On Tue, Sep 21, 2021 at 11:23 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > > 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)... > How would we communicate that issue? dmesg? I'm not very familiar with how sysctl change failure causes are communicated to users, so this might be a solved problem, but if I run `sysctl -w net.xdp.multibuffer 1` (or whatever ends up actually being the toggle) to active multi-buffer, and it fails because there's a loaded non-aware program, that seems like a potential for a lot of administrator pain. > >> 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. Maybe I missed something then, but you had stated that "while a particular driver knows if it's running in multi-buff mode" so I assumed that the driver would be able to tell when to toggle the mode on. I had been thinking that when a driver turned multi-buffer off, it could trigger a check of all drivers, but that also seems like it could just be a global refcount of all the drivers that have requested multi-buffer mode. When a driver enables multi-buffer for itself, it increments the refcount, and when it disables, it decrements. A non-zero count means the system is in multi-buffer mode. Obviously this is more complex than just requiring the administrator to enable the system-wide mode. > > 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 > --Zvi
