Kernel API wishlist

List of linux kernel work that would help rr.

In-kernel buffering of syscall outparam data

Syscallbuf+vsyscall hooking works OK in practice, but the kernel can do so much better, so much more easily. Essentially it "just" has to record copy_to_user() within syscall handlers, plus a bit of metadata. Perhaps could be exposed through perf_event.

Trapping on CPUID, like the RDTSC traps

Modern Intel chips can do this.

Expose Branch Trace Store to userspace

Probably through perf_event. There was a previous attempt to do this, but I think it fizzled out.

Efficient way for ptrace tracers to wait on a tracee's private futex

rr wants to use this for efficiently waiting on CLONE_CLEAR_CHILDTID futexes.

Bugfix: pwrite64/pread64 modify registers in a way visible to userspace

Counter to the syscall ABI. Bug that we shouldn't let slip through the cracks. Ubuntu bug isn't moving, need to test on kernel HEAD and upstream to kernel folks.

Bugfix: agree a kernel/userspace contract for ptrace traps of PR_TSC_SIGSEGV

See #692: the kernel munges sigstate to enable SIGSEGV traps when SIGSEGV is blocked, but apparently doesn't un-munge the state.