Re: [Xen-devel] [PATCH 1/5] purgatory: put variables altered by kexec in .data not .bss

David Vrabel
2013-09-17T10:00:06Z

On 17/09/13 09:17, Tim Deegan wrote:
> At 03:33 +0200 on 17 Sep (1379388789), Daniel Kiper wrote:
>> On Mon, Sep 16, 2013 at 01:48:07PM +0100, David Vrabel wrote:
>>> On 16/09/13 13:43, Daniel Kiper wrote:
>>>> On Thu, Sep 12, 2013 at 09:00:38PM +0100, David Vrabel wrote:
>>>>> From: David Vrabel <david.vrabel@citrix.com>
>>>>>
>>>>> elf_rel_set_symbol() fails if the symbol is in the .bss section.
>>>>>
>>>>> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
>>>>> ---
>>>>> purgatory/arch/i386/console-x86.c | 6 +++---
>>>>> purgatory/arch/i386/crashdump_backup.c | 8 +++++---
>>>>> purgatory/arch/x86_64/purgatory-x86_64.c | 6 +++---
>>>>> purgatory/include/purgatory.h | 4 ++++
>>>>> purgatory/purgatory.c | 4 ++--
>>>>> 5 files changed, 17 insertions(+), 11 deletions(-)
>>>>>
>>>>> diff --git a/purgatory/arch/i386/console-x86.c b/purgatory/arch/i386/console-x86.c
>>>>> index 9773573..40a734b 100644
>>>>> --- a/purgatory/arch/i386/console-x86.c
>>>>> +++ b/purgatory/arch/i386/console-x86.c
>>>>> @@ -55,9 +55,9 @@ static void putchar_vga(int ch)
>>>>> */
>>>>>
>>>>> /* Base Address */
>>>>> -uint8_t console_serial = 0;
>>>>> -uint16_t serial_base = 0x3f8; /* TTYS0 */
>>>>> -uint32_t serial_baud = 0;
>>>>> +uint8_t console_serial __data = 0;
>>>>> +uint16_t serial_base __data = 0x3f8; /* TTYS0 */
>>>>> +uint32_t serial_baud __data = 0;
>>>>
>>>> Hmmm... I do not know why we need this patch. I can build kexec-tools
>>>> in my env without it. I suppose that most of people can too.
>>>> Could you check what is wrong with your build system?
>>>
>>> Regardless of whether it works with some tools, it doesn't make a whole
>>> lot of sense to try and set the value of symbols in the .bss section.
>>> .bss is only for symbols with a value of zero.
>>
>> To be precise .bss is only for uninitialized symbols. Anyway, probably
>> your compiler is screwed. In my case it sees that a given symbol is
>> initialized and places it in .data section itself.
>
> David, are you using a compiler that doesn't support
> -fno-zero-initialized-in-bss? If that's a supported optoin, then the
> __data markers are indeed necessary.

I'm using 4.7.2 and it does support the option and it is correctly being
used when building purgatory. Not sure how I ended up with something
that wasn't working before.

I'll drop this patch.

David