Re: [PATCH 4/8] fwd: Move port exclusion handling from procfs_scan_listen() to callers

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Oct 30, 2025 at 09:24:04PM +0100, Stefano Brivio wrote:
> On Sat, 11 Oct 2025 15:48:23 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > To avoid forwarding loops, we need to exclude certain ports from being
> > auto-forwarded.  To accomplish this, procfs_scan_listen() takes a bitmap
> > of exclusions.  As it detects each port, it checks against that bitmap.
> > This is a complicated way of accomplishing what we need.  We can instead
> > mask out the excluded ports in the callers using a new bitmap_andc()
> > helper.
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  fwd.c  | 32 ++++++++++++++------------------
> >  util.c | 23 +++++++++++++++++++++++
> >  util.h |  1 +
> >  3 files changed, 38 insertions(+), 18 deletions(-)
> > 
> > diff --git a/fwd.c b/fwd.c
> > index 19309f14..c7b768d5 100644
> > --- a/fwd.c
> > +++ b/fwd.c
> > @@ -110,13 +110,11 @@ bool fwd_port_is_ephemeral(in_port_t port)
> >   * @fd:		fd for relevant /proc/net file
> >   * @lstate:	Code for listening state to scan for
> >   * @map:	Bitmap where numbers of ports in listening state will be set
> > - * @exclude:	Bitmap of ports to exclude from setting (and clear)
> >   *
> >   * #syscalls:pasta lseek
> >   * #syscalls:pasta ppc64le:_llseek ppc64:_llseek arm:_llseek
> >   */
> > -static void procfs_scan_listen(int fd, unsigned int lstate,
> > -			       uint8_t *map, const uint8_t *exclude)
> > +static void procfs_scan_listen(int fd, unsigned int lstate, uint8_t *map)
> >  {
> >  	struct lineread lr;
> >  	unsigned long port;
> > @@ -141,10 +139,7 @@ static void procfs_scan_listen(int fd, unsigned int lstate,
> >  		if (state != lstate)
> >  			continue;
> >  
> > -		if (bitmap_isset(exclude, port))
> > -			bitmap_clear(map, port);
> > -		else
> > -			bitmap_set(map, port);
> > +		bitmap_set(map, port);
> >  	}
> >  }
> >  
> > @@ -157,8 +152,9 @@ static void fwd_scan_ports_tcp(struct fwd_ports *fwd,
> >  			       const struct fwd_ports *rev)
> >  {
> >  	memset(fwd->map, 0, PORT_BITMAP_SIZE);
> > -	procfs_scan_listen(fwd->scan4, TCP_LISTEN, fwd->map, rev->map);
> > -	procfs_scan_listen(fwd->scan6, TCP_LISTEN, fwd->map, rev->map);
> > +	procfs_scan_listen(fwd->scan4, TCP_LISTEN, fwd->map);
> > +	procfs_scan_listen(fwd->scan6, TCP_LISTEN, fwd->map);
> > +	bitmap_andc(fwd->map, PORT_BITMAP_SIZE, fwd->map, rev->map);
> 
> I'm not entirely sure why I implemented it this way in 9657b6ed05cc
> ("conf, tcp: Periodic detection of bound ports for pasta port
> forwarding").
> 
> I guess the original idea was that only procfs_scan_listen() would
> manipulate bitmaps, for whatever reason. In any case, I'm fairly
> sure that this is equivalent (it obviously look like it, but I'm
> having a hard time to convince myself because of the weird way I
> implemented it originally).
> 
> >  }
> >  
> >  /**
> > @@ -173,26 +169,26 @@ static void fwd_scan_ports_udp(struct fwd_ports *fwd,
> >  			       const struct fwd_ports *tcp_fwd,
> >  			       const struct fwd_ports *tcp_rev)
> >  {
> > -	uint8_t exclude[PORT_BITMAP_SIZE];
> > -
> > -	bitmap_or(exclude, PORT_BITMAP_SIZE, rev->map, tcp_rev->map);
> > -
> >  	memset(fwd->map, 0, PORT_BITMAP_SIZE);
> > -	procfs_scan_listen(fwd->scan4, UDP_LISTEN, fwd->map, exclude);
> > -	procfs_scan_listen(fwd->scan6, UDP_LISTEN, fwd->map, exclude);
> > +	procfs_scan_listen(fwd->scan4, UDP_LISTEN, fwd->map);
> > +	procfs_scan_listen(fwd->scan6, UDP_LISTEN, fwd->map);
> >  
> >  	/* Also forward UDP ports with the same numbers as bound TCP ports.
> >  	 * This is useful for a handful of protocols (e.g. iperf3) where a TCP
> >  	 * control port is used to set up transfers on a corresponding UDP
> >  	 * port.
> > -	 *
> > +	 */
> > +	procfs_scan_listen(tcp_fwd->scan4, TCP_LISTEN, fwd->map);
> > +	procfs_scan_listen(tcp_fwd->scan6, TCP_LISTEN, fwd->map);
> > +
> > +	/*
> >  	 * This means we need to skip numbers of TCP ports bound on the other
> 
> Nit: /* This ...

Fixed.

> >  	 * side, too. Otherwise, we would detect corresponding UDP ports as
> >  	 * bound and try to forward them from the opposite side, but it's
> >  	 * already us handling them.
> >  	 */
> > -	procfs_scan_listen(tcp_fwd->scan4, TCP_LISTEN, fwd->map, exclude);
> > -	procfs_scan_listen(tcp_fwd->scan6, TCP_LISTEN, fwd->map, exclude);
> > +	bitmap_andc(fwd->map, PORT_BITMAP_SIZE, fwd->map, rev->map);
> > +	bitmap_andc(fwd->map, PORT_BITMAP_SIZE, fwd->map, tcp_rev->map);
> >  }
> >  
> >  /**
> > diff --git a/util.c b/util.c
> > index c492f904..eabadf7d 100644
> > --- a/util.c
> > +++ b/util.c
> > @@ -322,6 +322,7 @@ void bitmap_set(uint8_t *map, unsigned bit)
> >   * @map:	Pointer to bitmap
> >   * @bit:	Bit number to clear
> >   */
> > +/* cppcheck-suppress unusedFunction */
> >  void bitmap_clear(uint8_t *map, unsigned bit)
> >  {
> >  	unsigned long *word = (unsigned long *)map + BITMAP_WORD(bit);
> > @@ -351,6 +352,7 @@ bool bitmap_isset(const uint8_t *map, unsigned bit)
> >   * @a:		First operand
> >   * @b:		Second operand
> >   */
> > +/* cppcheck-suppress unusedFunction */
> 
> Should we just drop those? git stores them for us.

Eh, maybe?  It felt weird to remove a function that seemed like it
belonged in the bitmap "library" of functions.  Plus at the time I
thought I might use it again later in the series.  I didn't but I
still might use it relatively soon in follow up series.

> >  void bitmap_or(uint8_t *dst, size_t size, const uint8_t *a, const uint8_t *b)
> >  {
> >  	unsigned long *dw = (unsigned long *)dst;
> > @@ -365,6 +367,27 @@ void bitmap_or(uint8_t *dst, size_t size, const uint8_t *a, const uint8_t *b)
> >  		dst[i] = a[i] | b[i];
> >  }
> >  
> > +/**
> > + * bitmap_andc() - Logical conjunction with complement (AND NOT) of bitmap
> 
> Nit: this function name mixes classic logic terminology (conjunction,
> complement) and operator names (and, not), which makes it hard to guess,
> I think.

That's my POWER background showing:
    https://www.ibm.com/docs/en/aix/7.3.0?topic=set-andc-complement-instruction

> The Linux kernel has __bitmap_andnot(), which I find rather cacophonic,
> so what about bitmap_and_not() instead? It kind of fits with how we call
> these things in passt.

Makes sense, done.

> > + * @dst:	Pointer to result bitmap
> > + * @size:	Size of bitmaps, in bytes
> > + * @a:		First operand
> > + * @b:		Second operand
> > + */
> > +void bitmap_andc(uint8_t *dst, size_t size, const uint8_t *a, const uint8_t *b)
> > +{
> > +	unsigned long *dw = (unsigned long *)dst;
> > +	unsigned long *aw = (unsigned long *)a;
> > +	unsigned long *bw = (unsigned long *)b;
> > +	size_t i;
> > +
> > +	for (i = 0; i < size / sizeof(long); i++, dw++, aw++, bw++)
> > +		*dw = *aw & ~*bw;
> > +
> > +	for (i = size / sizeof(long) * sizeof(long); i < size; i++)
> > +		dst[i] = a[i] & ~b[i];
> > +}
> > +
> >  /**
> >   * ns_enter() - Enter configured user (unless already joined) and network ns
> >   * @c:		Execution context
> > diff --git a/util.h b/util.h
> > index 22eaac56..ac8339a3 100644
> > --- a/util.h
> > +++ b/util.h
> > @@ -213,6 +213,7 @@ void bitmap_set(uint8_t *map, unsigned bit);
> >  void bitmap_clear(uint8_t *map, unsigned bit);
> >  bool bitmap_isset(const uint8_t *map, unsigned bit);
> >  void bitmap_or(uint8_t *dst, size_t size, const uint8_t *a, const uint8_t *b);
> > +void bitmap_andc(uint8_t *dst, size_t size, const uint8_t *a, const uint8_t *b);
> >  char *line_read(char *buf, size_t len, int fd);
> >  void ns_enter(const struct ctx *c);
> >  bool ns_is_init(void);
> 
> -- 
> Stefano
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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