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

[Stefano Brivio <sbrivio@redhat.com>]

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);

-- 
Stefano