Re: [PATCH] x86, boot: access cmdline when loaded high

From: Yinghai Lu
Date: Tue Jun 11 2013 - 16:54:15 EST

On Tue, Jun 11, 2013 at 12:26 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Tue, Jun 11, 2013 at 12:17 PM, Yinghai Lu <yinghai@xxxxxxxxxx> wrote:
>> On Tue, Jun 11, 2013 at 11:41 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>> The __cmdline_find_option routine requires that the kernel cmdline be
>>> located under 0x10000. When running the compressed boot code, if the
>>> cmdline is loaded above this range, it will be ignored. This breaks
>>> recognition of things like "earlyprintk=...". Since the compressed boot
>>> code is already tricking the cmdline routines about locations, take it all
>>> the way and access the cmdline via offset instead of via actual location.
>> Hi,
>> What is point of the patch?
>> It looks like it try to fix one bug, but it doesn't,
>> as there is no bug out there.
>> Before the patch, command line that is loaded high
>> can be processed correctly in arch/x86/boot/compressed/cmdline.c.
> It can't. Here is the code from arch/x86/boot/cmdline.c:

Are you sure?

I tested that with kexec loaded second kernel high.

How do you test that?

> int __cmdline_find_option(unsigned long cmdline_ptr, const char
> *option, char *buffer, int bufsize)
> ...
> if (!cmdline_ptr)
> return -1; /* No command line */
> cptr = cmdline_ptr & 0xf;
> set_fs(cmdline_ptr >> 4);
> while (cptr < 0x10000 && (c = rdfs8(cptr++))) {
> This means that if get_cmd_line_ptr() returns an address >= 0x10000,
> __cmdline_find_option will simply ignore the cmdline.

No, you read the code wrong. you miss the

cptr = cmdline_ptr & 0xf;

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