Le 14/01/2020 à 22:09, Richard Henderson a écrit : > 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 > > target/i386/cpu.h | 6 +- > linux-user/i386/cpu_loop.c | 197 ++++++++++++++++++++++++++----------- > target/i386/translate.c | 16 ++- > 3 files changed, 155 insertions(+), 64 deletions(-) >
Thank you Richard. I'll take this series through the linux-user branch except if you prefer to do the pull request yourself via another branch (x86 tcg?). Thanks, Laurent
