */
>  		if (strcmp(buff, wwid) == 0) {

I'm confused about this. We should only have alias_old set if there
already exists a device that matches this WWID, right? That's what I
though alias_old means. Am I missing some way that alias_old could get
set to something other than the alias of an already existing device with
a matching WWID? Otherwise, once we verify that that this mapping also
exists in the bindings file, we should be fine to use it?

> -			alias = strdup(alias_old);
> +			if (!alias_already_taken(alias_old, wwid))
> +				alias = strdup(alias_old);
> +			else
> +				condlog(0, "ERROR: old alias [%s] for wwid [%s] is used by other map",
> +					alias_old, wwid);
>  			goto out;
> +
>  		} else {
>  			condlog(0, "alias %s already bound to wwid %s, cannot reuse",
>  				alias_old, buff);
> -			goto new_alias;

extra semicolon.

> +			goto new_alias;		     ;
>  		}
>  	}
>  
> +	/*
> +	 * Look for an existing alias in the bindings file.
> +	 * Pass prefix = NULL, so lookup_binding() won't try to allocate a new id.
> +	 */

There's no point in saving the return value here. We don't use if for
anything.

>  	id = lookup_binding(f, wwid, &alias, NULL, 0);
>  	if (alias) {
> -		condlog(3, "Use existing binding [%s] for WWID [%s]",
> -			alias, wwid);
> +		if (alias_already_taken(alias, wwid)) {
> +			free(alias);
> +			alias = NULL;
> +		} else
> +			condlog(3, "Use existing binding [%s] for WWID [%s]",
> +				alias, wwid);
>  		goto out;
>  	}
>  
>  	/* allocate the existing alias in the bindings file */
>  	id = scan_devname(alias_old, prefix);

Again, unless I'm overlooking something, I don't think we need to
check if the alias is already taken here. Since we know that a device
already exists with alias_old and the correct WWID, as long as alias_old
is a valid user_friendly_name we can just use it.

> +	if (id > 0 && id_already_taken(id, prefix, wwid)) {
> +		condlog(0, "ERROR: old alias [%s] for wwid [%s] is used by other map",
> +			alias_old, wwid);
> +		goto out;
> +	}
>  
>  new_alias:
>  	if (id <= 0) {
> +		/*
> +		 * no existing alias was provided, or allocating it
> +		 * failed. Try a new one.
> +		 */
>  		id = lookup_binding(f, wwid, &alias, prefix, 1);

If lookup_binding() was going to return (id == 0) it already would have
when we called it earlier in this function, so I don't think this check
is necessary either.

-Ben

> +		if (id == 0 && alias_already_taken(alias, wwid)) {
> +			free(alias);
> +			alias = NULL;
> +		}
>  		if (id <= 0)
>  			goto out;
>  		else
> diff --git a/tests/alias.c b/tests/alias.c
> index 3ca6c28..11f209e 100644
> --- a/tests/alias.c
> +++ b/tests/alias.c
> @@ -398,7 +398,7 @@ static void mock_self_alias(const char *alias, const char *wwid)
>  	will_return(__wrap_dm_get_uuid, wwid);
>  }
>  
> -#define USED_STR(alias_str, wwid_str) wwid_str ": alias '" alias_str "' already taken, but not in bindings file. reselecting alias\n"
> +#define USED_STR(alias_str, wwid_str) wwid_str ": alias '" alias_str "' already taken, reselecting alias\n"
>  
>  static void mock_failed_alias(const char *alias, char *msg)
>  {
> -- 
> 2.41.0
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