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

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

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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.

> >  
> >  	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.

> > +			err("$SHELL is too long (%u bytes)",
> > +			    strlen(arg.exe));
> > +			exit(EXIT_FAILURE);
> >  		}
> > +		sh_argv[0] = sh_arg0;
> > +		arg.argv = sh_argv;
> >  	}
> >  
> >  	pasta_child_pid = clone(pasta_setup_ns,
> 

-- 
David Gibson			| 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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