Hi Will, We are about to include the code signature required prior to restartable sequences abort handlers into glibc, which will make this ABI choice final. We need architecture maintainer input on that signature value.
That code signature is placed before each abort handler, so the kernel can validate that it is indeed jumping to an abort handler (and not some arbitrary attacker-chosen code). The signature is never executed. The current discussion thread on the glibc mailing list leads us towards using a trap with uncommon immediate operand, which simplifies integration with disassemblers, emulators, makes it easier to debug if the control flow gets redirected there by mistake, and is nicer for some architecture's speculative execution. We can have different signatures for each sub-architecture, as long as they don't have to co-exist within the same process. We can special-case with #ifdef for each sub-architecture and endianness if need be. If the architecture has instruction set extensions that can co-exist with the architecture instruction set within the same process (e.g. thumb for arm), we need to take into account to which instruction the chosen signature value would map (and possibly decide if we need to extend rseq to support many signatures). Here is an example of rseq signature definition template: /* * TODO: document trap instruction objdump output on each sub-architecture * instruction sets, as well as instruction set extensions. */ #define RSEQ_SIG 0x######## Ideally we'd need a patch on top of the Linux kernel tools/testing/selftests/rseq/rseq-arm.h file that updates the signature value, so I can then pick it up for the glibc patchset. Thanks! Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com