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List-Subscribe: List-Unsubscribe: +cc David. On Fri, Feb 27, 2026 at 03:56:01PM -0500, Steven Rostedt wrote: > On Fri, 27 Feb 2026 10:20:38 -0500 > Steven Rostedt wrote: > > > On Fri, 27 Feb 2026 11:22:22 +0000 > > Vincent Donnefort wrote: > > > > > > Ah right, Syzkaller is using madvise(MADVISE_DOFORK) which resets VM_DONTCOPY. > > > > > > As we are applying restrictive rules for this mapping, I believe setting VM_IO > > > might be a better fix. > > > > Agreed. > > > > Adding MM folks so we do this right. > > Dear MM folks, > > Here's the issue. When the ftrace ring buffer is memory mapped to user > space, we do not want anything "special" done to it. One of those things we > did not want done was to have it copied on fork. To do that, we added > VM_DONTCOPY, but we didn't know that an madvise() could disable that. It > looks like VM_IO will prevent that from happening. > > But looking at the various flags, I see there's a VM_SPECIAL. I'm wondering > if that is what we should use? VM_SPECIAL is not a VMA flag, it's a bitmask of all the flags which cause us not to permit things like splitting/merging of VMAs (because we can't safely do them), i.e. that are one or more of: VM_IO - Memory-mapped I/O range. VM_PFNMAP - A mapping without struct folio's/page's backing them, e.g. perhaps a raw kernel mapping. VM_MIXEDMAP - A combination of page/folio-backed memory and/or PFN-backed memory. VM_DONTEXPAND - Disallow expansion of memory in mremap(). You already set VM_DONTEXPAND so you get these semantics already. Setting VM_IO just to trigger a failure case in madvise() feels like a hack? I guess it'd do the trick though, but you're not going to be able to reclaim that memory, and you might get some unexpected behaviour in code paths that assume VM_IO means it's memory-mapped I/O... (for instance GUP will stop working, if you need that). I'd take a step back and wonder why you are wanting to not allow copying on fork? Is this kernel-allocated memory? In which case you should set VM_MIXEDMAP or VM_PFNMAP as appropriate... If not and it has a folio etc. then it seems like strange semantics. Are you really bothered also by users doing strange things? Maybe the solution is to tolerate a fork-copy even if it's broken? I presume somethings straight up breaks right now? Without more context that I don't really have much time to acquire it's hard to know what to advise. > > The effected code is here: > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/trace/ring_buffer.c#n7172 > > What's your thoughts? > > Thanks, > > -- Steve Cheers, Lorenzo