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

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

From: Mathieu Desnoyers

quoted

Sent: 12 January 2022 15:06

----- On Jan 12, 2022, at 9:58 AM, David Laight David.Laight@ACULAB.COM wrote:

quoted

quoted

 * [*] 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