Taking this very slowly because this is very fickle code. The frontend
needs to manage GDT and LDT, but before we get there, we need to
actually add support for LDT in the backend. Split the segments to two
arrays so the JIT can actually update their cached values correctly.
Still treats GDT and LDT as mirrors like how the JIT previously did (By
it ignoring the selector's TI bit).
Previously we were only storing the 32-bit base address which isn't
actually how segment descriptors work.
In reality segment descriptors are 64-bit descriptors that are laid out
in a particular layout depending on the 4-bit type value. In reality we
only care about code and data segment layouts since the rest are
bonkers.
Describe these descriptors correctly and setup a default code descriptor
for the operating mode that FEX is starting in.
Seccomp is a relatively complex feature that was added to Linux back in
2005, and was further extended in 2013 to support BPF based protections.
Once seccomp is enabled, you can no longer disable seccomp but
additional protections can be placed on top of existing seccomp filters.
Additionally seccomp filters are inherited in child processes, which
ensures the process tree can't escape from the secure computing
environment through child processes.
The basis of this feature is a shim that lives between userspace and the
kernel at the syscall entrypoint.
In "strict" mode, seccomp only allows read, write, exit, exit_group, and {rt_,}sigreturn to function.
When in "filter" mode, a BPF filter is run on syscall entrypoint and
returns state about if the syscall should be allowed or not. Multiple
filters can be installed in this mode, all of which get executed. The
result that is the most restricted is the action that occurs at the end.
There are some significant limitations in filter mode that must be
adhered to which makes executing this code inside of kernel space a
non-issue and effectively limits how much cpu time is spent in the filters.
Although these filters are free to do basically anything with the
provided data, just can't do any loops.
FEX needs to implement seccomp because there are multiple applications
using the feature, the primary one being Chromium which some games embed
without disabling the sandbox. WINE also uses seccomp for capturing
games that do raw Windows system calls. Apparently Red Dead Redemption
is one of the games that requires this.
While FEX implements seccomp, it is not yet all encompassing, which is
one of the reasons why it isn't enabled by default and requires a config
option.
**seccomp_unotify is not implemented**
This is a relatively new feature for seccomp which lets the seccomp
filter signal an FD for multiple things. Luckily Chromium and WINE don't
use this. This will be tricky to implement under FEX since it
requires ioctl trapping and some other behaviour
**ptrace isn't supported**
One feature of seccomp is that it can raise ptrace events. Since FEX
doesn't support ptrace at all, this isn't handled. Again Chromium and
WINE don't use this.
**kill-thread not quite correct**
This isn't directly related to seccomp but more about how we do thread
shutdown in FEX. This will require some more changes around thread state
tracking before fully supporting this. Chromium and WINE don't use this.
kill-process also falls under this
Features that are supported:
- Strict mode and seccomp-bpf mode supported
- All BFP instructions that seccomp-bpf understands
- Inheriting seccomp through execve
- This means we serialize and deserialize the calling thread's
seccomp filters
- An execve that escapes FEX will also escape seccomp. Not much we
can do about it
- TSync - Allowing post-mortem seccomp insertion which allows threads to
synchronize seccomp filters after the fact
Features that are not supported:
- Different arch qualifiers depending on syscall entrypoint
- Just like our syscall handler, we are hardcoded to the arch that the
application starts with
- user_notif
- ptrace
- Runtime code cache invalidation when seccomp is installed
- Currently we must ensure all syscalls go through the frontend
syscall handler
- Runtime invalidation of code cache with inline syscalls will get
fixed in the future.
This currently isn't enabled by default because of the minor feature
problems that haven't been resolved. Currently the Linux Kernel's test
application works for the features that FEX supports, and WINE's usage
can be handled by FEX. Chromium's sandbox doesn't yet work with this PR,
but it only fails due to features unrelated to seccomp.
Having this open for merging now so we can work to resolve the remaining
issues without this bitrotting.
Two primary things here:
- Remove the static `GlobalDelegator`
- Move the thread_local SignalDelegator::ThreadState information
directly in to ThreadStateObject
Having the ThreadStateObject and the SignalDelegator information
disjoint was confusing but was required when we didn't have any object
in the frontend that could have its own independent data. Since we fixed
this with the `ThreadStateObject` type we can now move this over.
The `GlobalDelegator` object is now instead stored in
`ThreadStateObject` instead.
Instead of using a thread_local variable, we now just consume 8-bytes of
the signal alt-stack since the kernel gives us that information about
where it lives. This then converts all the thread_local usage to use
either the passed in CPU state if it exists, or fetching it from the
alt-stack offset.
Very minor changes in behaviour here, will help when trying to improve
FEX's behaviour around signals.
This has been leaked state to FEXCore for quite a while. FEXCore never
actually needed this information, moves the bits to the frontend that
are necessary.
Minor behaviour change that `RunUntilExit` now just assumes the primary
thread is using it. This behaviour is on the chopping block to get
removed next anyway.
Reimagining of #3355 without any json generators or new concepts.
Fixes some mislabeling of system calls. Some getting inlined when they
shouldn't be, a lot not getting inlined when they can be.
This really cleans up the syscall implementation, all syscalls that can
be passthrough implementations require a very small two line
declaration.
Additionally cleans up a bit of implementation cruft where some
passthrough syscalls were using the glibc syscall handler, and some were
using the glibc implementation. We have had multiple issues in the past
where the glibc implementation does something subtly different than the
raw syscall and breaks things. Now all passthrough handlers do a system
call directly, removing at least one indirection and some ambiguity.
This makes it significantly easier to add new passthrough syscalls as
well. Only need to do a version check and add the three lines per
syscall. Which there are new syscalls incoming that we will want to add.
Tangible improvements:
- Syscalls are lower overhead than ever.
- When I'm adding more syscalls I have less chance of mucking it up.
Previously we were only storing the 32-bit base address which isn't
actually how segment descriptors work.
In reality segment descriptors are 64-bit descriptors that are laid out
in a particular layout depending on the 4-bit type value. In reality we
only care about code and data segment layouts since the rest are
bonkers.
Describe these descriptors correctly and setup a default code descriptor
for the operating mode that FEX is starting in.
Which requires moving LinuxEmulation to its own independent folder as
well. Since both IRLoader and FEXLoader rely on it.
No functional change, just moves the the code around.
LC