Re: [PATCH 0/3] linux-user: Implement x86_64 vsyscalls

[Laurent Vivier]

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