On 2025-09-22 17:14:18+0200, Johannes Berg wrote: > On Mon, 2025-09-22 at 16:01 +0200, Thomas Weißschuh wrote: > > Right now it does not provide any advantage over a regular syscall. > > Essentially it is just overhead. That said, if you do want to make a > > real vDSO out of it, I'd be happy to help in that. > > I don't know if I'd say "just overhead" - depends on which path is more > optimised in a typical libc implementation? I'd basically think it's > identical, no? You either link to the vDSO, or a __weak same function in > the libc?
The code also needs to be built and maintained. AFAIK __weak is only for the compile-time linker. The vDSO call will be an indirect call. > > > I mean ... on the one hand, sure, it doesn't really do much after this, > > > but OTOH it lets userspace actually use that path? So might be useful. > > > > What advantage does userspace have from it? > > Right now, none? But it's easier to play with if you have the > infrastructure, and I'm not convinced there's a _disadvantage_? So far that hasn't happened. The disadvantages are the ones from above, nothing critical. But of course it is your subsystem and your call to make. > > > > Also the functionality to map the host vDSO and vsyscall page into UML > > > > userspace looks very weird and error-prone. Maybe it can also go away. > > > > > > Surely host vDSO etc. is never mapped into UML userspace and never is, > > > not sure what you're thinking of, but clearly that's wrong as written. > > > > This is how I understand the 32bit implementation using > > ARCH_REUSE_HOST_VSYSCALL_AREA and NEW_AUX_ENT(AT_SYSINFO_EHDR, > > vsyscall_ehdr) > > where vsyscall_ehdr comes from the hosts getauxval(AT_SYSINFO_EHDR). > > Huh, hm, yeah I forgot about that ... 32-bit. Yeah, agree we should just > kill that. I'm not even sure it works with the host kernel trapping > there? Oh well. Ack, do you want me to send a patch? This was my real gripe with the UM vDSO. I want to enable time namespaces for all architectures but these need to be handled in the vDSO properly. For the 64-bit stub vDSO it's not a problem as the syscalls will work correctly. But the interaction with the weird 32-bit logic on the other hand... Thomas
