Hi Andy, Andy Lutomirski <luto@xxxxxxxxxx> writes: > On Wed, Mar 24, 2021 at 10:39 AM Vasily Gorbik <gor@xxxxxxxxxxxxx> wrote: >> >> Hi Andy, >> >> On Sat, Mar 20, 2021 at 08:48:34PM -0700, Andy Lutomirski wrote: >> > Hi all- >> > >> > I'm working on my kentry patchset, and I encountered: >> > >> > commit 56e62a73702836017564eaacd5212e4d0fa1c01d >> > Author: Sven Schnelle <svens@xxxxxxxxxxxxx> >> > Date: Sat Nov 21 11:14:56 2020 +0100 >> > >> > s390: convert to generic entry >> > >> > As part of this work, I was cleaning up the generic syscall helpers, >> > and I encountered the goodies in do_syscall() and __do_syscall(). >> > >> > I'm trying to wrap my head around the current code, and I'm rather confused. >> > >> > 1. syscall_exit_to_user_mode_work() does *all* the exit work, not just >> > the syscall exit work. So a do_syscall() that gets called twice will >> > do the loopy part of the exit work (e.g. signal handling) twice. Is >> > this intentional? If so, why? >> > >> > 2. I don't understand how this PIF_SYSCALL_RESTART thing is supposed >> > to work. Looking at the code in Linus' tree, if a signal is pending >> > and a syscall returns -ERESTARTSYS, the syscall will return back to >> > do_syscall(). The work (as in (1)) gets run, calling do_signal(), >> > which will notice -ERESTARTSYS and set PIF_SYSCALL_RESTART. >> > Presumably it will also push the signal frame onto the stack and aim >> > the return address at the svc instruction mentioned in the commit >> > message from "s390: convert to generic entry". Then __do_syscall() >> > will turn interrupts back on and loop right back into do_syscall(). >> > That seems incorrect. >> > >> > Can you enlighten me? My WIP tree is here: >> > https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/log/?h=x86/kentry >> > >> >> For all the details to that change we'd have to wait for Sven, who is back >> next week. >> >> > Here are my changes to s390, and I don't think they're really correct: >> > >> > >> > https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/diff/arch/s390/kernel/syscall.c?h=x86/kentry&id=58a459922be0fb8e0f17aeaebcb0ac8d0575a62c >> >> Couple of things: syscall_exit_to_user_mode_prepare is static, >> and there is another code path in arch/s390/kernel/traps.c using >> enter_from_user_mode/exit_to_user_mode. >> >> Anyhow I gave your branch a spin and got few new failures on strace test >> suite, in particular on restart_syscall test. I'll try to find time to >> look into details. > > I refreshed the branch, but I confess I haven't compile tested it. :) > > I would guess that the new test case failures are a result of the > buggy syscall restart logic. I think that all of the "restart" cases > except execve() should just be removed. Without my patch, I suspect > that signal delivery with -ERESTARTSYS would create the signal frame, > do an accidental "restarted" syscall that was a no-op, and then > deliver the signal. With my patch, it may simply repeat the original > interrupted signal forever. PIF_SYSCALL_RESTART is set in arch_do_signal_or_restart(), but only if there's no signal handler registered. In that case we don't need a signal frame, so that should be fine. The problem why your branch is not working is that arch_do_signal_or_restart() gets called from exit_to_user_mode_prepare(), and that is after the check whether PIF_SYSCALL_RESTART is set in __do_syscall(). So i'm wondering how to fix that. x86 simply rewinds the pc, so the system call instruction gets re-executed when returning to user space. For s390 that doesn't work, as the s390 svc instruction might have the syscall number encoded. If we would have to restart a system call with restart_systemcall(), we need to change the syscall number to __NR_restart_syscall. As we cannot change the hardcoded system call number, we somehow need to handle that inside of the kernel. So i wonder whether we should implement the PIF_SYSCALL_RESTART check in entry.S after all the return to user space entry code was run but before doing the real swtch back to user space. If PIF_SYSCALL_RESTART is set we would then just jump back to the entry code and pretend we came from user space. That would have the benefit that the entry C code looks the same like other architectures and that amount of code to add in entry.S shouldn't be much. Any thoughts? Regards Sven
