linux-kernel - Re: Edited kexec_load(2) [kexec_file

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Date:	Fri, 30 Jan 2015 16:25:06 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Scot Doyle <lkml14@...tdoyle.com>
CC:	mtk.manpages@...il.com, Vivek Goyal <vgoyal@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	"linux-man@...r.kernel.org" <linux-man@...r.kernel.org>,
	Kexec Mailing List <kexec@...ts.infradead.org>,
	Andy Lutomirski <luto@...capital.net>,
	Dave Young <dyoung@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: Edited kexec_load(2) [kexec_file_load()] man page for review

On 01/29/2015 05:06 PM, Scot Doyle wrote:
> On Thu, 29 Jan 2015, Michael Kerrisk (man-pages) wrote:
>> On 29 January 2015 at 02:27, Scot Doyle <lkml14@...tdoyle.com> wrote:
>>> On Wed, 28 Jan 2015, Vivek Goyal wrote:
>>>> On Wed, Jan 28, 2015 at 10:10:59PM +0000, Scot Doyle wrote:
>>>>> On Wed, 28 Jan 2015, Vivek Goyal wrote:
>>>>>> On Wed, Jan 28, 2015 at 09:14:03PM +0000, Scot Doyle wrote:
>>>>>>> When I tested, kexec_file_load required CONFIG_RELOCATABLE. Is the same
>>>>>>> true for kexec_load? Would it make sense to note this in the man pages
>>>>>>> along with the need for CONFIG_KEXEC_FILE, etc? Or as an error message?
>>>>>>
>>>>>> Hmm.., I can't see an explicity dependency between RELOCATABLE and
>>>>>> KEXEC. Both KEXEC and KEXEC_FILE should be able to load a kernel
>>>>>> even if it had RELOCATABLE=n.
>>>>>>
>>>>>> Just that kernel will run from the address it has been built for.
>>>>>>
>>>>>> Thanks
>>>>>> Vivek
>>>>>
>>>>> Confusing, right? kexec_file_load returns -ENOEXEC and dmesg says
>>>>> "kexec-bzImage64: XLF_CAN_BE_LOADED_ABOVE_4G is not set." which leads to
>>>>> arch/x86/boot/header.S line 396:
>>>>>
>>>>> #if defined(CONFIG_RELOCATABLE) && defined(CONFIG_X86_64)
>>>>>    /* kernel/boot_param/ramdisk could be loaded above 4g */
>>>>> # define XLF1 XLF_CAN_BE_LOADED_ABOVE_4G
>>>>> #else
>>>>> # define XLF1 0
>>>>> #endif
>>>>
>>>> Ah, this one. Actually generic kexec file loading implementation does not
>>>> impose this restriction. It is the image specific loader part which
>>>> decides what kind of bzImage it can load.
>>>>
>>>> Current implementation (kexec-bzimage64.c), is only supporting loading
>>>> bzImages which are 64bit and can be loaded above 4G. This simplifies
>>>> the implementation of loader.
>>>>
>>>> But there is nothing which prevents one from implementing other image
>>>> loaders.
>>>>
>>>> So instead of saying that kexec_file_load() depends on CONFIG_RELOCATABLE,
>>>> it might be better to say in man page that currently this system call
>>>> supports only loading a bzImage which is 64bit and which can be loaded
>>>> above 4G too.
>>>>
>>>> Thanks
>>>> Vivek
>>>
>>> Thanks, I agree, and think it would make sense to list them as part of the
>>> page's ENOEXEC error.
>>
>> Scott, could you then phras a couple of sentences that capture thge
>> details, so I can add it to the ENOEXEC error?
>>
>> Thanks,
>>
>> Michael
> 
> Yes, maybe something like "kernel_fd does not refer to an open file, or 
> the file type is not supported. Currently, the file must be a bzImage
> and contain an x86 kernel loadable above 4G in memory (see 
> Documentation/x86/boot.txt)."?
> 
> boot.txt explains that loading above 4G implies 64-bit and is specified 
> via a bit in xloadflags added in Linux 3.8.

Added and pushed. Thanks, Scott.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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