Richard Henderson <richard.hender...@linaro.org> writes:
> The x86_64 abi has a legacy vsyscall page. The kernel folk > have been trying to deprecate this since at least v3.1, but > > (1) We don't implement the vdso that replaces vsyscalls, > (2) As of v5.5, the vsyscall page is still enabled by default. > > This lack is affecting Peter's linux-user testing. > > The dependency is not obvious because Peter is running the tests > on x86_64, so the host is providing a vsyscall page to qemu. > > Because of how user-only memory operations are handled, with no > validation of guest vs host pages, so long as qemu chooses to > run with guest_base == 0, the guest may Just So Happen to read > the host's vsyscall page. > > Complicating this, new OS releases may use a kernel configured > with CONFIG_LEGACY_VSYSCALL_XONLY=y, which means the the vsyscall > page cannot be read, only executed. Which means that the guest > then cannot read the host vsyscall page during translation and > will SIGSEGV. > > Exactly which of these many variables is affecting Peter's testing > with Ubuntu 18.04 of my TCG merge, I'm not exactly sure. I suspect > that it is the change to drop the textseg_addr adjustment to user-only > static binaries. IIRC bionic does not support -static-pie, which is > the preferred replacement. This could mean that the host and guest > binaries overlap, which leads to guest_base != 0. > > I vaguely remember someone (Paolo?) implementing something like > this many years ago, but clearly it never got merged. > > In any case, this emulation has been missing for too long. > > > r~ > > > Richard Henderson (3): > target/i386: Renumber EXCP_SYSCALL > linux-user/i386: Split out gen_signal > linux-user/i386: Emulate x86_64 vsyscalls And realising I didn't read it properly: /x86_64-linux-user/qemu-x86_64 ~/lsrc/linux.git/tools/testing/selftests/x86/test_vsyscall_64 [WARN] failed to find vDSO no vsyscall map in /proc/self/maps [RUN] test gettimeofday() [RUN] test time() [RUN] getcpu() on CPU 0 [RUN] getcpu() on CPU 1 [RUN] Checking read access to the vsyscall page [OK] We do not have read access: #PF(0x4) [RUN] Make sure that vsyscalls really page fault ** ERROR:/home/alex/lsrc/qemu.git/linux-user/x86_64/../i386/cpu_loop.c:185:emulate_vsyscall: assertion failed: (ret != -TARGET_EFAULT) qemu:handle_cpu_signal received signal outside vCPU context @ pc=0x7f6eed31b613 -- Alex Bennée