Michal Suchánek <[email protected]> writes: > On Thu, Jul 02, 2026 at 02:01:02PM +0200, Sven Schnelle wrote: >> Michal Suchánek <[email protected]> writes: >> >> > On Thu, Jul 02, 2026 at 10:12:35AM +0200, Sven Schnelle wrote: >> >> Michal Suchánek <[email protected]> writes: >> >> >> >> > The return value of syscall_enter_from_user_mode is used both for the >> >> > adjusted syscall number and the indicator that a syscall should be >> >> > skipped. >> >> > >> >> > As seccomp can be invoked on any syscall, including invalid ones this >> >> > somewhat undermines seccomp. >> >> > >> >> > While the seccomp variants that terminate the process do not need to >> >> > care about this for the filter that sets the syscall return value this >> >> > disctinction is required. >> >> > >> >> > Pass the syscall number as a pointer to the inline entry functions, and >> >> > use the return value exclusively for the indication that the syscall is >> >> > already handled. >> >> > >> >> > This should avoid the need for the s390 PIF_SYSCALL_RET_SET which is the >> >> > workaround for exactly this deficiency. >> >> >> >> I'm not sure whether PIF_SYSCALL_RET_SET can be removed - the syscall >> >> return might still get set by PTRACE_SET_SYSCALL_INFO when the tracee is >> >> stopped. This might be a positive number which can't be distinguished >> >> from a syscall number. But maybe i'm missing something? It's been quite >> >> a while since I touched all that ptrace stuff. >> > >> > When the syscall return value is set (in the registers) the return value >> > which is also the modified syscall number is set to -1 indicating the >> > syscall was handled. At least that's how the API is described. >> > >> > So yes, if the syscall number range is restricted or the syscall number >> > is returned through a path different from the function return value the >> > flag should not be needed in the entry path because the case can be >> > detected through the return value alone. >> >> I'm still failing to see how this would work without an additional >> flag. Assume a program (the tracee) is stopped because of a syscall >> entry. The tracer then decides to skip the syscall and changes >> regs->gpr2 (which contains either the syscall number or return value) >> to contain 42. When the tracer than restarts the syscall, how does >> do_syscall() know that gpr2 is now a return value and not a syscall number? > > Because then the return value from the syscall_enter_from_user_mode > machinery would be -1 indicating the syscall should be skipped. That is > how the return value of syscall_enter_from_user_mode is documented, I > did not verify that it actually works that way for the tracing case on > s390.
I read the code and tested - I think I confused the way how syscall are intercepted by seccomp vs. ptrace. The PIF_SYSCALL_RET_SET is indeed only required to indicate syscalls skipped via seccomp and not ptrace.
