On Wed, Mar 25, 2026 at 01:56:22AM +0100, Stefano Brivio wrote:
> On Mon, 23 Mar 2026 18:37:22 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > Extend the dynamic update protocol to expose the pif indices and names
> > from a running passt/pasta to the pesto tool.  pesto records that data
> > and (for now) prints it out.
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  conf.c      | 38 +++++++++++++++++++++
> >  pesto.c     | 98 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
> >  serialise.c | 21 ++++++++++++
> >  serialise.h |  3 ++
> >  4 files changed, 159 insertions(+), 1 deletion(-)
> > 
> > diff --git a/conf.c b/conf.c
> > index 7b960fe9..b878db94 100644
> > --- a/conf.c
> > +++ b/conf.c
> > @@ -2303,6 +2303,41 @@ void conf(struct ctx *c, int argc, char **argv)
> >  		conf_print(c);
> >  }
> >  
> > +/**
> > + * conf_send_pifs() - Send list of pifs to dynamic update client (pesto)
> > + * @c:		Execution context
> > + * @fd:		Socket to the client
> > + *
> > + * Return: 0 on success, -1 on failure
> > + */
> > +static int conf_send_pifs(const struct ctx *c, int fd)
> > +{
> > +	uint32_t num = 0;
> > +	unsigned pif;
> > +
> > +	/* First count the number of pifs with tables */
> > +	for (pif = 0; pif < PIF_NUM_TYPES; pif++) {
> > +		if (c->fwd[pif])
> > +			num++;
> > +	}
> > +
> > +	if (write_u32(fd, num))
> > +		return -1;
> > +
> > +	for (pif = 0; pif < PIF_NUM_TYPES; pif++) {
> > +		if (!c->fwd[pif])
> > +			continue;
> > +
> > +		if (write_u8(fd, pif))
> > +			return -1;
> > +
> > +		if (write_str(fd, pif_name(pif)) < 0)
> > +			return -1;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> >  /**
> >   * conf_listen_handler() - Handle events on configuration listening socket
> >   * @c:		Execution context
> > @@ -2359,6 +2394,9 @@ void conf_listen_handler(struct ctx *c, uint32_t events)
> >  "Warning: Using experimental unsupported configuration protocol");
> >  	}
> >  
> > +	if (conf_send_pifs(c, fd) < 0)
> > +		goto fail;
> > +
> >  	return;
> >  
> >  fail:
> > diff --git a/pesto.c b/pesto.c
> > index f8c9e01d..ed4f2aab 100644
> > --- a/pesto.c
> > +++ b/pesto.c
> > @@ -36,6 +36,8 @@
> >  #include "serialise.h"
> >  #include "pesto.h"
> >  
> > +static int verbosity = 1;
> > +
> >  #define die(...)							\
> >  	do {								\
> >  		FPRINTF(stderr, __VA_ARGS__);				\
> > @@ -51,6 +53,19 @@
> >  		}							\
> >  	} while (0)
> >  
> > +/**
> > + * xmalloc() - Allocate memory, with fatal error on failure
> > + * @size:	Number of bytes to allocate
> > + */
> > +static void *xmalloc(size_t size)
> > +{
> > +	void *p = malloc(size);
> 
> I would really really prefer to skip this unless it's absolutely
> necessary, for a number of reasons:
> 
> 1. this thing is talking to passt which talks to untrusted workloads,
>    and there's a risk of arbitrary code execution, which implies the
>    possibility of specially crafted messages that might make us
>    allocate:
>
>    a. in specific regions and build a heap spraying attack based on that
> 
>    b. a lot, and cause a denial of service

Eh, I suppose.

>    ...and, while this process should be short lived, we can't assume
>    that, if somebody achieves arbitrary code execution in passt and
>    malicious messages, because we risk looping on rules, or having
>    passt send us information very slowly (minutes / hours), etc.

...ah, those are good points.

> 
> 2. besides, we might want to add a "daemon" mode one day, and then it's
>    not short lived anymore, which opens the door to leaks and
>    double-frees (adding to a. and b. above)

I really don't see any reason we'd want that, but I guess that's
irrelevant given that previous paragraph.

> 
> 3. we might need to reuse functions from passt and not notice right
>    away that they call this xmalloc(), or have to eventually adapt them
>    anyway

I mean.. when we test the path in question, we should hit the seccomp
filter, which should make it pretty obvious.

> 4. consistency, in messaging ("passt doesn't allocate dynamic memory",
>    without caveats) and for auditing reasons

Yeah, fair.

> 
> Strings can be 32 bytes, or 1024 if we want to splurge. Unless we need
> to pile a lot of them on the stack (but it's not the case in pesto) a
> malloc() doesn't really bring any advantage compared to static buffers
> here and there. It's not megabytes.

So, you've convinced me in principle, but I'm not putting this at the
top of my priorities.  Using malloc() makes things a bit easier while
we're playing around with the protocol a bunch.  Once we look
reasonably close to a v1 protocol, I'll do the malloc() removal.

> 
> > +
> > +	if (!p)
> > +		die("Memory allocation failure");
> > +	return p;
> > +}
> > +
> >  /**
> >   * usage() - Print usage, exit with given status code
> >   * @name:	Executable name
> > @@ -69,6 +84,83 @@ static void usage(const char *name, FILE *f, int status)
> >  	exit(status);
> >  }
> >  
> > +/**
> > + * pesto_recv_str() - Receive a string from passt/pasta
> > + * @fd:		Control socket
> > + *
> > + * Return: pointer to malloc()ed string
> > + */
> > +static const char *pesto_recv_str(int fd)
> > +{
> > +	uint32_t len;
> > +	char *buf;
> > +
> > +	if (read_u32(fd, &len) < 0)
> > +		die("Error reading from control socket");
> > +
> > +	buf = xmalloc(len);
> > +	if (read_all_buf(fd, buf, len) < 0)
> > +		die("Error reading from control socket");
> > +
> > +	return buf;
> > +}
> > +
> > +struct pif_state {
> > +	uint8_t pif;
> > +	const char *name;
> > +};
> > +
> > +struct conf_state {
> 
> More as a to-do list item rather than as a review comment: we need
> documentation for those structs.

Ah, yes.

> 
> > +	uint32_t npifs;
> > +	struct pif_state pif[];
> > +};
> > +
> > +/**
> > + * pesto_read_pifs() - Read pif names and IDs from passt/pasta
> > + * @fd:		Control socket
> > + */
> > +static const struct conf_state *pesto_read_pifs(int fd)
> > +{
> > +	uint32_t num;
> > +	struct conf_state *state;
> > +	unsigned i;
> > +
> > +	if (read_u32(fd, &num) < 0)
> > +		die("Error reading from control socket");
> > +
> > +	debug("Receiving %"PRIu32" interface names", num);
> > +
> > +	state = xmalloc(sizeof(*state) + num * sizeof(struct pif_state));
> > +	state->npifs = num;
> > +
> > +	for (i = 0; i < num; i++) {
> > +		struct pif_state *ps = &state->pif[i];
> > +
> > +		if (read_u8(fd, &ps->pif) < 0)
> > +			die("Error reading from control socket");
> > +		ps->name = pesto_recv_str(fd);
> > +
> > +		debug("%u: %s", ps->pif, ps->name);
> > +	}
> > +
> > +	return state;
> > +}
> > +
> > +/**
> > + * show_state() - Show current rule state obtained from passt/pasta
> > + * @pifs:	PIF name information
> > + */
> > +static void show_state(const struct conf_state *state)
> > +{
> > +	unsigned i;
> > +
> > +	for (i = 0; i < state->npifs; i++) {
> > +		const struct pif_state *ps = &state->pif[i];
> > +		printf("Forwarding rules for %s interface\n", ps->name);
> > +		printf("\tTBD\n");
> > +	}
> > +}
> > +
> >  /**
> >   * main() - Entry point and whole program with loop
> >   * @argc:	Argument count
> > @@ -91,12 +183,12 @@ int main(int argc, char **argv)
> >  		{ 0 },
> >  	};
> >  	struct sockaddr_un a = { AF_UNIX, "" };
> > +	const struct conf_state *state;
> >  	const char *optstring = "vh";
> >  	struct pesto_hello hello;
> >  	struct sock_fprog prog;
> >  	int optname, ret, s;
> >  	uint32_t s_version;
> > -	int verbosity = 1;
> >  
> >  	prog.len = (unsigned short)sizeof(filter_pesto) /
> >  				   sizeof(filter_pesto[0]);
> > @@ -172,5 +264,9 @@ int main(int argc, char **argv)
> >  "Warning: Using experimental protocol version, client and server must match\n");
> >  	}
> >  
> > +	state = pesto_read_pifs(s);
> > +
> > +	show_state(state);
> > +
> >  	exit(0);
> >  }
> > diff --git a/serialise.c b/serialise.c
> > index e3ea86e3..94c34d3c 100644
> > --- a/serialise.c
> > +++ b/serialise.c
> > @@ -19,6 +19,7 @@
> >  #include <endian.h>
> >  #include <errno.h>
> >  #include <stdint.h>
> > +#include <string.h>
> >  #include <unistd.h>
> >  
> >  #include "serialise.h"
> > @@ -121,6 +122,26 @@ int write_all_buf(int fd, const void *buf, size_t len)
> >  		return write_all_buf(fd, &beval, sizeof(beval));	\
> >  	}
> >  
> > +#define be8toh(x)	(x)
> > +#define htobe8(x)	(x)
> > +
> > +SERIALISE_UINT(8)
> >  SERIALISE_UINT(32)
> >  
> >  #undef SERIALISE_UNIT
> > +
> > +/**
> > + * write_str() - Write a string to an fd in length/value format
> > + * @fd:		Socket to the client
> > + * @s:		String to send
> > + *
> > + * Return: 0 on success, -1 on error
> > + */
> > +int write_str(int fd, const char *s)
> > +{
> > +	uint32_t len = strlen(s) + 1; /* Include \0 */
> > +
> > +	if (write_u32(fd, len) < 0)
> > +		return -1;
> 
> Even if we don't need to allocate here, I would feel more comfortable
> if we passed fixed-size strings only to passt to have a slightly lower
> risk of buffer overflows, in general.

That's fair.  I'd basically already decided to move to fixed length
pif names, just haven't implemented it yet.

> > +	return write_all_buf(fd, s, len);
> > +}
> > diff --git a/serialise.h b/serialise.h
> > index 0a4ed086..7ef35786 100644
> > --- a/serialise.h
> > +++ b/serialise.h
> > @@ -16,6 +16,9 @@ int write_all_buf(int fd, const void *buf, size_t len);
> >  	int read_u##bits(int fd, uint##bits##_t *val);			\
> >  	int write_u##bits(int fd, uint##bits##_t val);
> >  
> > +SERIALISE_UINT_DECL(8)
> >  SERIALISE_UINT_DECL(32)
> >  
> > +int write_str(int fd, const char *s);
> > +
> >  #endif /* _SERIALISE_H */
> 
> -- 
> 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