Messages in this thread | | | Date | Wed, 12 Jan 2022 10:24:13 -0500 (EST) | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH v2 1/2] rseq: x86: implement abort-at-ip extension |
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----- 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 > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, > UK > Registration No: 1397386 (Wales)
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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