Zvi Effron <zeffron@xxxxxxxxxxxxx> writes: > On Tue, Sep 21, 2021 at 9:06 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: >> >> Hi Lorenz (Cc. the other people who participated in today's discussion) >> >> Following our discussion at the LPC session today, I dug up my previous >> summary of the issue and some possible solutions[0]. Seems no on >> actually replied last time, which is why we went with the "do nothing" >> approach, I suppose. I'm including the full text of the original email >> below; please take a look, and let's see if we can converge on a >> consensus here. >> >> First off, a problem description: If an existing XDP program is exposed >> to an xdp_buff that is really a multi-buffer, while it will continue to >> run, 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? >> > > I think there's another potential problem with this as well: what happens to > already loaded programs that are not MB-aware? Are they forcibly unloaded? I'd say probably the opposite: You can't toggle whatever switch we end up with if there are any non-MB-aware programs (you'd have to unload them first)... >> 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. >> > > Could we combine the last two bits here into a global toggle that doesn't > require a sysctl? If any driver is put into multi-buffer mode, then the system > switches to requiring all programs be multi-buffer? When the last multi-buffer > enabled driver switches out of multi-buffer, remove the system-wide > restriction? Well, the trouble here is that we don't necessarily have an explicit "multi-buf mode" for devices. For instance, you could raise the MTU of a device without it necessarily involving any XDP multi-buffer stuff (if you're not running XDP on that device). So if we did turn "raising the MTU" into such a mode switch, we would end up blocking any MTU changes if any XDP programs are loaded. Or having an MTU change cause a force-unload of all XDP programs. Neither of those are desirable outcomes, I think; and if we add a separate "XDP multi-buff" switch, we might as well make it system-wide? > Regarding my above question, if non-MB-aware XDP programs are not forcibly > unloaded, then a global toggle is also insufficient. An existing non-MB-aware > XDP program would still beed to be rejected at attach time by the > driver. See above. -Toke
