Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes: > On Tue, Jun 15, 2021 at 12:36 PM Eric W. Biederman > <ebiederm@xxxxxxxxxxxx> wrote: >> >> I looked and there nothing I can do that is not arch specific, so >> whack the moles with a minimal backportable fix. >> >> This change survives boot testing on qemu-system-alpha. > > So as mentioned in the other thread, I think this patch is exactly right. > > However, the need for this part > >> @@ -785,6 +785,7 @@ ret_from_kernel_thread: >> mov $9, $27 >> mov $10, $16 >> jsr $26, ($9) >> + lda $sp, SWITCH_STACK_SIZE($sp) >> br $31, ret_to_user >> .end ret_from_kernel_thread > > obviously eluded me in my "how about something like this", and I had > to really try to figure out why we'd ever return. > > Which is why I came to that "oooh - kernel_execve()" realization. > > It might be good to comment on that somewhere. And if you can think of > some other case, that should be mentioned too. > > Anyway, thanks for looking into this odd case. And if you have a > test-case for this all, it really would be a good thing. Yes, it > should only affect a couple of odd-ball architectures, but still... It > would also be good to hear that you actually did verify the behavior > of this patch wrt that ptrace-of-io-worker-threads case.. *Grumble* So just going through and looking to see what it takes to instrument and put in warnings when things go wrong I have found another issue. Today there exists: PTRACE_EVENT_FORK PTRACE_EVENT_VFORK PTRACE_EVENT_CLONE Which happens after the actual fork operation in the kernel. The following code wraps those operations in arch/alpha/kernel/entry.S .macro fork_like name .align 4 .globl alpha_\name .ent alpha_\name alpha_\name: .prologue 0 bsr $1, do_switch_stack jsr $26, sys_\name ldq $26, 56($sp) lda $sp, SWITCH_STACK_SIZE($sp) ret .end alpha_\name .endm The code in the kernel when calls in fork.c calls ptrace_event_pid which ultimately calls ptrace_stop. So userspace can reasonably expect to stop the process and change it's registers. With unconditionally popping the switch stack any of those registers that are modified are lost. So I will update my changes to handle that case as well. Eric
