Messages in this thread |  | | Date | Fri, 26 Jul 2019 14:37:39 +0200 | From | Christian Brauner <> | Subject | Re: [RFC][PATCH 1/5] exit: kill struct waitid_info |
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On Fri, Jul 26, 2019 at 06:59:39AM -0500, Eric W. Biederman wrote: > Christian Brauner <christian@brauner.io> writes: > > > On Thu, Jul 25, 2019 at 07:46:50AM -0500, Eric W. Biederman wrote: > >> Linus Torvalds <torvalds@linux-foundation.org> writes: > >> > >> > On Wed, Jul 24, 2019 at 7:47 AM Christian Brauner <christian@brauner.io> wrote: > >> >> > >> >> The code here uses a struct waitid_info to catch basic information about > >> >> process exit including the pid, uid, status, and signal that caused the > >> >> process to exit. This information is then stuffed into a struct siginfo > >> >> for the waitid() syscall. That seems like an odd thing to do. We can > >> >> just pass down a siginfo_t struct directly which let's us cleanup and > >> >> simplify the whole code quite a bit. > >> > > >> > Ack. Except I'd like the commit message to explain where this comes > >> > from instead of that "That seems like an odd thing to do". > >> > > >> > The _original_ reason for "struct waitid_info" was that "siginfo_t" is > >> > huge because of all the insane padding that various architectures do. > >> > > >> > So it was introduced by commit 67d7ddded322 ("waitid(2): leave copyout > >> > of siginfo to syscall itself") very much to avoid stack usage issues. > >> > And I quote: > >> > > >> > collect the information needed for siginfo into > >> > a small structure (waitid_info) > >> > > >> > simply because "sigset_t" was big. > >> > > >> > But that size came from the explicit "pad it out to 128 bytes for > >> > future expansion that will never happen", and the kernel using the > >> > same exact sigset_t that was exposed to user space. > >> > > >> > Then in commit 4ce5f9c9e754 ("signal: Use a smaller struct siginfo in > >> > the kernel") we got rid of the insane padding for in-kernel use, > >> > exactly because it causes issues like this. > >> > > >> > End result: that "struct waitid_info" no longer makes sense. It's not > >> > appreciably smaller than kernel_siginfo_t is today, but it made sense > >> > at the time. > >> > >> Apologies. I meant to reply yesterday but I was preempted by baby > >> issues. > >> > >> I strongly disagree that this direction makes sense. The largest > >> value that I see from struct waitid_info is that it makes it possible to > >> reason about which values are returned where struct kernel_siginfo does > >> not. > >> > >> One of the details the existence of struct waitid_info makes clear is > >> that unlike the related child death path the wait code does not > >> fillin si_utime and si_stime. Which is very important to know when you > >> are dealing with y2038 issues and Arnd Bergmann is. > >> > >> The most egregious example I know of using siginfo wrong is: > >> 70f1b0d34bdf ("signal/usb: Replace kill_pid_info_as_cred with > >> kill_pid_usb_asyncio"). But just by moving struct siginfo out of the > >> program logic and into dedicated little functions that just deal with > >> the craziness of struct siginfo I have found lots of little bugs. > >> > >> We don't need that kind of invitation to bugs in the wait logic. > > > > I don't think it's a strong enough argument for rejecting this change. > > Suspecting that something might go wrong if we simplify something is a > > valid call to proceed with caution and be on the lookout for potential > > regressions so we can react fast. I respect that. But it's not > > necessarily a good argument to reject a change. > > Except your change was not a simplification. Your change was > a substitution to do the work of filling in struct kernel_siginfo in 4 > locations instead of just 2. > > The only simplification came from not using unsafe_put_user. Which is > valid but has nothing to do with struct waitid_info. > > > I'm happy to switch from an initializer (which is not even clear is a > > bug) to using clear_siginfo(). > > I just double checked the definitions in signal_types.h and > uapi/asm-generic/siginfo.h and there is definitely padding on 64bit. > So yes barring magic compiler plug ins it is a bug. > > > And I'm also going to split this patch out of the P_PIDFD patch but I'm > > going to send this out again. I haven't heard a sound argument why > > this > > patch is worse than what we have right now in there. > > Then I am afraid I have not expressed myself well. > > When I read through this patch I saw. > - A bug when dealing with struct kernel_siginfo. > - A substitution from of struct waitid_info to struct kernel_siginfo. > - An actual simplification in replacing several unsafe_put_user calls > with copy_siginfo_to_user. > - A gratuitous change in change the order of several of the statements. > - No simplification in the logic of do_wait.
I'm not going to feel bad for trying to improve a piece of code and not getting it right the first time. Removal of a custom struct that is only used to copy bits of information into another struct for which we have a dedicated in-kernel struct to avoid exactly that seems like a valid use of s/<custom-struct>/<dedicated-kernel-struct>/ to me; especially since I needed to touch that file anyway.
> > Or in short I saw you did "s/struct waitid_info/struct kernel_siginfo/" > and introduced bugs. Further you increased the number of locations that > we need to be very careful with struct kernel_siginfo from 2 to 4.
If you're concerned about siginfo_t being used in more places than it is right now, then please put a comment above it that it should be avoided and that because of architectural nuances clear_signfo() needs to be used to initialize it correctly.
Christian
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