Re: [PATCH] linux-user: make binfmt flag O require P

[Laurent Vivier]

Le 14/12/2019 à 13:20, Joakim Tjernlund a écrit :
> From: Joakim Tjernlund <address@hidden>
> 
> QEMU can autodetect if it is started from Linux binfmt loader
> when binfmt flag O is on.
> Use that and require binfmt flag P as well which will enable QEMU
> to pass in correct argv0 to the application.

I agree it's a good idea to try to detect the P flag if we use the O but
it changes the current behavior and breaks the compatibility with
previous version. This will prevent to run old and new version of QEMU
on the same system.

I already try to find a solution to this problem.

The first one is at QEMU level:

https://patchew.org/QEMU/address@hidden/

Another would be at linux level to provide a way to detect binfmt flags
(like you try to do with AT_EXECFD):

https://lore.kernel.org/patchwork/patch/1158151/

I also found another one from another author:

https://patchwork.kernel.org/patch/10902935/

Thanks,
Laurent


> Signed-off-by: Joakim Tjernlund <address@hidden>
> ---
>  linux-user/main.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/linux-user/main.c b/linux-user/main.c
> index 6ff7851e86..1b626e5762 100644
> --- a/linux-user/main.c
> +++ b/linux-user/main.c
> @@ -544,7 +544,7 @@ static void usage(int exitcode)
>      exit(exitcode);
>  }
>  
> -static int parse_args(int argc, char **argv)
> +static int parse_args(int argc, char **argv, int assume_P_flag)
>  {
>      const char *r;
>      int optind;
> @@ -560,7 +560,17 @@ static int parse_args(int argc, char **argv)
>              arginfo->handle_opt(r);
>          }
>      }
> -
> +    if (assume_P_flag) {
> +        /* Assume started by binmisc and binfmt P flag is set */
> +        if (argc < 3) {
> +            fprintf(stderr, "%s: Please use me through binfmt with P flag\n",
> +                    argv[0]);
> +            exit(1);
> +        }
> +        exec_path = argv[1];
> +        /* Next argv must be argv0 for the app */
> +        return 2;
> +    }
>      optind = 1;
>      for (;;) {
>          if (optind >= argc) {
> @@ -659,7 +669,8 @@ int main(int argc, char **argv, char **envp)
>      qemu_add_opts(&qemu_trace_opts);
>      qemu_plugin_add_opts();
>  
> -    optind = parse_args(argc, argv);
> +    execfd = qemu_getauxval(AT_EXECFD);
> +    optind = parse_args(argc, argv,  execfd > 0);
>  
>      if (!trace_init_backends()) {
>          exit(1);
> @@ -682,7 +693,6 @@ int main(int argc, char **argv, char **envp)
>  
>      init_qemu_uname_release();
>  
> -    execfd = qemu_getauxval(AT_EXECFD);
>      if (execfd == 0) {
>          execfd = open(exec_path, O_RDONLY);
>          if (execfd < 0) {
>