Re: [PATCH 02/10] pasta: More general way of starting spawned shell as a login shell

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 13 Oct 2022 19:22:27 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Thu, Oct 13, 2022 at 04:16:59AM +0200, Stefano Brivio wrote:
> > Just nits here:
> > 
> > On Tue, 11 Oct 2022 16:40:10 +1100
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> >   
> > > When invoked so as to spawn a shell, pasta checks explicitly for the
> > > shell being bash and if so, adds a "-l" option to make it a login shell.
> > > This is not ideal, since this is a bash specific option and requires pasta
> > > to know about specific shell variants.
> > > 
> > > There's a general convention for starting a login shell, which is to
> > > prepend a "-" to argv[0].  Use this approach instead, so we don't need bash
> > > specific logic.  
> > 
> > Hah, I didn't know that was the meaning.
> >   
> > > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > > ---
> > >  pasta.c | 32 ++++++++++++++++++++------------
> > >  1 file changed, 20 insertions(+), 12 deletions(-)
> > > 
> > > diff --git a/pasta.c b/pasta.c
> > > index 1dd8267..7c3acef 100644
> > > --- a/pasta.c
> > > +++ b/pasta.c
> > > @@ -148,10 +148,12 @@ void pasta_open_ns(struct ctx *c, const char *netns)
> > >  
> > >  /**
> > >   * struct pasta_setup_ns_arg - Argument for pasta_setup_ns()
> > > + * @exe:	Executable to run
> > >   * @argv:	Command and arguments to run
> > >   */
> > >  struct pasta_setup_ns_arg {
> > > -	char **argv;
> > > +	const char *exe;
> > > +	char *const *argv;
> > >  };
> > >  
> > >  /**
> > > @@ -162,12 +164,13 @@ struct pasta_setup_ns_arg {
> > >   */
> > >  static int pasta_setup_ns(void *arg)
> > >  {
> > > -	struct pasta_setup_ns_arg *a = (struct pasta_setup_ns_arg *)arg;
> > > +	const struct pasta_setup_ns_arg *a
> > > +		= (const struct pasta_setup_ns_arg *)arg;  
> > 
> > At this point the assignment could be split onto another line.  
> 
> Uh.. I'm not sure what you mean by that.

const struct pasta_setup_ns_arg *a;

a = (const struct pasta_setup_ns_arg *)arg;

> 
> > >  
> > >  	FWRITE("/proc/sys/net/ipv4/ping_group_range", "0 0",
> > >  	       "Cannot set ping_group_range, ICMP requests might fail");
> > >  
> > > -	execvp(a->argv[0], a->argv);
> > > +	execvp(a->exe, a->argv);
> > >  
> > >  	perror("execvp");
> > >  	exit(EXIT_FAILURE);
> > > @@ -182,26 +185,31 @@ static int pasta_setup_ns(void *arg)
> > >  void pasta_start_ns(struct ctx *c, int argc, char *argv[])
> > >  {
> > >  	struct pasta_setup_ns_arg arg = {
> > > +		.exe = argv[0],
> > >  		.argv = argv,
> > >  	};
> > > -	char *shell = getenv("SHELL");
> > > -	char *sh_argv[] = { shell, NULL };
> > > -	char *bash_argv[] = { shell, "-l", NULL };
> > > +	char *sh_argv[] = { NULL, NULL };
> > >  	char ns_fn_stack[NS_FN_STACK_SIZE];  
> > 
> > If you respin, it would be nice to have the usual ordering here
> > (sh_argv[] after ns_fn_stack).  
> 
> Done.
> 
> > > +	char sh_arg0[PATH_MAX + 1];
> > >  
> > >  	c->foreground = 1;
> > >  	if (!c->debug)
> > >  		c->quiet = 1;
> > >  
> > > -	if (!shell)
> > > -		shell = "/bin/sh";
> > >  
> > >  	if (argc == 0) {
> > > -		if (strstr(shell, "/bash")) {
> > > -			arg.argv = bash_argv;
> > > -		} else {
> > > -			arg.argv = sh_argv;
> > > +		arg.exe = getenv("SHELL");
> > > +		if (!arg.exe)
> > > +			arg.exe = "/bin/sh";
> > > +
> > > +		if ((size_t)snprintf(sh_arg0, sizeof(sh_arg0),
> > > +				     "-%s", arg.exe) >= sizeof(sh_arg0)) {  
> > 
> > This is completely specified and looks safe, but it also looks more
> > complicated than it actually is, at a glance.
> > 
> > Maybe a separate length check before snprintf() would make it look
> > more natural. Not a strong preference though.  
> 
> Uh.. not sure what you mean by that either.

Ah, sorry.

if ((len = strlen(arg.exe)) + 1 /* - */ + 1 /* \0 */ > sizeof(sh_arg0))
	err("$SHELL is too long (%u bytes)", strlen(arg.exe));

(void)snprintf(sh_arg0, sizeof(sh_arg0), "-%s", arg.exe);

I don't know, it doesn't look so... cramped. Maybe it's just me. If it
doesn't make sense just disregard ;)

-- 
Stefano