Changing the subject to address this point specifically: > Right that was my conclusion as well. Existing programs might have > subtle side effects if they start running on multibuffer drivers as > is. I don't have any good ideas though on how to handle this. So I had a chat about this with Lorenzo, Eelco and Jesper today, and promised I'd summarise our discussion to you all, so this is my attempt at that. Please excuse the long email, I'm just trying to be comprehensive :) So first off, a problem description: If an existing XDP program is exposed to an xdp_buff that is really a multi-buffer, it may end up with subtle and hard-to-debug bugs: If it's parsing the packet it'll only see part of the payload and not be aware of that fact, and if it's calculating the packet length, that will also only be wrong (only counting the first fragment). So what to do about this? First of all, to do anything about it, XDP programs need to be able to declare themselves "multi-buffer aware" (but see point 1 below). We could try to auto-detect it in the verifier by which helpers the program is using, but since existing programs could be perfectly happy to just keep running, it probably needs to be something the program communicates explicitly. One option is to use the expected_attach_type to encode this; programs can then declare it in the source by section name, or the userspace loader can set the type for existing programs if needed. With this, the kernel will know if a given XDP program is multi-buff aware and can decide what to do with that information. For this we came up with basically three options: 1. Do nothing. This would make it up to users / sysadmins to avoid anything breaking by manually making sure to not enable multi-buffer support while loading any XDP programs that will malfunction if presented with an mb frame. This will probably break in interesting ways, but it's nice and simple from an implementation PoV. With this we don't need the declaration discussed above either. 2. Add a check at runtime and drop the frames if they are mb-enabled and the program doesn't understand it. This is relatively simple to implement, but it also makes for difficult-to-understand issues (why are my packets suddenly being dropped?), and it will incur runtime overhead. 3. Reject loading of programs that are not MB-aware when running in an MB-enabled mode. This would make things break in more obvious ways, and still allow a userspace loader to declare a program "MB-aware" to force it to run if necessary. The problem then becomes at what level to block this? Doing this at the driver level is not enough: while a particular driver knows if it's running in multi-buff mode, we can't know for sure if a particular XDP program is multi-buff aware at attach time: it could be tail-calling other programs, or redirecting packets to another interface where it will be processed by a non-MB aware program. So another option is to make it a global toggle: e.g., create a new sysctl to enable multi-buffer. If this is set, reject loading any XDP program that doesn't support multi-buffer mode, and if it's unset, disable multi-buffer mode in all drivers. This will make it explicit when the multi-buffer mode is used, and prevent any accidental subtle malfunction of existing XDP programs. The drawback is that it's a mode switch, so more configuration complexity. None of these options are ideal, of course, but I hope the above explanation at least makes sense. If anyone has any better ideas (or can spot any flaws in the reasoning above) please don't hesitate to let us know! -Toke
