The implementation of madvise (linux-user/syscall.c:11331, tag v5.0.0-rc4) always returns zero (i.e. success). However, an application requesting (at least) MADV_WIPEONFORK may need to know whether the call was actually successful. If not (because the kernel doesn't support WIPEONFORK) then it will need to take other measures to provide fork-safety (such as drawing entropy from the kernel in every case). But, if the application believes that WIPEONFORK is supported (because madvise returned zero), but it actually isn't (as in qemu), then it may forego those protections on the assumption that WIPEONFORK will provide fork-safety.
Roughly, the comment in qemu that says "This is a hint, so ignoring and returning success is ok." is no longer accurate in the presence of MADV_WIPEONFORK.
(This is not purely academic: BoringSSL is planning on acting in this way. We found the qemu behaviour in pre-release testing and are planning on making an madvise call with advice=-1 first to test whether unknown advice values actually produce EINVAL.)
The QEMU project is currently moving its bug tracking to another system.
For this we need to know which bugs are still valid and which could be
closed already. Thus we are setting older bugs to "Incomplete" now.
If you still think this bug report here is valid, then please switch
the state back to "New" within the next 60 days, otherwise this report
will be marked as "Expired". Or please mark it as "Fix Released" if
the problem has been solved with a newer version of QEMU already.
Thank you and sorry for the inconvenience.