----- On Jan 7, 2022, at 5:27 PM, David Laight David.Laight@xxxxxxxxxx wrote: >> That being said, there might be an architecture agnostic alternative available. >> On abort of a RSEQ_CS_FLAG_ABORT_AT_IP critical section, we could let the kernel >> decrement/increment the stack pointer to make room for a pointer (depending if >> the >> stack grows down or up). It would then store the abort-at-ip value at the top of >> stack. > > Stack redzone in a leaf function? Good point! For x86-64 for instance, which has a redzone of 128 bytes, there are a few alternatives. On abort: Approach A) Move the stack pointer as little as possible 1. On abort in kernel: 1.1 Subtract 8 bytes from the stack pointer 1.2 Store the abort-at-ip value at stack pointer - 128 2. In abort handler (userspace) 2.1 user-space knows that it can find the abort-at-ip value at stack pointer - 128 2.2 after using that value, the abort handler needs to add 8 bytes to the stack pointer Approach B) Move the stack pointer to skip the entire redzone 1. On abort in kernel: 1.1 Subtract 128 bytes from the stack pointer 1.2 Store the abort-at-ip value at stack pointer - 8 (basically within a new red zone) 2. In abort handler (userspace) 2.1 user-space knows that it can find the abort-at-ip value at stack pointer - 8 2.2 after using that value, the abort handler needs to add 128 bytes to the stack pointer I suspect both approaches should work. Any preference or other ideas ? Thanks, Mathieu > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, > UK > Registration No: 1397386 (Wales) -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
