Skip to content

Commit

Permalink
Store userspace context in proper struct when handling syscalls
Browse files Browse the repository at this point in the history 
The idea here is that once we have the userspace context stored in a proper struct (instead of on the temporary kernel stack), we can later persist it and resume paused processes.

It's the first time I'm writing assembly (http://memegen/9887503231543599), so for now this just leaves the existing logic for saving registers as is. We should replace it with xsave later on probably.

Change-Id: Id9887487e4fd2ec36206359cd0050bab2c989df3
  • Loading branch information
@jul-sh
jul-sh committed Jun 26, 2024
1 parent 28f8b82 commit 10eb046
Show file tree
Hide file tree
Showing 2 changed files with 92 additions and 84 deletions.
2 changes: 2 additions & 0 deletions oak_restricted_kernel/src/lib.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -37,6 +37,8 @@
#![feature(naked_functions)]
#![feature(c_size_t)]
#![feature(never_type)]
#![feature(offset_of_nested)]
#![feature(asm_const)]

mod acpi;
mod args;
Expand Down
174 changes: 90 additions & 84 deletions oak_restricted_kernel/src/syscall/mod.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -29,7 +29,7 @@ mod switch_process;
mod tests;

use alloc::boxed::Box;
use core::{arch::asm, ffi::c_void};
use core::{arch::asm, ffi::c_void, mem::offset_of};

use oak_channel::Channel;
use oak_restricted_kernel_interface::{Errno, Syscall};
Expand All @@ -51,24 +51,30 @@ use self::{
use crate::mm;

/// State we need to track for system calls.
///
/// Do not change the order of the fields here, as this is accessed from
/// assembly!
#[repr(C)]
#[repr(C, align(16))]
#[derive(Debug)]
struct GsData {
/// Kernel stack pointer (what to set in RSP after saving user RSP).
kernel_sp: VirtAddr,

/// User stack pointer. Saved from RSP after SYSCALL.
user_sp: VirtAddr,

/// User instruction pointer (where to return after SYSCALL). Saved from
/// RCX.
user_ip: VirtAddr,
/// User context data
user_ctx: UserContext,
}

/// User flags. Saved from R11.
user_flags: usize,
#[repr(C, align(16))]
#[derive(Debug, Default)]
struct UserContext {
rsp: u64,
rcx: u64,
rdi: u64,
rsi: u64,
rdx: u64,
rbx: u64,
r8: u64,
r9: u64,
r10: u64,
r11: u64,
ymm: [[u64; 4]; 16],
}

pub fn enable_syscalls(channel: Box<dyn Channel>, dice_data: dice_data::DiceData) {
Expand All @@ -85,9 +91,7 @@ pub fn enable_syscalls(channel: Box<dyn Channel>, dice_data: dice_data::DiceData
let gsdata = Box::leak(Box::new(GsData {
// Stack grows down, so SP points to the end of the page
kernel_sp,
user_sp: VirtAddr::zero(),
user_ip: VirtAddr::zero(),
user_flags: 0,
user_ctx: UserContext::default(),
}));

KernelGsBase::write(VirtAddr::from_ptr(gsdata));
Expand Down Expand Up @@ -170,43 +174,38 @@ extern "C" fn syscall_entrypoint() {
asm! {
// Switch to the syscall stack
"swapgs", // switch to kernel GS
"mov gs:[0x8], rsp", // save user RSP
"mov gs:[0x10], rcx", // save user RIP
"mov gs:[0x18], r11", // save user RFLAGS
"mov rsp, gs:[0x0]", // switch to kernel stack

// Save mutable registers other than RAX, RCX and R11 (the first is trashed for the return value;
// the latter two are are stored in gsdata).
"push rsi",
"push rdi",
"push rdx",
"push rbx",
"push r8",
"push r9",
"push r10",
// Save user context to GsData
"mov gs:[{OFFSET_RSP}], rsp",
"mov gs:[{OFFSET_RCX}], rcx",
"mov gs:[{OFFSET_RDI}], rdi",
"mov gs:[{OFFSET_RSI}], rsi",
"mov gs:[{OFFSET_RDX}], rdx",
"mov gs:[{OFFSET_RBX}], rbx",
"mov gs:[{OFFSET_R8}], r8",
"mov gs:[{OFFSET_R9}], r9",
"mov gs:[{OFFSET_R10}], r10",
"mov gs:[{OFFSET_R11}], r11",

// Make sure the stack is 16-byte aligned.
"sub rsp, 8",
// Save AVX registers to GsData
"vmovups gs:[{OFFSET_YMM} + 0*32], YMM0",
"vmovups gs:[{OFFSET_YMM} + 1*32], YMM1",
"vmovups gs:[{OFFSET_YMM} + 2*32], YMM2",
"vmovups gs:[{OFFSET_YMM} + 3*32], YMM3",
"vmovups gs:[{OFFSET_YMM} + 4*32], YMM4",
"vmovups gs:[{OFFSET_YMM} + 5*32], YMM5",
"vmovups gs:[{OFFSET_YMM} + 6*32], YMM6",
"vmovups gs:[{OFFSET_YMM} + 7*32], YMM7",
"vmovups gs:[{OFFSET_YMM} + 8*32], YMM8",
"vmovups gs:[{OFFSET_YMM} + 9*32], YMM9",
"vmovups gs:[{OFFSET_YMM} + 10*32], YMM10",
"vmovups gs:[{OFFSET_YMM} + 11*32], YMM11",
"vmovups gs:[{OFFSET_YMM} + 12*32], YMM12",
"vmovups gs:[{OFFSET_YMM} + 13*32], YMM13",
"vmovups gs:[{OFFSET_YMM} + 14*32], YMM14",
"vmovups gs:[{OFFSET_YMM} + 15*32], YMM15",

// Back up the AVX registers.
// TODO(#3329): Update interrupt handler macro to support AVX, SSE or neither.
"sub rsp, 16*32",
"vmovups [rsp + 0*32], YMM0",
"vmovups [rsp + 1*32], YMM1",
"vmovups [rsp + 2*32], YMM2",
"vmovups [rsp + 3*32], YMM3",
"vmovups [rsp + 4*32], YMM4",
"vmovups [rsp + 5*32], YMM5",
"vmovups [rsp + 6*32], YMM6",
"vmovups [rsp + 7*32], YMM7",
"vmovups [rsp + 8*32], YMM8",
"vmovups [rsp + 9*32], YMM9",
"vmovups [rsp + 10*32], YMM10",
"vmovups [rsp + 11*32], YMM11",
"vmovups [rsp + 12*32], YMM12",
"vmovups [rsp + 13*32], YMM13",
"vmovups [rsp + 14*32], YMM14",
"vmovups [rsp + 15*32], YMM15",
"mov rsp, gs:[{OFFSET_KERNEL_STACK_POINTER}]", // switch to kernel stack

// Shuffle around register values to match sysv calling convention, and escape into
// proper Rust code from the assembly.
Expand All @@ -222,47 +221,54 @@ extern "C" fn syscall_entrypoint() {
"pop r9",
"add rsp, 8",

// Restore AVX registers.
"vmovups YMM0, [rsp + 0*32]",
"vmovups YMM1, [rsp + 1*32]",
"vmovups YMM2, [rsp + 2*32]",
"vmovups YMM3, [rsp + 3*32]",
"vmovups YMM4, [rsp + 4*32]",
"vmovups YMM5, [rsp + 5*32]",
"vmovups YMM6, [rsp + 6*32]",
"vmovups YMM7, [rsp + 7*32]",
"vmovups YMM8, [rsp + 8*32]",
"vmovups YMM9, [rsp + 9*32]",
"vmovups YMM10, [rsp + 10*32]",
"vmovups YMM11, [rsp + 11*32]",
"vmovups YMM12, [rsp + 12*32]",
"vmovups YMM13, [rsp + 13*32]",
"vmovups YMM14, [rsp + 14*32]",
"vmovups YMM15, [rsp + 15*32]",
"add rsp, 16*32",
// Restore AVX registers from GsData.
"vmovups YMM0, gs:[{OFFSET_YMM} + 0*32]",
"vmovups YMM1, gs:[{OFFSET_YMM} + 1*32]",
"vmovups YMM2, gs:[{OFFSET_YMM} + 2*32]",
"vmovups YMM3, gs:[{OFFSET_YMM} + 3*32]",
"vmovups YMM4, gs:[{OFFSET_YMM} + 4*32]",
"vmovups YMM5, gs:[{OFFSET_YMM} + 5*32]",
"vmovups YMM6, gs:[{OFFSET_YMM} + 6*32]",
"vmovups YMM7, gs:[{OFFSET_YMM} + 7*32]",
"vmovups YMM8, gs:[{OFFSET_YMM} + 8*32]",
"vmovups YMM9, gs:[{OFFSET_YMM} + 9*32]",
"vmovups YMM10, gs:[{OFFSET_YMM} + 10*32]",
"vmovups YMM11, gs:[{OFFSET_YMM} + 11*32]",
"vmovups YMM12, gs:[{OFFSET_YMM} + 12*32]",
"vmovups YMM13, gs:[{OFFSET_YMM} + 13*32]",
"vmovups YMM14, gs:[{OFFSET_YMM} + 14*32]",
"vmovups YMM15, gs:[{OFFSET_YMM} + 15*32]",

// Undo stack alignment.
"add rsp, 8",
// Restore scratch registers.
"pop r10",
"pop r9",
"pop r8",
"pop rbx",
"pop rdx",
"pop rdi",
"pop rsi",
// Restore scratch registers from GsData
"mov rcx, gs:[{OFFSET_RCX}]",
"mov rdi, gs:[{OFFSET_RDI}]",
"mov rsi, gs:[{OFFSET_RSI}]",
"mov rdx, gs:[{OFFSET_RDX}]",
"mov rbx, gs:[{OFFSET_RBX}]",
"mov r8, gs:[{OFFSET_R8}]",
"mov r9, gs:[{OFFSET_R9}]",
"mov r10, gs:[{OFFSET_R10}]",
"mov r11, gs:[{OFFSET_R11}]", // restore user RFLAGS

// Restore user values in preparation for SYSRET.
// We don't save the kernel stack value; we'll just overwrite what's there next time
// a syscall is invoked.
"mov rsp, gs:[0x8]", // restore user RSP
"mov rcx, gs:[0x10]", // restore user RIP
"mov r11, gs:[0x18]", // restore user RFLAGS
// Restore user RSP in preparation for SYSRET.
"mov rsp, gs:[{OFFSET_RSP}]",
"swapgs", // restore user GS

// Back to user code in Ring 3.
"sysretq",
HANDLER = sym syscall_handler,
OFFSET_KERNEL_STACK_POINTER = const(offset_of!(GsData, kernel_sp)),
OFFSET_RSP = const(offset_of!(GsData, user_ctx.rsp)),
OFFSET_RCX = const(offset_of!(GsData, user_ctx.rcx)),
OFFSET_RDI = const(offset_of!(GsData, user_ctx.rdi)),
OFFSET_RSI = const(offset_of!(GsData, user_ctx.rsi)),
OFFSET_RDX = const(offset_of!(GsData, user_ctx.rdx)),
OFFSET_RBX = const(offset_of!(GsData, user_ctx.rbx)),
OFFSET_R8 = const(offset_of!(GsData, user_ctx.r8)),
OFFSET_R9 = const(offset_of!(GsData, user_ctx.r9)),
OFFSET_R10 = const(offset_of!(GsData, user_ctx.r10)),
OFFSET_R11 = const(offset_of!(GsData, user_ctx.r11)),
OFFSET_YMM = const(offset_of!(GsData, user_ctx.ymm)),
options(noreturn)
}
}
Expand Down

0 comments on commit 10eb046

Please sign in to comment.