Comment 2 for bug 1783362

I did some research and found that this bug is present since 2003:

- 2003/05/13: https://github.com/qemu/qemu/commit/54936004fddc52c321cb3f9a9a51140e782bed5d#diff-2bf4728e0473404c39c97190bd02b2f8
  - https://github.com/qemu/qemu/blob/54936004fddc52c321cb3f9a9a51140e782bed5d/linux-user/mmap.c#L182-L183
- 2008/06/02: https://github.com/qemu/qemu/commit/c8a706fe6242a553960ccc3071a4e75ceba6f3d2#diff-2bf4728e0473404c39c97190bd02b2f8
  - https://github.com/qemu/qemu/blob/c8a706fe6242a553960ccc3071a4e75ceba6f3d2/linux-user/mmap.c#L284-L285
  - https://github.com/qemu/qemu/blob/c8a706fe6242a553960ccc3071a4e75ceba6f3d2/linux-user/mmap.c#L400-L410

It is present in versions 2.11.2, 2.12.0 and master:

- https://github.com/qemu/qemu/blob/v2.11.2/linux-user/mmap.c#L401-L402
- https://github.com/qemu/qemu/blob/v2.12.0/linux-user/mmap.c#L401-L402
- https://github.com/qemu/qemu/blob/master/linux-user/mmap.c#L400-L401

I think that a possible fix is:

@@ -397,8 +397,10 @@ abi_long target_mmap(abi_ulong start, abi_ulong len, int prot,
     }

     len = TARGET_PAGE_ALIGN(len);
- if (len == 0)
- goto the_end;
+ if (len == 0) {
+ errno = EINVAL;
+ goto fail;
+ }
     real_start = start & qemu_host_page_mask;
     host_offset = offset & qemu_host_page_mask;