Re: [Xen-devel] [PATCH v4 1/9] livepatch: Clear .bss when payload is reverted

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.09.16 at 15:50, <konrad.wilk@xxxxxxxxxx> wrote:
> On Fri, Sep 09, 2016 at 02:33:18PM +0100, Ross Lagerwall wrote:
>> On 08/24/2016 03:22 AM, Konrad Rzeszutek Wilk wrote:
>> > So that when we apply the patch again the .bss is cleared.
>> > Otherwise we may find some variables containing old values.
>> > 
>> > The payloads may contain various .bss - especially if -fdata-sections
>> > is used which can create .bss.<name> sections.
>> > 
>> 
>> After having thought about this again, I'm not sure it makes much sense. Any
>> data sections in the payload are not reset to their initial values, so
>> resetting the bss only may result in an unexpected combination of new & old
>> data/bss.
> 
> Regardless of that I think clearing the .bss upon applying the livepatch is
> still the right thing to do.
> 
> Regarding of the .data - we could have a copy of the .data the first time
> we load - and then during application copy over it from the original one?.
> 
>> 
>> Perhaps it just needs to be documented that a payload's bss/data is
>> untouched across revert/apply?
> 
> It really cuts down on bugs if we clear the .bss. It is kind of ingrained in 
> every
> developer that the .bss is zero-ed out at startup.

I don't think Ross meant to suggest to not clear it at all: Clearing it
at load time is the equivalent of loading actual data into initialized
sections.

Jan


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