This is an interesting vdso implementation because it only exists in
x86-64 with kernel v6.11. For Arm64 the implementation is likely to land
in a couple kernel versions from now.
Some differences with vdso_getrandom versus the regular getrandom
syscall
- Has two additional arguments, opaque_state and opaque_len
- Expects the userspace to mmap/munmap this opaque state structure
- Lets userspace query information about this structure upfront
- If the opaque data structure isn't provided then it falls back to
regular SYS_getrandom
With the "glibc" implementation, we can tell the interface to allocate a
single page that gets unused (otherwise glibc ends up not using the
interface), and fallback to the regular SYS_getrandom.
In the case of the vdso interface, currently the arguments just get
passed through.
Keeping this as a WIP until the ARM64 kernel patches land and I can
actually test the things. Running the "glibc" path works today with the
selftest, but the vdso path is currently completely untested.
These need to be bit-exact following exactly what is shown in the
assembly.
libunwind parses where EIP is to see if it is in a stack frame.
Also needsto live in VDSO otherwise backtrace doesn't work.
The `mov ebp, ecx` was breaking vsyscall and was expected to be used
with the `syscall` instruction rather than `int 0x80`.
Remove that to fix it.
Also remove the pushes and pops around the syscall instruction, these
are unnecessary in an emulated environment, we won't clobber the
registers.
Fixes Steam execution with VDSO.
VDSO is heavily abused by Proton games to the point it is showing up as
CPU time.
Implement a guest-facing only thunk library using the hardcoded VDSO
interface in Thunks.
If available this will always be loaded on application load and set the
auxv value to support it.
This requires a bit of special treatment as our first user of linker
scripts since the format of the ELF must be careful crafted to not break
applications trying to parse it.
This library exposes a handful of symbols:
- clock_gettime
- clock_getres
- gettimeofday
- time
- getcpu
- All previous with `__vdso_` prefix
- LINUX_2.6
All of these symbols get routed directly to the host architecture VDSO
interface if they exist.
AArch64 doesn't have getcpu or time VDSO.
In a microbench, VDSO improved bench times substantially
x86-64 host: 3.612s -> 1.369s - 2.63x speed
AArch64 host: 3.821s -> 2.284s - 1.67x speed
- AArch64 isn't as improved due to missing VDSO symbols
This is also our first /always/ enabled thunk as long as the file exists
Ryan Houdek