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Re: [PATCH] x86/livepatch: enable livepatching assembly source files
- To: Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 18 Apr 2023 13:48:38 +0100
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- Cc: Wei Liu <wl@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 18 Apr 2023 12:50:10 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 18/04/2023 12:00 pm, Jan Beulich wrote:
> On 18.04.2023 11:24, Roger Pau Monne wrote:
>> Some of the assembly entry points cannot be safely patched until it's
>> safe to use jmp, as livepatch can replace a whole block with a jmp to
>> a new address, and that won't be safe until speculative mitigations
>> have been applied.
> Isn't the issue only with indirect JMP, whereas livepatch uses only
> direct ones?
We already have direct jumps prior to speculation safety logic.
Livepatching putting more in doesn't change our safety.
>> --- a/xen/arch/x86/include/asm/config.h
>> +++ b/xen/arch/x86/include/asm/config.h
>> @@ -44,6 +44,20 @@
>> /* Linkage for x86 */
>> #ifdef __ASSEMBLY__
>> #define ALIGN .align 16,0x90
>> +#ifdef CONFIG_LIVEPATCH
>> +#define START_LP(name) \
>> + jmp name; \
>> + .pushsection .text.name, "ax", @progbits; \
> In how far is livepatch susceptible to two .text.* sections of the same
> name? This can result here and perhaps also for static C functions.
Well - the section is the unit of binary-diffing noticing a difference.
If we have a naming collision here, then I expect the linker will merge
the two section, and the livepatch will end up bigger than it strictly
needs to be.
~Andrew
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