Re: [PATCH] linux-user: manage binfmt-misc preserve-arg[0] flag

[Michael Tokarev <mjt@tls.msk.ru>]

> Add --preserve-argv0 in qemu-binfmt-conf.sh to configure the preserve-argv0
> flag.
...
> diff --git a/linux-user/main.c b/linux-user/main.c
> @@ -697,6 +707,20 @@ int main(int argc, char **argv, char **envp)
>          }
>      }
>  
> +    /*
> +     * get binfmt_misc flags
> +     */
> +    preserve_argv0 = !!(qemu_getauxval(AT_FLAGS) & AT_FLAGS_PRESERVE_ARGV0);
> +
> +    /*
> +     * Manage binfmt-misc preserve-arg[0] flag
> +     *    argv[optind]     full path to the binary
> +     *    argv[optind + 1] original argv[0]
> +     */
> +    if (optind + 1 < argc && preserve_argv0) {
> +        optind++;
> +    }

Please note: this code is executed after parse_args() which is called
way up. And parse_args were able to mess up with the options & optind.

This is sort of a corner case really, but we rely on argv[1][0] being
!= '-'.  I think it is better to explicitly omit a call to parse_args()
for the AT_FLAGS_PRESERVE_ARGV0 case.  But parse_args() apparently is
a misnomer, since it also parses $ENVironment variables - this part
apparently should be run either way.

I noticed this because this interferes with my change in this area that
enables similar functionality (detecting the binfmt usage) but without
requiring any kernel changes and working with any version of kernel (it
has been discussed previously) - with both my code and this code in place
and the patched kernel, we update optind TWICE, one in parse_args() and
second here.  This caused someone filesystem to be wiped out already due
to wrong options being used.

Thanks,

/mjt