Re: [PATCH] pasta.c: modify hostname when detaching new namespace

[Stefano Brivio <sbrivio@redhat.com>]

Hi Danish,

On Fri, 24 May 2024 19:39:18 +0200
Stefano Brivio <sbrivio@redhat.com> wrote:

> On Fri, 24 May 2024 18:18:53 +0530
> Danish Prakash <contact@danishpraka.sh> wrote:
> 
> > When invoking pasta without any arguments, it's difficult
> > to tell whether we are in the new namespace or not leaving
> > users a bit confused. This change modifies the host namespace
> > to add a prefix "pasta-" to make it a bit more obvious.
> > 
> > Signed-off-by: Danish Prakash <contact@danishpraka.sh>
> > ---
> >  pasta.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > diff --git a/pasta.c b/pasta.c
> > index 31e1e00..90afd74 100644
> > --- a/pasta.c
> > +++ b/pasta.c
> > @@ -50,6 +50,8 @@
> >  #include "netlink.h"
> >  #include "log.h"
> >  
> > +#define HOSTNAME_PREFIX		"pasta-"
> > +
> >  /* PID of child, in case we created a namespace */
> >  int pasta_child_pid;
> >  
> > @@ -178,6 +180,7 @@ struct pasta_spawn_cmd_arg {
> >   */
> >  static int pasta_spawn_cmd(void *arg)
> >  {
> > +	char hostname[HOST_NAME_MAX + 1] = HOSTNAME_PREFIX;
> >  	const struct pasta_spawn_cmd_arg *a;
> >  	sigset_t set;
> >  
> > @@ -188,6 +191,12 @@ static int pasta_spawn_cmd(void *arg)
> >  	if (write_file("/proc/sys/net/ipv4/ping_group_range", "0 0"))
> >  		warn("Cannot set ping_group_range, ICMP requests might fail");
> >  
> > +	if (!gethostname(hostname + sizeof(HOSTNAME_PREFIX) - 1, HOST_NAME_MAX + 1 - sizeof(HOSTNAME_PREFIX)) ||  
> 
> Following the Linux kernel coding style also means we try to stick into
> 80 columns where possible:
> 
>   https://www.kernel.org/doc/html/latest/process/coding-style.html#breaking-long-lines-and-strings
> 
> ...so there was a reason why I proposed this line like I did, with the
> line splits. These subtleties, I can also fix them up on merge.
> 
> > +            errno == ENAMETOOLONG ) {  
> 
> Excess whitespace between ENAMETOOLONG and ). Same here, I would fix
> this up on merge.
> 
> > +		if (sethostname(hostname, strlen(hostname)))  
> 
> So, I mentioned before that you don't really need to set a NULL
> terminating byte for sethostname() itself, because it takes a length.
> 
> But strlen() needs it. If gethostname() truncated the hostname,
> according to POSIX, it's unspecified whether we'll have a NULL byte at
> the end of 'hostname', and strlen() would read out-of-bounds, past the
> end of 'hostname'.
> 
> That's not an issue with glibc, but if POSIX says it's not guaranteed,
> we shouldn't take anything for granted.
> 
> I would suggest that you simply add a NULL byte at HOST_NAME_MAX,
> unconditionally, that should cover the normal case as well as the
> ENAMETOOLONG case. I haven't tested this by the way.
> 
> > +			warn("Unable to set pasta-prefixed hostname");
> > +	}
> > +
> >  	/* Wait for the parent to be ready: see main() */
> >  	sigemptyset(&set);
> >  	sigaddset(&set, SIGUSR1);  
> 

I didn't get any follow up on this patch. Did you actually mean to
abandon it?

If you prefer, I can also pick it up from there.

-- 
Stefano