Messages in this thread | | | Subject | Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads | From | Michael Schmitz <> | Date | Tue, 22 Jun 2021 12:01:06 +1200 |
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Hi Linus,
On 22/06/21 11:14 am, Linus Torvalds wrote: > On Mon, Jun 21, 2021 at 12:45 PM Al Viro <viro@zeniv.linux.org.uk> wrote: >>> Looks like sys_exit() and do_group_exit() would be the two places to >>> do it (do_group_exit() would handle the signal case and >>> sys_group_exit()). >> Maybe... I'm digging through that pile right now, will follow up when >> I get a reasonably complete picture > We might have another possible way to solve this: > > (a) make it the rule that everybody always saves the full (integer) > register set in pt_regs > > (b) make m68k just always create that switch-stack for all system > calls (it's really not that big, I think it's like six words or > something)
Correct - six words for registers, one for the return address. Probably still a win compared to setting and clearing flag bits all over the place in an attempt to catch any as yet undetected unsafe cases of ptrace_stop.
I'll have to see how much of a performance impact I can see (not that I can even remotely measure that accurately - it's more of a 'does it now feel real sluggish' thing).
Cheers,
Michael
> > (c) admit that alpha is broken, but nobody really cares > >> In the meanwhile, do kernel/kthread.c uses look even remotely sane? >> Intentional - sure, but it really looks wrong to use thread exit code >> as communication channel there... > I really doubt that it is even "intentional". > > I think it's "use some errno as a random exit code" and nobody ever > really thought about it, or thought about how that doesn't really > work. People are used to the error numbers, not thinking about how > do_exit() doesn't take an error number, but a signal number (and an > 8-bit positive error code in bits 8-15). > > Because no, it's not even remotely sane. > > I think the do_exit(-EINTR) could be do_exit(SIGINT) and it would make > more sense. And the -ENOMEM might be SIGBUS, perhaps. > > It does look like the usermode-helper code does save the exit code > with things like > > kernel_wait(pid, &sub_info->retval); > > and I see call_usermodehelper_exec() doing > > retval = sub_info->retval; > > and treating it as an error code. But I think those have never been > tested with that (bogus) exit code thing from kernel_wait(), because > it wouldn't have worked. It has only ever been tested with the (real) > exit code things like > > if (pid < 0) { > sub_info->retval = pid; > > which does actually assign a negative error code to it. > > So I think that > > kernel_wait(pid, &sub_info->retval); > > line is buggy, and should be something like > > int wstatus; > kernel_wait(pid, &wstatus); > sub_info->retval = WEXITSTATUS(wstatus) ? -EINVAL : 0; > > or something. > > Linus
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