Re: [RFC PATCH v2 1/2] rseq: x86: implement abort-at-ip extension

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jan 12, 2022, at 10:15 AM, David Laight David.Laight@ACULAB.COM wrote:

> From: Mathieu Desnoyers
>> Sent: 12 January 2022 15:06
>> 
>> ----- On Jan 12, 2022, at 9:58 AM, David Laight David.Laight@ACULAB.COM wrote:
>> 
>> >>  * [*] The openrisc, powerpc64 and x86-64 architectures define a "redzone" as a
>> >>  *     stack area beyond the stack pointer which can be used by the compiler
>> >>  *     to store local variables in leaf functions.
>> >
>> > I wonder if that is really worth the trouble it causes!
>> > By the time a function is spilling values to stack the cost
>> > of a %sp update is almost certainly noise.
>> >
>> > Someone clearly thought it was a 'good idea (tm)'.
>> 
>> I must admit that I've been surprised to learn about these redzones. Thanks for
>> pointing them out to me, it was clearly a blind spot. I suspect it would be
>> useful
>> to introduce per-architecture KERNEL_REDZONE, USER_REDZONE and
>> COMPAT_USER_REDZONE
>> with a asm-generic version defining them to 0, with proper documentation. It
>> would
>> make it clearer to kernel developers working on stuff similar to signal handler
>> delivery that they need to consider these carefully.
> 
> They can never be used in kernel - any ISR would overwrite them.

arch/powerpc/include/asm/ptrace.h define those for ppc64:

113:#define USER_REDZONE_SIZE	512
114:#define KERNEL_REDZONE_SIZE	288

and then uses the kernel redzone size for:

#define STACK_INT_FRAME_SIZE    (sizeof(struct pt_regs) + \
                                 STACK_FRAME_OVERHEAD + KERNEL_REDZONE_SIZE)

which AFAIU should ensure that ISR don't overwrite the redzone within the kernel.

Thanks,

Mathieu

> 
>	David
> 
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-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com