Re: [Xen-devel] [PATCH v2] xen/arch/x86: Fix early boot command line parsing

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 03/12/13 20:28, Daniel Kiper wrote:
> There is no reliable way to encode NUL character as a character so encode
> it as a number. Read: http://sourceware.org/binutils/docs/as/Characters.html.
> Octal and hex encoding do not work on at least one system (GNU assembler
> version 2.22 (x86_64-linux-gnu) using BFD version (GNU Binutils for Debian) 
> 2.22).
> Without this fix e.g. no-real-mode option at the end of xen.gz command line
> is not detected.
>
> v2 - suggestions/fixes:
>    - add relevant comment to assembly file
>      (suggested by Andrew Cooper),
>    - leave encoding of other characters as is for readability
>      (suggested by Jan Beulich).
>
> Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>

I was thinking more along the lines of /* NULL check (as $'\0' == 0x30
in GAS) */

Either way, at least it is obvious.

Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

> ---
>  xen/arch/x86/boot/cmdline.S |   10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/xen/arch/x86/boot/cmdline.S b/xen/arch/x86/boot/cmdline.S
> index 05ffb94..b83975e 100644
> --- a/xen/arch/x86/boot/cmdline.S
> +++ b/xen/arch/x86/boot/cmdline.S
> @@ -138,7 +138,15 @@
>          call    .Lstrlen
>          add     $4,%esp
>          xadd    %eax,%ebx
> -        cmpb    $'\0',(%ebx)
> +        /*
> +         * There is no reliable way to encode NUL character as a character 
> so encode
> +         * it as a number. Read: 
> http://sourceware.org/binutils/docs/as/Characters.html.
> +         * Octal and hex encoding do not work on at least one system (GNU 
> assembler
> +         * version 2.22 (x86_64-linux-gnu) using BFD version (GNU Binutils 
> for Debian) 2.22).
> +         * Without this fix e.g. no-real-mode option at the end of xen.gz 
> command line
> +         * is not detected.
> +         */
> +        cmpb    $0,(%ebx)
>          je      3f
>          cmpb    $' ',(%ebx)
>          je      3f


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