From: Theo Buehler <tb@theobuehler.org>
Subject: Re: httpd: fix scan-build dead stores findings
To: Rafael Sadowski <rafael@sizeofvoid.org>
Cc: tech@openbsd.org
Date: Fri, 2 Jan 2026 20:48:09 +0100

On Fri, Jan 02, 2026 at 08:14:29PM +0100, Rafael Sadowski wrote:
> On Thu Jan 01, 2026 at 10:12:57AM +0100, Stefan Sperling wrote:
> > On Sun, Dec 28, 2025 at 06:55:28AM +0100, Rafael Sadowski wrote:
> > > Hi,
> > > 
> > > scan-build flagged a few dead stores in httpd:
> > > 
> > > - config.c:636 – s incremented but never used after
> > > - httpd.c:533 – q assigned but immediately overwritten in the loop
> > > - server.c:891 – inrd/inwr assigned, then reassigned, never actually needed
> > > - server_fcgi.c:678,690 – kv result unused
> > > 
> > > OK?
> > > 
> > > Diff removes all of them. OK?
> > > 1. src/usr.sbin/httpd/config.c:636:3: warning: Value stored to 's' is never read [deadcode.DeadStores]
> > >   636 |                 s += srv_conf->return_uri_len;
> > > 
> > > 2. src/usr.sbin/httpd/httpd.c:533:6: warning: Although the value stored to 'q' is used in the enclosing expression, the value is never actually read from 'q' [deadcode.DeadStores]
> > >   533 |         p = q = label;
> > >       |             ^   ~~~~~
> > > 
> > > 3. src/usr.sbin/httpd/server.c:891:15: warning: Value stored to 'inrd' during its initialization is never read [deadcode.DeadStores]
> > >   891 |         evbuffercb               inrd = server_read;
> > >       |                                  ^~~~   ~~~~~~~~~~~
> > > 
> > > 4. src/usr.sbin/httpd/server_fcgi.c:678:9: warning: Although the value stored to 'kv' is used in the enclosing expression, the value is never actually read from 'kv' [deadcode.DeadStores]
> > >   678 |                         if ((kv = kv_find(&resp->http_headers, &key)) == NULL) {
> > >       |                              ^    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > src/usr.sbin/httpd/server_fcgi.c:690:8: warning: Although the value stored to 'kv' is used in the enclosing expression, the value is never actually read from 'kv' [deadcode.DeadStores]
> > >   690 |                 if ((kv = kv_find(&resp->http_headers, &key)) != NULL) {
> > >       |                      ^    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > 
> > > 
> > > diff --git a/usr.sbin/httpd/config.c b/usr.sbin/httpd/config.c
> > > index b45081129b7..aa2cd382ed3 100644
> > > --- a/usr.sbin/httpd/config.c
> > > +++ b/usr.sbin/httpd/config.c
> > > @@ -531,7 +531,6 @@ config_getserver_config(struct httpd *env, struct server *srv,
> > >  		if ((srv_conf->return_uri = get_data(p + s,
> > >  		    srv_conf->return_uri_len)) == NULL)
> > >  			goto fail;
> > > -		s += srv_conf->return_uri_len;
> > 
> > I am not 100% sure about this one. On one hand scan-build is right that the
> > value of s is never used in the remainder of this function. On the other
> > hand, this code seems to be written with the assumption in mind that more
> > optional chuks of data with variable length could be added in the future.
> > If we ever grow the server config structure and process more data via the
> > 'p + s' idiom then this line would need to be added back in to avoid a bug.
> > So perhaps this line is worth keeping?
> > 
> > The rest is ok stsp@
> 
> Thanks Stefan,
> 
> I've committed the rest of the dead store fixes but left this one out for now.
> 
> I see your point about future expansion, but I'd lean towards removing
> it. The comment already documents that more variable-length fields could
> be added. If someone later adds TLS data or other fields, they will need
> to modify this function anyway.

I think the more important thing to fix here is that there is no check
whether the imsg contains enough data to copy reurn_uri_len bytes from
p + s.

> 
> However, I don't really have a strong opinion on this either.
>