Linaro-mm-sig May 2024

linaro-mm-sig@lists.linaro.org

27 participants
53 discussions

Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes

by David Laight

From: Christian Brauner > Sent: 10 May 2024 11:55 > > > For the uapi issue you describe below my take would be that we should just > > try, and hope that everyone's been dutifully using O_CLOEXEC. But maybe > > I'm biased from the gpu world, where we've been hammering it in that > > "O_CLOEXEC or bust" mantra since well over a decade. Really the only valid > > Oh, we're very much on the same page. All new file descriptor types that > I've added over the years are O_CLOEXEC by default. IOW, you need to > remove O_CLOEXEC explicitly (see pidfd as an example). And imho, any new > fd type that's added should just be O_CLOEXEC by default. For fd a shell redirect creates you may want so be able to say 'this fd will have O_CLOEXEC set after the next exec'. Also (possibly) a flag that can't be cleared once set and that gets kept by dup() etc. But maybe that is excessive? I've certainly used: # ip netns exec ns command 3</sys/class/net in order to be able to (easily) read status for interfaces in the default namespace and a specific namespace. The would be hard if the O_CLOEXEC flag had got set by default. (Especially without a shell builtin to clear it.) David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)

7 months, 2 weeks

Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes

by Linus Torvalds

On Thu, 9 May 2024 at 04:39, Christian Brauner <brauner(a)kernel.org> wrote: > > Not worth it without someone explaining in detail why imho. First pass > should be to try and replace kcmp() in scenarios where it's obviously > not needed or overkill. Ack. > I've added a CLASS(fd_raw) in a preliminary patch since we'll need that > anyway which means that your comparison patch becomes even simpler imho. > I've also added a selftest patch: > > https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git/log/?h=vfs.misc LGTM. Maybe worth adding an explicit test for "open same file, but two separate opens, F_DUPFD_QUERY returns 0? Just to clarify the "it's not testing the file on the filesystem for equality, but the file pointer itself". Linus

7 months, 2 weeks

Re: Safety of opening up /dev/dma_heap/* to physically present users (udev uaccess tag) ?

by Dmitry Baryshkov

On Tue, 7 May 2024 at 18:15, Bryan O'Donoghue <bryan.odonoghue(a)linaro.org> wrote: > > On 07/05/2024 16:09, Dmitry Baryshkov wrote: > > Ah, I see. Then why do you require the DMA-ble buffer at all? If you are > > providing data to VPU or DRM, then you should be able to get the buffer > > from the data-consuming device. > > Because we don't necessarily know what the consuming device is, if any. > > Could be VPU, could be Zoom/Hangouts via pipewire, could for argument > sake be GPU or DSP. > > Also if we introduce a dependency on another device to allocate the > output buffers - say always taking the output buffer from the GPU, then > we've added another dependency which is more difficult to guarantee > across different arches. Yes. And it should be expected. It's a consumer who knows the restrictions on the buffer. As I wrote, Zoom/Hangouts should not require a DMA buffer at all. Applications should be able to allocate the buffer out of the generic memory. GPUs might also have different requirements. Consider GPUs with VRAM. It might be beneficial to allocate a buffer out of VRAM rather than generic DMA mem. -- With best wishes Dmitry

7 months, 2 weeks

Re: Safety of opening up /dev/dma_heap/* to physically present users (udev uaccess tag) ?

by Daniel Vetter

On Tue, May 07, 2024 at 04:07:39PM -0400, Nicolas Dufresne wrote: > Hi, > > Le mardi 07 mai 2024 à 21:36 +0300, Laurent Pinchart a écrit : > > Shorter term, we have a problem to solve, and the best option we have > > found so far is to rely on dma-buf heaps as a backend for the frame > > buffer allocatro helper in libcamera for the use case described above. > > This won't work in 100% of the cases, clearly. It's a stop-gap measure > > until we can do better. > > Considering the security concerned raised on this thread with dmabuf heap > allocation not be restricted by quotas, you'd get what you want quickly with > memfd + udmabuf instead (which is accounted already). > > It was raised that distro don't enable udmabuf, but as stated there by Hans, in > any cases distro needs to take action to make the softISP works. This > alternative is easy and does not interfere in anyway with your future plan or > the libcamera API. You could even have both dmabuf heap (for Raspbian) and the > safer memfd+udmabuf for the distro with security concerns. > > And for the long term plan, we can certainly get closer by fixing that issue > with accounting. This issue also applied to v4l2 io-ops, so it would be nice to > find common set of helpers to fix these exporters. Yeah if this is just for softisp, then memfd + udmabuf is also what I was about to suggest. Not just as a stopgap, but as the real official thing. udmabuf does kinda allow you to pin memory, but we can easily fix that by adding the right accounting and then either let mlock rlimits or cgroups kernel memory limits enforce good behavior. -Sima -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch

7 months, 2 weeks

Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes

by Linus Torvalds

On Sat, 4 May 2024 at 02:37, Christian Brauner <brauner(a)kernel.org> wrote: > > --- a/drivers/dma-buf/dma-buf.c > +++ b/drivers/dma-buf/dma-buf.c > @@ -244,13 +244,18 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll) > if (!dmabuf || !dmabuf->resv) > return EPOLLERR; > > + if (!get_file_active(&dmabuf->file)) > + return EPOLLERR; [...] I *really* don't think anything that touches dma-buf.c can possibly be right. This is not a dma-buf.c bug. This is purely an epoll bug. Lookie here, the fundamental issue is that epoll can call '->poll()' on a file descriptor that is being closed concurrently. That means that *ANY* driver that relies on *any* data structure that is managed by the lifetime of the 'struct file' will have problems. Look, here's sock_poll(): static __poll_t sock_poll(struct file *file, poll_table *wait) { struct socket *sock = file->private_data; and that first line looks about as innocent as it possibly can, right? Now, imagine that this is called from 'epoll' concurrently with the file being closed for the last time (but it just hasn't _quite_ reached eventpoll_release() yet). Now, imagine that the kernel is built with preemption, and the epoll thread gets preempted _just_ before it loads 'file->private_data'. Furthermore, the machine is under heavy load, and it just stays off its CPU a long time. Now, during this TOTALLY INNOCENT sock_poll(), in another thread, the file closing completes, eventpoll_release() finishes, and the preemption of the poll() thing just takes so long that you go through an RCU period too, so that the actual file has been released too. So now that totally innoced file->private_data load in the poll() is probably going to get random data. Yes, the file is allocated as SLAB_TYPESAFE_BY_RCU, so it's probably still a file. Not guaranteed, even the slab will get fully free'd in some situations. And yes, the above case is impossible to hit in practice. You have to hit quite the small race window with an operation that practically never happens in the first place. But my point is that the fact that the problem with file->f_count lifetimes happens for that dmabuf driver is not the fault of the dmabuf code. Not at all. It is *ENTIRELY* a bug in epoll, and the dmabuf code is probably just easier to hit because it has a poll() function that does things that have longer lifetimes than most things, and interacts more directly with that f_count. So I really don't understand why Al thinks this is "dmabuf does bad things with f_count". It damn well does not. dma-buf is the GOOD GUY here. It's doing things *PROPERLY*. It's taking refcounts like it damn well should. The fact that it takes ref-counts on something that the epoll code has messed up is *NOT* its fault. Linus

7 months, 2 weeks

Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes

by Daniel Vetter

On Wed, May 08, 2024 at 12:08:57PM +0200, Christian Brauner wrote: > On Mon, May 06, 2024 at 04:29:44PM +0200, Christian König wrote: > > Am 04.05.24 um 20:20 schrieb Linus Torvalds: > > > On Sat, 4 May 2024 at 08:32, Linus Torvalds > > > <torvalds(a)linux-foundation.org> wrote: > > > > Lookie here, the fundamental issue is that epoll can call '->poll()' > > > > on a file descriptor that is being closed concurrently. > > > Thinking some more about this, and replying to myself... > > > > > > Actually, I wonder if we could *really* fix this by simply moving the > > > eventpoll_release() to where it really belongs. > > > > > > If we did it in file_close_fd_locked(), it would actually make a > > > *lot* more sense. Particularly since eventpoll actually uses this: > > > > > > struct epoll_filefd { > > > struct file *file; > > > int fd; > > > } __packed; > > > > > > ie it doesn't just use the 'struct file *', it uses the 'fd' itself > > > (for ep_find()). > > > > > > (Strictly speaking, it should also have a pointer to the 'struct > > > files_struct' to make the 'int fd' be meaningful). > > > > While I completely agree on this I unfortunately have to ruin the idea. > > > > Before we had KCMP some people relied on the strange behavior of eventpoll > > to compare struct files when the fd is the same. > > > > I just recently suggested that solution to somebody at AMD as a workaround > > when KCMP is disabled because of security hardening and I'm pretty sure I've > > seen it somewhere else as well. > > > > So when we change that it would break (undocumented?) UAPI behavior. > > I've worked on that a bit yesterday and I learned new things about epoll > and ran into some limitations. > > Like, what happens if process P1 has a file descriptor registered in an > epoll instance and now P1 forks and creates P2. So every file that P1 > maintains gets copied into a new file descriptor table for P2. And the > same file descriptors refer to the same files for both P1 and P2. So this is pretty similar to any other struct file that has resources hanging off the struct file instead of the underlying inode. Like drm chardev files, where all the buffers, gpu contexts and everything else hangs off the file and there's no other way to get at them (except when exporting to some explicitly meant-for-sharing file like dma-buf). If you fork() that it's utter hilarity, which is why absolutely everyone should set O_CLOEXEC on these. Or EPOLL_CLOEXEC for epoll_create. For the uapi issue you describe below my take would be that we should just try, and hope that everyone's been dutifully using O_CLOEXEC. But maybe I'm biased from the gpu world, where we've been hammering it in that "O_CLOEXEC or bust" mantra since well over a decade. Really the only valid use-case is something like systemd handing open files to a service, where it drops priviledges even well before the exec() call. But we can't switch around the defaults for any of these special open files with anything more than just a current seek position as state, since that breaks uapi. -Sima > > So there's two interesting cases here: > > (1) P2 explicitly removes the file descriptor from the epoll instance > via epoll_ctl(EPOLL_CTL_DEL). That removal affects both P1 and P2 > since the <fd, file> pair is only registered once and it isn't > marked whether it belongs to P1 and P2 fdtable. > > So effectively fork()ing with epoll creates a weird shared state > where removal of file descriptors that were registered before the > fork() affects both child and parent. > > I found that surprising even though I've worked with epoll quite > extensively in low-level userspace. > > (2) P2 doesn't close it's file descriptors. It just exits. Since removal > of the file descriptor from the epoll instance isn't done during > close() but during last fput() P1's epoll state remains unaffected > by P2's sloppy exit because P1 still holds references to all files > in its fdtable. > > (Sidenote, if one ends up adding every more duped-fds into epoll > instance that one doesn't explicitly close and all of them refer to > the same file wouldn't one just be allocating new epitems that > are kept around for a really long time?) > > So if the removal of the fd would now be done during close() or during > exit_files() when we call close_files() and since there's currently no > way of differentiating whether P1 or P2 own that fd it would mean that > (2) collapses into (1) and we'd always alter (1)'s epoll state. That > would be a UAPI break. > > So say we record the fdtable to get ownership of that file descriptor so > P2 doesn't close anything in (2) that really belongs to P1 to fix that > problem. > > But afaict, that would break another possible use-case. Namely, where P1 > creates an epoll instance and registeres fds and then fork()s to create > P2. Now P1 can exit and P2 takes over the epoll loop of P1. This > wouldn't work anymore because P1 would deregister all fds it owns in > that epoll instance during exit. I didn't see an immediate nice way of > fixing that issue. > > But note that taking over an epoll loop from the parent doesn't work > reliably for some file descriptors. Consider man signalfd(2): > > epoll(7) semantics > If a process adds (via epoll_ctl(2)) a signalfd file descriptor to an epoll(7) instance, > then epoll_wait(2) returns events only for signals sent to that process. In particular, > if the process then uses fork(2) to create a child process, then the child will be able > to read(2) signals that are sent to it using the signalfd file descriptor, but > epoll_wait(2) will not indicate that the signalfd file descriptor is ready. In this > scenario, a possible workaround is that after the fork(2), the child process can close > the signalfd file descriptor that it inherited from the parent process and then create > another signalfd file descriptor and add it to the epoll instance. Alternatively, the > parent and the child could delay creating their (separate) signalfd file descriptors and > adding them to the epoll instance until after the call to fork(2). > > So effectively P1 opens a signalfd and registers it in an epoll > instance. Then it fork()s and creates P2. Now both P1 and P2 call > epoll_wait(). Since signalfds are always relative to the caller and P1 > did call signalfd_poll() to register the callback only P1 can get > events. So P2 can't take over signalfds in that epoll loop. > > Honestly, the inheritance semantics of epoll across fork() seem pretty > wonky and it would've been better if an epoll fd inherited across > would've returned ESTALE or EINVAL or something. And if that inheritance > of epoll instances would really be a big use-case there'd be some > explicit way to enable this. -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch

7 months, 2 weeks

Re: [PATCH] dmabuf: fix dmabuf file poll uaf issue

by Christian König

Am 18.04.24 um 03:33 schrieb zhiguojiang: > 在 2024/4/15 19:57, Christian König 写道: >> [Some people who received this message don't often get email from >> christian.koenig(a)amd.com. Learn why this is important at >> https://aka.ms/LearnAboutSenderIdentification ] >> >> Am 15.04.24 um 12:35 schrieb zhiguojiang: >>> 在 2024/4/12 14:39, Christian König 写道: >>>> [Some people who received this message don't often get email from >>>> christian.koenig(a)amd.com. Learn why this is important at >>>> https://aka.ms/LearnAboutSenderIdentification ] >>>> >>>> Am 12.04.24 um 08:19 schrieb zhiguojiang: >>>>> [SNIP] >>>>> -> Here task 2220 do epoll again where internally it will get/put >>>>> then >>>>> start to free twice and lead to final crash. >>>>> >>>>> Here is the basic flow: >>>>> >>>>> 1. Thread A install the dma_buf_fd via dma_buf_export, the fd >>>>> refcount >>>>> is 1 >>>>> >>>>> 2. Thread A add the fd to epoll list via epoll_ctl(EPOLL_CTL_ADD) >>>>> >>>>> 3. After use the dma buf, Thread A close the fd, then here fd >>>>> refcount >>>>> is 0, >>>>> and it will run __fput finally to release the file >>>> >>>> Stop, that isn't correct. >>>> >>>> The fs layer which calls dma_buf_poll() should make sure that the file >>>> can't go away until the function returns. >>>> >>>> Then inside dma_buf_poll() we add another reference to the file while >>>> installing the callback: >>>> >>>> /* Paired with fput in dma_buf_poll_cb */ >>>> get_file(dmabuf->file); >>> Hi, >>> >>> The problem may just occurred here. >>> >>> Is it possible file reference count already decreased to 0 and fput >>> already being in progressing just before calling >>> get_file(dmabuf->file) in dma_buf_poll()? >> >> No, exactly that isn't possible. >> >> If a function gets a dma_buf pointer or even more general any reference >> counted pointer which has already decreased to 0 then that is a major >> bug in the caller of that function. >> >> BTW: It's completely illegal to work around such issues by using >> file_count() or RCU functions. So when you suggest stuff like that it >> will immediately face rejection. >> >> Regards, >> Christian. > Hi, > > Thanks for your kind comment. > > 'If a function gets a dma_buf pointer or even more general any reference > > counted pointer which has already decreased to 0 then that is a major > > bug in the caller of that function.' > > According to the issue log, we can see the dma buf file close and > epoll operation running in parallel. > > Apparently at the moment caller calls epoll, although another task > caller already called close on the same fd, but this fd was still in > progress to close, so fd is still valid, thus no EBADF returned to > caller. No, exactly that can't happen either. As far as I can see the EPOLL holds a reference to the files it contains. So it is perfectly valid to add the file descriptor to EPOLL and then close it. In this case the file won't be closed, but be kept alive by it's reference in the EPOLL file descriptor. > > Then the two task contexts operate on same dma buf fd(one is close, > another is epoll) in kernel space. > > Please kindly help to comment whether above scenario is possible. > > If there is some bug in the caller logic, Can u help to point it out? :) Well what could be is that the EPOLL implementation is broken somehow, but I have really doubts on that. As I said before the most likely explanation is that you have a broken device driver which messes up the DMA-buf file reference count somehow. Regards, Christian. > > Regards, > Zhiguo >> >>> >>>> >>>> This reference is only dropped after the callback is completed in >>>> dma_buf_poll_cb(): >>>> >>>> /* Paired with get_file in dma_buf_poll */ >>>> fput(dmabuf->file); >>>> >>>> So your explanation for the issue just seems to be incorrect. >>>> >>>>> >>>>> 4. Here Thread A not do epoll_ctl(EPOLL_CTL_DEL) manunally, so it >>>>> still resides in one epoll_list. >>>>> Although __fput will call eventpoll_release to remove the file from >>>>> binded epoll list, >>>>> but it has small time window where Thread B jumps in. >>>> >>>> Well if that is really the case then that would then be a bug in >>>> epoll_ctl(). >>>> >>>>> >>>>> 5. During the small window, Thread B do the poll action for >>>>> dma_buf_fd, where it will fget/fput for the file, >>>>> this means the fd refcount will be 0 -> 1 -> 0, and it will call >>>>> __fput again. >>>>> This will lead to __fput twice for the same file. >>>>> >>>>> 6. So the potenial fix is use get_file_rcu which to check if file >>>>> refcount already zero which means under free. >>>>> If so, we just return and no need to do the dma_buf_poll. >>>> >>>> Well to say it bluntly as far as I can see this suggestion is >>>> completely >>>> nonsense. >>>> >>>> When the reference to the file goes away while dma_buf_poll() is >>>> executed then that's a massive bug in the caller of that function. >>>> >>>> Regards, >>>> Christian. >>>> >>>>> >>>>> Here is the race condition: >>>>> >>>>> Thread A Thread B >>>>> dma_buf_export >>>>> fd_refcount is 1 >>>>> epoll_ctl(EPOLL_ADD) >>>>> add dma_buf_fd to epoll list >>>>> close(dma_buf_fd) >>>>> fd_refcount is 0 >>>>> __fput >>>>> dma_buf_poll >>>>> fget >>>>> fput >>>>> fd_refcount is zero again >>>>> >>>>> Thanks >>>>> >>>> >>> >> >

7 months, 2 weeks

Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes

by Christian König

Am 08.05.24 um 10:23 schrieb Christian Brauner: > On Tue, May 07, 2024 at 07:45:02PM +0200, Christian König wrote: >> Am 07.05.24 um 18:46 schrieb Linus Torvalds: >>> On Tue, 7 May 2024 at 04:03, Daniel Vetter <daniel(a)ffwll.ch> wrote: >>>> It's really annoying that on some distros/builds we don't have that, and >>>> for gpu driver stack reasons we _really_ need to know whether a fd is the >>>> same as another, due to some messy uniqueness requirements on buffer >>>> objects various drivers have. >>> It's sad that such a simple thing would require two other horrid >>> models (EPOLL or KCMP). >>> >>> There'[s a reason that KCMP is a config option - *some* of that is >>> horrible code - but the "compare file descriptors for equality" is not >>> that reason. >>> >>> Note that KCMP really is a broken mess. It's also a potential security >>> hole, even for the simple things, because of how it ends up comparing >>> kernel pointers (ie it doesn't just say "same file descriptor", it >>> gives an ordering of them, so you can use KCMP to sort things in >>> kernel space). >>> >>> And yes, it orders them after obfuscating the pointer, but it's still >>> not something I would consider sane as a baseline interface. It was >>> designed for checkpoint-restore, it's the wrong thing to use for some >>> "are these file descriptors the same". >>> >>> The same argument goes for using EPOLL for that. Disgusting hack. >>> >>> Just what are the requirements for the GPU stack? Is one of the file >>> descriptors "trusted", IOW, you know what kind it is? >>> >>> Because dammit, it's *so* easy to do. You could just add a core DRM >>> ioctl for it. Literally just >>> >>> struct fd f1 = fdget(fd1); >>> struct fd f2 = fdget(fd2); >>> int same; >>> >>> same = f1.file && f1.file == f2.file; >>> fdput(fd1); >>> fdput(fd2); >>> return same; >>> >>> where the only question is if you also woudl want to deal with O_PATH >>> fd's, in which case the "fdget()" would be "fdget_raw()". >>> >>> Honestly, adding some DRM ioctl for this sounds hacky, but it sounds >>> less hacky than relying on EPOLL or KCMP. >>> >>> I'd be perfectly ok with adding a generic "FISAME" VFS level ioctl >>> too, if this is possibly a more common thing. and not just DRM wants >>> it. >>> >>> Would something like that work for you? >> Well the generic approach yes, the DRM specific one maybe. IIRC we need to >> be able to compare both DRM as well as DMA-buf file descriptors. >> >> The basic problem userspace tries to solve is that drivers might get the >> same fd through two different code paths. >> >> For example application using OpenGL/Vulkan for rendering and VA-API for >> video decoding/encoding at the same time. >> >> Both APIs get a fd which identifies the device to use. It can be the same, >> but it doesn't have to. >> >> If it's the same device driver connection (or in kernel speak underlying >> struct file) then you can optimize away importing and exporting of buffers >> for example. >> >> Additional to that it makes cgroup accounting much easier because you don't >> count things twice because they are shared etc... > One thing to keep in mind is that a generic VFS level comparing function > will only catch the obvious case where you have dup() equivalency as > outlined above by Linus. That's what most people are interested in and > that could easily replace most kcmp() use-cases for comparing fds. > > But, of course there's the case where you have two file descriptors > referring to two different files that reference the same underlying > object (usually stashed in file->private_data). > > For most cases that problem can ofc be solved by comparing the > underlying inode. But that doesn't work for drivers using the generic > anonymous inode infrastructure because it uses the same inode for > everything or for cases where the same underlying object can even be > represented by different inodes. > > So for such cases a driver specific ioctl() to compare two fds will > be needed in addition to the generic helper. At least for the DRM we already have some solution for that, see drmGetPrimaryDeviceNameFromFd() for an example. Basically the file->private_data is still something different, but we use this to figure out if we have two file descriptors (with individual struct files underneath) pointing to the same hw driver. This is important if you need to know if just importing/exporting of DMA-buf handles between the two file descriptors is enough or if you deal with two different hw devices and need to do stuff like format conversion etc... Regards, Christian.

7 months, 2 weeks

Re: get_file() unsafe under epoll (was Re: [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove)

by Linus Torvalds

On Mon, 6 May 2024 at 10:46, Stefan Metzmacher <metze(a)samba.org> wrote: > > I think it's a very important detail that epoll does not take > real references. Otherwise an application level 'close()' on a socket > would not trigger a tcp disconnect, when an fd is still registered with > epoll. Yes, exactly. epoll() ends up actually wanting the lifetime of the ep item be the lifetime of the file _descriptor_, not the lifetime of the file itself. We approximate that - badly - with epoll not taking a reference on the file pointer, and then at final fput() it tears things down. But that has two real issues, and one of them is that "oh, now epoll has file pointers that are actually dead" that caused this thread. The other issue is that "approximates" thing, where it means that duplicating the file pointer (dup*() and fork() end unix socket file sending etc) will not mean that the epoll ref is also out of sync with the lifetime of the file descriptor. That's why I suggested that "clean up epoll references at file_close_fd() time instead: https://lore.kernel.org/all/CAHk-=wj6XL9MGCd_nUzRj6SaKeN0TsyTTZDFpGdW34R+zM… because it would actually really *fix* the lifetime issue of ep items. In the process, it would make it possible to actually take a f_count reference at EPOLL_CTL_ADD time, since now the lifetime of the EP wouldn't be tied to the lifetime of the 'struct file *' pointer, it would be properly tied to the lifetime of the actual file descriptor that you are adding. So it would be a huge conceptual cleanup too. (Of course - at that point EPOLL_CTL_ADD still doesn't actually _need_ a reference to the file, since the file being open in itself is already that reference - but the point here being that there would *be* a reference that the epoll code would effectively have, and you'd never be in the situation we were in where there would be stale "dead" file pointers that just haven't gone through the cleanup yet). But I'd rather not touch the epoll code more than I have to. Which is why I applied the minimal patch for just "refcount over vfs_poll()", and am just mentioning my suggestion in the hope that some eager beaver would like to see how painful it would do to make the bigger surgery... Linus

7 months, 2 weeks

Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes

by David Laight

From: Christian Brauner > Sent: 06 May 2024 09:45 > > > The fact is, it's not dma-buf that is violating any rules. It's epoll. > > I agree that epoll() not taking a reference on the file is at least > unexpected and contradicts the usual code patterns for the sake of > performance and that it very likely is the case that most callers of > f_op->poll() don't know this. > > Note, I cleary wrote upthread that I'm ok to do it like you suggested > but raised two concerns a) there's currently only one instance of > prolonged @file lifetime in f_op->poll() afaict and b) that there's > possibly going to be some performance impact on epoll(). > > So it's at least worth discussing what's more important because epoll() > is very widely used and it's not that we haven't favored performance > before. > > But you've already said that you aren't concerned with performance on > epoll() upthread. So afaict then there's really not a lot more to > discuss other than take the patch and see whether we get any complaints. Surely there isn't a problem with epoll holding a reference to the file structure - it isn't really any different to a dup(). 'All' that needs to happen is that the 'magic' that makes epoll() remove files on the last fput happen when the close is done. I'm sure there are horrid locking issues it that code (separate from it calling ->poll() after ->release()) eg if you call close() concurrently with EPOLL_CTL_ADD. I'm not at all sure it would have mattered if epoll kept the file open. But it can't do that because it is documented not to. As well as poll/select holding a reference to all their fd for the duration of the system call, a successful mmap() holds a reference until the pages are all unmapped - usually by process exit. We (dayjob) have code that uses epoll() to monitor large numbers of UDP sockets. I was doing some tests (trying to) receive RTP (audio) data concurrently on 10000 sockets with typically one packet every 20ms. There are 10000 associated RCTP sockets that are usually idle. A more normal limit would be 1000 RTP sockets. All the data needs to go into a single (multithreaded) process. Just getting all the packets queued on the sockets was non-trivial. epoll is about the only way to actually read the data. (That needed multiple epoll fd so each thread could process all the events from one epoll fd then look for another unprocessed fd.) David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)

7 months, 2 weeks

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Linaro-mm-sig May 2024