Re: [Qemu-devel] [PATCH v2] linux-user: fix mmap_find_vma_reserved()

[Richard Henderson]

On 07/11/2018 09:40 AM, Laurent Vivier wrote:
> The value given by mmap_find_vma_reserved() is used with mmap(),
> so it is needed to be aligned with the host page size.
> 
> Since commit 18e80c55bb, reserved_va is only aligned to TARGET_PAGE_SIZE,
> and it works well if this size is greater or equal to the host page size.
> 
> But ppc64 hosts have 64kB page size and when we start a 4kiB page size
> guest (like i386), it fails when it tries to mmap the stack:
> 
>     mmap stack: Invalid argument
> 
> Fixes: 18e80c55bb (linux-user: Tidy and enforce reserved_va initialization)
> Signed-off-by: Laurent Vivier <address@hidden>
> ---
> 
> Notes:
>     v2:
>       fix typo s/has/as/
> 
>  linux-user/main.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/linux-user/main.c b/linux-user/main.c
> index 52b5a618fe..15299e9dd7 100644
> --- a/linux-user/main.c
> +++ b/linux-user/main.c
> @@ -689,6 +689,11 @@ int main(int argc, char **argv, char **envp)
>      target_environ = envlist_to_environ(envlist, NULL);
>      envlist_free(envlist);
>  
> +    /* reserved_va must be aligned with the host page size
> +     * as it is used with mmap()
> +     */
> +    reserved_va &= qemu_host_page_mask;
> +

So... this silently overrides the command-line argument.  The current code is
only a problem because we assign the default to a global variable, which must
be a compile-time constant.

I wonder if it's worth add an error message in handle_arg_reserved_va, and
moving the default initialization logic from the global variable to here, as

    if (HOST_LONG_BITS == 64 &&
        TARGET_VIRT_ADDR_SPACE_BITS <= 32 &&
        reserved_va == 0) {
        reserved_va = MAX_RESERVED_VA & qemu_host_page_mask;
    }

merging your comment with the moved comment from the global variable init.


r~