----- On Nov 18, 2021, at 1:48 PM, Cristian Rodríguez crrodriguez@xxxxxxxxxxxx wrote: > On Thu, Nov 18, 2021 at 7:17 AM Florian Weimer via Libc-alpha > <libc-alpha@xxxxxxxxxxxxxx> wrote: > >> 4. Add public symbols __rseq_abi_offset, __rseq_abi_size (currently 32 >> or 0), __rseq_abi_flags (currently 0). __rseq_abi_offset is the >> offset to add to the thread pointer (see __builtin_thread_pointer) to >> get to the rseq area. They will be public ABI symbols. These >> variables are initialized before user code runs, and changing the >> results in undefined behavior. > > Why not then __get_rseq_whatwever functions and not variables ? or > maybe writing to these variables results in a compiler or linker error > instead of UB ? rseq critical sections cannot issue function calls, and also function calls are noticeably expensive compared to an rseq critical section. So all users would end up needing to make a local copy of the information fetched by those getters. So rather than require all those extra per-user copies, I suspect exposing a single copy through public glibc symbols is more efficient. The downside is indeed that writing to those variables is UB. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
