Re: [Qemu-devel] [PATCH] Fix commandline handling for ARM semihosted executables, on Linux and BSD hosts

[Peter Maydell <peter.maydell@linaro.org>]

On 23 November 2010 15:26, Schildbach, Wolfgang <WSCHI@dolby.com> wrote:
> When running an ARM semihosted executable on a linux machine, the
> command line is not delivered to the guest (see
> https://bugs.launchpad.net/qemu/+bug/673613).

I've tested this, and it does work; however I don't think the code
you have to handle the "empty command line" case is right.

> +                if (!host_cmdline_len) {
> +                    /* is arcg==0 even a possibility? */
> +                    arm_cmdline_buffer[0] = 0;
> +                    host_cmdline_len=1;
> +                }

To answer the question in the comment:
You can certainly start a native binary with argc==0 (ie an argv with only
the terminating NULL) using execv(). The qemu loader_exec() function
similarly separates the binary to be started from the argv array in its
API. As it happens the command line parsing in linux-user/main.c
doesn't provide the user with a way to give an empty array to
loader_exec(), but I think it's more robust to just handle the special
case here rather than impose an undocumented restriction on use
of loader_exec(). (After all ARM semihosting is definitely a minority
sport and this isn't exactly a performance critical bit of code :-))

In any case, I think that your handling for this case is being done
too late. By this point you have already done the "is the buffer
big enough" check and locked the user buffers with a host_cmdline_len
of zero even though you're now writing a byte here.

My suggestion would be just to handle this case early, by putting
something like this before the 'command line too long' test:

 if (host_cmdline_len == 0) {
     /* Simpler to treat the "empty command line" case specially */
     if (arm_cmdline_len < 1) {
         return -1;
     }
     arm_cmdline_buffer = lock_user(VERIFY_WRITE, ARG(0), 1, 0);
     arm_cmdline_buffer[0] = 0;
     SET_ARG(1, 0);
     unlock_user(arm_cmdline_buffer, ARG(0), 1);
     return 0;
 }

-- PMM