wasmtime: Refactor trap-handling (bytecodealliance#9087)

This commit groups together the registers that have to be collected from
a signal handler to correctly report a trap: namely, the program counter
and frame pointer, as of the time that the trap occurred.

I also moved the call to set_jit_trap inside test_if_trap for every
platform that uses both methods. Only the implementation for Mach ports
still needs to call set_jit_trap because it doesn't use test_if_trap.

In addition I'm fixing an unrelated doc comment that I stumbled across
while working on this.

crates/wasmtime/src/runtime/func.rs

@@ -984,8 +984,8 @@ impl Func {
     /// # Panics
     ///
     /// This function will panic if called on a function belonging to an async
-    /// store. Asynchronous stores must always use `call_async`.
-    /// initiates a panic. Also panics if `store` does not own this function.
+    /// store. Asynchronous stores must always use `call_async`. Also panics if
+    /// `store` does not own this function.
     ///
     /// [`WasmBacktrace`]: crate::WasmBacktrace
     pub fn call(

crates/wasmtime/src/runtime/vm/sys/custom/traphandlers.rs

@@ -1,4 +1,4 @@
-use crate::runtime::vm::traphandlers::{tls, TrapTest};
+use crate::runtime::vm::traphandlers::{tls, TrapRegisters, TrapTest};
 use crate::runtime::vm::VMContext;
 use core::mem;
 
@@ -31,7 +31,7 @@ impl TrapHandler {
     pub fn validate_config(&self, _macos_use_mach_ports: bool) {}
 }
 
-extern "C" fn handle_trap(ip: usize, fp: usize, has_faulting_addr: bool, faulting_addr: usize) {
+extern "C" fn handle_trap(pc: usize, fp: usize, has_faulting_addr: bool, faulting_addr: usize) {
     tls::with(|info| {
         let info = match info {
             Some(info) => info,
@@ -42,17 +42,14 @@ extern "C" fn handle_trap(ip: usize, fp: usize, has_faulting_addr: bool, faultin
         } else {
             None
         };
-        let ip = ip as *const u8;
-        let test = info.test_if_trap(ip, |_handler| {
+        let regs = TrapRegisters { pc, fp };
+        let test = info.test_if_trap(regs, faulting_addr, |_handler| {
             panic!("custom signal handlers are not supported on this platform");
         });
         match test {
             TrapTest::NotWasm => {}
             TrapTest::HandledByEmbedder => unreachable!(),
-            TrapTest::Trap { jmp_buf, trap } => {
-                info.set_jit_trap(ip, fp, faulting_addr, trap);
-                unsafe { wasmtime_longjmp(jmp_buf) }
-            }
+            TrapTest::Trap { jmp_buf } => unsafe { wasmtime_longjmp(jmp_buf) },
         }
     })
 }

crates/wasmtime/src/runtime/vm/sys/unix/machports.rs

@@ -35,7 +35,7 @@
 
 use crate::runtime::module::lookup_code;
 use crate::runtime::vm::sys::traphandlers::wasmtime_longjmp;
-use crate::runtime::vm::traphandlers::tls;
+use crate::runtime::vm::traphandlers::{tls, TrapRegisters};
 use mach2::exc::*;
 use mach2::exception_types::*;
 use mach2::kern_return::*;
@@ -384,21 +384,16 @@ unsafe fn handle_exception(request: &mut ExceptionRequest) -> bool {
 /// a native backtrace once we've switched back to the thread itself. After
 /// the backtrace is captured we can do the usual `longjmp` back to the source
 /// of the wasm code.
-unsafe extern "C" fn unwind(
-    wasm_pc: *const u8,
-    wasm_fp: usize,
-    fault1: usize,
-    fault2: usize,
-    trap: u8,
-) -> ! {
+unsafe extern "C" fn unwind(pc: usize, fp: usize, fault1: usize, fault2: usize, trap: u8) -> ! {
     let jmp_buf = tls::with(|state| {
         let state = state.unwrap();
+        let regs = TrapRegisters { pc, fp };
         let faulting_addr = match fault1 {
             0 => None,
             _ => Some(fault2),
         };
         let trap = Trap::from_u8(trap).unwrap();
-        state.set_jit_trap(wasm_pc, wasm_fp, faulting_addr, trap);
+        state.set_jit_trap(regs, faulting_addr, trap);
         state.take_jmp_buf()
     });
     debug_assert!(!jmp_buf.is_null());

crates/wasmtime/src/runtime/vm/sys/unix/signals.rs

@@ -1,6 +1,6 @@
 //! Trap handling on Unix based on POSIX signals.
 
-use crate::runtime::vm::traphandlers::{tls, TrapTest};
+use crate::runtime::vm::traphandlers::{tls, TrapRegisters, TrapTest};
 use crate::runtime::vm::VMContext;
 use std::cell::RefCell;
 use std::io;
@@ -166,23 +166,24 @@ unsafe extern "C" fn trap_handler(
         // Otherwise flag ourselves as handling a trap, do the trap
         // handling, and reset our trap handling flag. Then we figure
         // out what to do based on the result of the trap handling.
-        let (pc, fp) = get_pc_and_fp(context, signum);
-        let test = info.test_if_trap(pc, |handler| handler(signum, siginfo, context));
+        let faulting_addr = match signum {
+            libc::SIGSEGV | libc::SIGBUS => Some((*siginfo).si_addr() as usize),
+            _ => None,
+        };
+        let regs = get_trap_registers(context, signum);
+        let test = info.test_if_trap(regs, faulting_addr, |handler| {
+            handler(signum, siginfo, context)
+        });
 
         // Figure out what to do based on the result of this handling of
         // the trap. Note that our sentinel value of 1 means that the
         // exception was handled by a custom exception handler, so we
         // keep executing.
-        let (jmp_buf, trap) = match test {
+        let jmp_buf = match test {
             TrapTest::NotWasm => return false,
             TrapTest::HandledByEmbedder => return true,
-            TrapTest::Trap { jmp_buf, trap } => (jmp_buf, trap),
-        };
-        let faulting_addr = match signum {
-            libc::SIGSEGV | libc::SIGBUS => Some((*siginfo).si_addr() as usize),
-            _ => None,
+            TrapTest::Trap { jmp_buf } => jmp_buf,
         };
-        info.set_jit_trap(pc, fp, faulting_addr, trap);
         // On macOS this is a bit special, unfortunately. If we were to
         // `siglongjmp` out of the signal handler that notably does
         // *not* reset the sigaltstack state of our signal handler. This
@@ -247,20 +248,20 @@ unsafe extern "C" fn trap_handler(
     }
 }
 
-unsafe fn get_pc_and_fp(cx: *mut libc::c_void, _signum: libc::c_int) -> (*const u8, usize) {
+unsafe fn get_trap_registers(cx: *mut libc::c_void, _signum: libc::c_int) -> TrapRegisters {
     cfg_if::cfg_if! {
         if #[cfg(all(any(target_os = "linux", target_os = "android"), target_arch = "x86_64"))] {
             let cx = &*(cx as *const libc::ucontext_t);
-            (
-                cx.uc_mcontext.gregs[libc::REG_RIP as usize] as *const u8,
-                cx.uc_mcontext.gregs[libc::REG_RBP as usize] as usize
-            )
+            TrapRegisters {
+                pc: cx.uc_mcontext.gregs[libc::REG_RIP as usize] as usize,
+                fp: cx.uc_mcontext.gregs[libc::REG_RBP as usize] as usize,
+            }
         } else if #[cfg(all(any(target_os = "linux", target_os = "android"), target_arch = "aarch64"))] {
             let cx = &*(cx as *const libc::ucontext_t);
-            (
-                cx.uc_mcontext.pc as *const u8,
-                cx.uc_mcontext.regs[29] as usize,
-            )
+            TrapRegisters {
+                pc: cx.uc_mcontext.pc as usize,
+                fp: cx.uc_mcontext.regs[29] as usize,
+            }
         } else if #[cfg(all(target_os = "linux", target_arch = "s390x"))] {
             // On s390x, SIGILL and SIGFPE are delivered with the PSW address
             // pointing *after* the faulting instruction, while SIGSEGV and
@@ -277,46 +278,46 @@ unsafe fn get_pc_and_fp(cx: *mut libc::c_void, _signum: libc::c_int) -> (*const
                 _ => 0,
             };
             let cx = &*(cx as *const libc::ucontext_t);
-            (
-                (cx.uc_mcontext.psw.addr - trap_offset) as *const u8,
-                *(cx.uc_mcontext.gregs[15] as *const usize),
-            )
+            TrapRegisters {
+                pc: (cx.uc_mcontext.psw.addr - trap_offset) as usize,
+                fp: *(cx.uc_mcontext.gregs[15] as *const usize),
+            }
         } else if #[cfg(all(target_os = "macos", target_arch = "x86_64"))] {
             let cx = &*(cx as *const libc::ucontext_t);
-            (
-                (*cx.uc_mcontext).__ss.__rip as *const u8,
-                (*cx.uc_mcontext).__ss.__rbp as usize,
-            )
+            TrapRegisters {
+                pc: (*cx.uc_mcontext).__ss.__rip as usize,
+                fp: (*cx.uc_mcontext).__ss.__rbp as usize,
+            }
         } else if #[cfg(all(target_os = "macos", target_arch = "aarch64"))] {
             let cx = &*(cx as *const libc::ucontext_t);
-            (
-                (*cx.uc_mcontext).__ss.__pc as *const u8,
-                (*cx.uc_mcontext).__ss.__fp as usize,
-            )
+            TrapRegisters {
+                pc: (*cx.uc_mcontext).__ss.__pc as usize,
+                fp: (*cx.uc_mcontext).__ss.__fp as usize,
+            }
         } else if #[cfg(all(target_os = "freebsd", target_arch = "x86_64"))] {
             let cx = &*(cx as *const libc::ucontext_t);
-            (
-                cx.uc_mcontext.mc_rip as *const u8,
-                cx.uc_mcontext.mc_rbp as usize,
-            )
+            TrapRegisters {
+                pc: cx.uc_mcontext.mc_rip as usize,
+                fp: cx.uc_mcontext.mc_rbp as usize,
+            }
         } else if #[cfg(all(target_os = "linux", target_arch = "riscv64"))] {
             let cx = &*(cx as *const libc::ucontext_t);
-            (
-                cx.uc_mcontext.__gregs[libc::REG_PC] as *const u8,
-                cx.uc_mcontext.__gregs[libc::REG_S0] as usize,
-            )
+            TrapRegisters {
+                pc: cx.uc_mcontext.__gregs[libc::REG_PC] as usize,
+                fp: cx.uc_mcontext.__gregs[libc::REG_S0] as usize,
+            }
         } else if #[cfg(all(target_os = "freebsd", target_arch = "aarch64"))] {
             let cx = &*(cx as *const libc::mcontext_t);
-            (
-                cx.mc_gpregs.gp_elr as *const u8,
-                cx.mc_gpregs.gp_x[29] as usize,
-            )
+            TrapRegisters {
+                pc: cx.mc_gpregs.gp_elr as usize,
+                fp: cx.mc_gpregs.gp_x[29] as usize,
+            }
         } else if #[cfg(all(target_os = "openbsd", target_arch = "x86_64"))] {
             let cx = &*(cx as *const libc::ucontext_t);
-            (
-                cx.sc_rip as *const u8,
-                cx.sc_rbp as usize,
-            )
+            TrapRegisters {
+                pc: cx.sc_rip as usize,
+                fp: cx.sc_rbp as usize,
+            }
         }
         else {
             compile_error!("unsupported platform");

crates/wasmtime/src/runtime/vm/sys/windows/traphandlers.rs

@@ -1,4 +1,4 @@
-use crate::runtime::vm::traphandlers::{tls, TrapTest};
+use crate::runtime::vm::traphandlers::{tls, TrapRegisters, TrapTest};
 use crate::runtime::vm::VMContext;
 use std::ffi::c_void;
 use std::io;
@@ -96,13 +96,18 @@ unsafe extern "system" fn exception_handler(exception_info: *mut EXCEPTION_POINT
             Some(info) => info,
             None => return ExceptionContinueSearch,
         };
+        let context = &*(*exception_info).ContextRecord;
         cfg_if::cfg_if! {
             if #[cfg(target_arch = "x86_64")] {
-                let ip = (*(*exception_info).ContextRecord).Rip as *const u8;
-                let fp = (*(*exception_info).ContextRecord).Rbp as usize;
+                let regs = TrapRegisters {
+                    pc: context.Rip as usize,
+                    fp: context.Rbp as usize,
+                };
             } else if #[cfg(target_arch = "aarch64")] {
-                let ip = (*(*exception_info).ContextRecord).Pc as *const u8;
-                let fp = (*(*exception_info).ContextRecord).Anonymous.Anonymous.Fp as usize;
+                let regs = TrapRegisters {
+                    pc: context.Pc as usize,
+                    fp: context.Anonymous.Anonymous.Fp as usize,
+                };
             } else {
                 compile_error!("unsupported platform");
             }
@@ -117,13 +122,10 @@ unsafe extern "system" fn exception_handler(exception_info: *mut EXCEPTION_POINT
         } else {
             None
         };
-        match info.test_if_trap(ip, |handler| handler(exception_info)) {
+        match info.test_if_trap(regs, faulting_addr, |handler| handler(exception_info)) {
             TrapTest::NotWasm => ExceptionContinueSearch,
             TrapTest::HandledByEmbedder => ExceptionContinueExecution,
-            TrapTest::Trap { jmp_buf, trap } => {
-                info.set_jit_trap(ip, fp, faulting_addr, trap);
-                wasmtime_longjmp(jmp_buf)
-            }
+            TrapTest::Trap { jmp_buf } => wasmtime_longjmp(jmp_buf),
         }
     })
 }

crates/wasmtime/src/runtime/vm/traphandlers.rs

@@ -219,6 +219,11 @@ impl From<wasmtime_environ::Trap> for TrapReason {
     }
 }
 
+pub(crate) struct TrapRegisters {
+    pub pc: usize,
+    pub fp: usize,
+}
+
 /// Return value from `test_if_trap`.
 pub(crate) enum TrapTest {
     /// Not a wasm trap, need to delegate to whatever process handler is next.
@@ -230,8 +235,6 @@ pub(crate) enum TrapTest {
     Trap {
         /// How to longjmp back to the original wasm frame.
         jmp_buf: *const u8,
-        /// The trap code of this trap.
-        trap: wasmtime_environ::Trap,
     },
 }
 
@@ -455,7 +458,8 @@ impl CallThreadState {
     #[cfg_attr(miri, allow(dead_code))] // miri doesn't handle traps yet
     pub(crate) fn test_if_trap(
         &self,
-        pc: *const u8,
+        regs: TrapRegisters,
+        faulting_addr: Option<usize>,
         call_handler: impl Fn(&SignalHandler) -> bool,
     ) -> TrapTest {
         // If we haven't even started to handle traps yet, bail out.
@@ -473,19 +477,20 @@ impl CallThreadState {
         }
 
         // If this fault wasn't in wasm code, then it's not our problem
-        let Some((code, text_offset)) = lookup_code(pc as usize) else {
+        let Some((code, text_offset)) = lookup_code(regs.pc) else {
             return TrapTest::NotWasm;
         };
 
         let Some(trap) = code.lookup_trap_code(text_offset) else {
             return TrapTest::NotWasm;
         };
 
+        self.set_jit_trap(regs, faulting_addr, trap);
+
         // If all that passed then this is indeed a wasm trap, so return the
         // `jmp_buf` passed to `wasmtime_longjmp` to resume.
         TrapTest::Trap {
             jmp_buf: self.take_jmp_buf(),
-            trap,
         }
     }
 
@@ -496,17 +501,16 @@ impl CallThreadState {
     #[cfg_attr(miri, allow(dead_code))] // miri doesn't handle traps yet
     pub(crate) fn set_jit_trap(
         &self,
-        pc: *const u8,
-        fp: usize,
+        TrapRegisters { pc, fp, .. }: TrapRegisters,
         faulting_addr: Option<usize>,
         trap: wasmtime_environ::Trap,
     ) {
-        let backtrace = self.capture_backtrace(self.limits, Some((pc as usize, fp)));
-        let coredump = self.capture_coredump(self.limits, Some((pc as usize, fp)));
+        let backtrace = self.capture_backtrace(self.limits, Some((pc, fp)));
+        let coredump = self.capture_coredump(self.limits, Some((pc, fp)));
         unsafe {
             (*self.unwind.get()).as_mut_ptr().write((
                 UnwindReason::Trap(TrapReason::Jit {
-                    pc: pc as usize,
+                    pc,
                     faulting_addr,
                     trap,
                 }),