Stream: git-wasmtime

Topic: wasmtime / issue #3061 UD2 is executed and Valgrind emits...

Wasmtime GitHub notifications bot (Jul 04 2021 at 16:27):

Encrylize edited issue #3061:

Test Case

trap.c:

#include <wasm.h>
#include <wasmtime.h>

int main(void) {
    wasm_engine_t* engine = wasm_engine_new();
    wasm_store_t* store = wasm_store_new(engine);

    wasm_byte_vec_t binary;
    const char* wat = "(module (func (export \"trap\") (unreachable)))";
    wasmtime_wat2wasm(wat, strlen(wat), &binary);
    wasm_module_t* module = wasm_module_new(store, &binary);

    wasm_byte_vec_delete(&binary);

    wasm_extern_vec_t imports = WASM_EMPTY_VEC;
    wasm_instance_t* instance = wasm_instance_new(store, module, &imports, NULL);

    wasm_extern_vec_t exports;
    wasm_instance_exports(instance, &exports);

    const wasm_func_t* func = wasm_extern_as_func(exports.data[0]);

    const wasm_val_vec_t args = WASM_EMPTY_VEC;
    wasm_val_vec_t results = WASM_EMPTY_VEC;
    wasm_trap_t* trap = wasm_func_call(func, &args, &results);
    wasm_trap_delete(trap);

    wasm_extern_vec_delete(&exports);
    wasm_module_delete(module);
    wasm_instance_delete(instance);
    wasm_store_delete(store);
    wasm_engine_delete(engine);

    return 0;
}

Steps to Reproduce

• Compile program: gcc trap.c -lwasmtime -o trap
• Execute program using LLDB: lldb ./trap
• Execute program using Valgrind: valgrind ./trap

Expected Results

The UD2 instruction should not be executed and no warnings should be emitted by Valgrind.

Actual Results

The UD2 instruction is executed as shown by LLDB:

$ lldb ./trap
(lldb) target create "./trap"
Current executable set to '/home/jfb/projects/wasm/trap' (x86_64).
(lldb) r
Process 301185 launched: '/home/jfb/projects/wasm/trap' (x86_64)
Process 301185 stopped
* thread #1, name = 'trap', stop reason = signal SIGILL: illegal instruction operand
    frame #0: 0x00007ffff7fb7004
->  0x7ffff7fb7004: ud2
    0x7ffff7fb7006: pushq  %rbp
    0x7ffff7fb7007: movq   %rsp, %rbp
    0x7ffff7fb700a: callq  *%rdx
(lldb) c
Process 301185 resuming
Process 301185 exited with status = 0 (0x00000000)
(lldb) exit

Valgrind emits a warning about the client switching stacks:

$ valgrind ./trap
==302700== Memcheck, a memory error detector
==302700== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==302700== Using Valgrind-3.17.0 and LibVEX; rerun with -h for copyright info
==302700== Command: ./trap
==302700==
==302700== valgrind: Unrecognised instruction at address 0x4850004.
==302700==    at 0x4850004: ???
==302700==    by 0x485000B: ???
==302700==    by 0x4E2548F: wasmtime_setjmp (in /usr/lib/libwasmtime.so)
==302700==    by 0x4980838: wasmtime_runtime::traphandlers::catch_traps (in /usr/lib/libwasmtime.so)
==302700==    by 0x496DE16: wasmtime::func::Func::call (in /usr/lib/libwasmtime.so)
==302700==    by 0x4A22DE3: wasm_func_call (in /usr/lib/libwasmtime.so)
==302700==    by 0x10935D: main (in /home/jfb/projects/wasm/trap)
==302700== Your program just tried to execute an instruction that Valgrind
==302700== did not recognise.  There are two possible reasons for this.
==302700== 1. Your program has a bug and erroneously jumped to a non-code
==302700==    location.  If you are running Memcheck and you just saw a
==302700==    warning about a bad jump, it's probably your program's fault.
==302700== 2. The instruction is legitimate but Valgrind doesn't handle it,
==302700==    i.e. it's Valgrind's fault.  If you think this is the case or
==302700==    you are not sure, please let us know and we'll try to fix it.
==302700== Either way, Valgrind will now raise a SIGILL signal which will
==302700== probably kill your program.
==302700== Warning: client switching stacks?  SP change: 0x5aaa068 --> 0x1ffefffd00
==302700==          to suppress, use: --max-stackframe=137327107224 or greater
==302700==
==302700== HEAP SUMMARY:
==302700==     in use at exit: 920 bytes in 7 blocks
==302700==   total heap usage: 659 allocs, 652 frees, 152,738 bytes allocated
==302700==
==302700== LEAK SUMMARY:
==302700==    definitely lost: 0 bytes in 0 blocks
==302700==    indirectly lost: 0 bytes in 0 blocks
==302700==      possibly lost: 0 bytes in 0 blocks
==302700==    still reachable: 920 bytes in 7 blocks
==302700==         suppressed: 0 bytes in 0 blocks
==302700== Rerun with --leak-check=full to see details of leaked memory
==302700==
==302700== For lists of detected and suppressed errors, rerun with: -s
==302700== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

Versions and Environment

Wasmtime version or commit: Commit c71ad9490

Operating system: Arch Linux

Architecture: x86_64

Wasmtime GitHub notifications bot (Jul 04 2021 at 21:19):

cfallin commented on issue #3061:

Hi @Encrylize,

The undefined instruction (ud2 on x86-64) is how wasmtime transfers control to its trap handlers; executing this instruction is the intended behavior when a trap occurs.

Normally, wasmtime will install a signal handler for SIGILL (or the equivalent mechanism with Mach ports on macOS or Win32 APIs on Windows), catch the signal, and longjmp back to the top-level entry point, at which point it can return the trap error-code to the caller.

However, if you are running under a debugger or under Valgrind, it's possible that this tool will catch the signal before wasmtime does. There is nothing that we can really do about this, as far as I know: the tools have complete control over inspecting the process (gdb/lldb via the ptrace() syscall, Valgrind because it actually emulates execution with a JIT layer), and have full visibility of signals.

I know that at least Valgrind has the concept of "suppressions", and gdb allows setting the default action for a signal (such as SIGILL) to "ignore"; perhaps one of these mechanisms would help in your case?

In any case, using signals to catch traps is standard practice for JIT engines and other runtimes, and is a fully correct and supported use of the signal-handler mechanism, so this is not a bug that we can resolve, I think. Thank you for filing the report regardless!

Wasmtime GitHub notifications bot (Jul 04 2021 at 21:20):

cfallin closed issue #3061:

Test Case

trap.c:

#include <wasm.h>
#include <wasmtime.h>

int main(void) {
    wasm_engine_t* engine = wasm_engine_new();
    wasm_store_t* store = wasm_store_new(engine);

    wasm_byte_vec_t binary;
    const char* wat = "(module (func (export \"trap\") (unreachable)))";
    wasmtime_wat2wasm(wat, strlen(wat), &binary);
    wasm_module_t* module = wasm_module_new(store, &binary);

    wasm_byte_vec_delete(&binary);

    wasm_extern_vec_t imports = WASM_EMPTY_VEC;
    wasm_instance_t* instance = wasm_instance_new(store, module, &imports, NULL);

    wasm_extern_vec_t exports;
    wasm_instance_exports(instance, &exports);

    const wasm_func_t* func = wasm_extern_as_func(exports.data[0]);

    const wasm_val_vec_t args = WASM_EMPTY_VEC;
    wasm_val_vec_t results = WASM_EMPTY_VEC;
    wasm_trap_t* trap = wasm_func_call(func, &args, &results);
    wasm_trap_delete(trap);

    wasm_extern_vec_delete(&exports);
    wasm_module_delete(module);
    wasm_instance_delete(instance);
    wasm_store_delete(store);
    wasm_engine_delete(engine);

    return 0;
}

Steps to Reproduce

• Compile program: gcc trap.c -lwasmtime -o trap
• Execute program using LLDB: lldb ./trap
• Execute program using Valgrind: valgrind ./trap

Expected Results

The UD2 instruction should not be executed and no warnings should be emitted by Valgrind.

Actual Results

The UD2 instruction is executed as shown by LLDB:

$ lldb ./trap
(lldb) target create "./trap"
Current executable set to '/home/jfb/projects/wasm/trap' (x86_64).
(lldb) r
Process 301185 launched: '/home/jfb/projects/wasm/trap' (x86_64)
Process 301185 stopped
* thread #1, name = 'trap', stop reason = signal SIGILL: illegal instruction operand
    frame #0: 0x00007ffff7fb7004
->  0x7ffff7fb7004: ud2
    0x7ffff7fb7006: pushq  %rbp
    0x7ffff7fb7007: movq   %rsp, %rbp
    0x7ffff7fb700a: callq  *%rdx
(lldb) c
Process 301185 resuming
Process 301185 exited with status = 0 (0x00000000)
(lldb) exit

Valgrind emits a warning about the client switching stacks:

$ valgrind ./trap
==302700== Memcheck, a memory error detector
==302700== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==302700== Using Valgrind-3.17.0 and LibVEX; rerun with -h for copyright info
==302700== Command: ./trap
==302700==
==302700== valgrind: Unrecognised instruction at address 0x4850004.
==302700==    at 0x4850004: ???
==302700==    by 0x485000B: ???
==302700==    by 0x4E2548F: wasmtime_setjmp (in /usr/lib/libwasmtime.so)
==302700==    by 0x4980838: wasmtime_runtime::traphandlers::catch_traps (in /usr/lib/libwasmtime.so)
==302700==    by 0x496DE16: wasmtime::func::Func::call (in /usr/lib/libwasmtime.so)
==302700==    by 0x4A22DE3: wasm_func_call (in /usr/lib/libwasmtime.so)
==302700==    by 0x10935D: main (in /home/jfb/projects/wasm/trap)
==302700== Your program just tried to execute an instruction that Valgrind
==302700== did not recognise.  There are two possible reasons for this.
==302700== 1. Your program has a bug and erroneously jumped to a non-code
==302700==    location.  If you are running Memcheck and you just saw a
==302700==    warning about a bad jump, it's probably your program's fault.
==302700== 2. The instruction is legitimate but Valgrind doesn't handle it,
==302700==    i.e. it's Valgrind's fault.  If you think this is the case or
==302700==    you are not sure, please let us know and we'll try to fix it.
==302700== Either way, Valgrind will now raise a SIGILL signal which will
==302700== probably kill your program.
==302700== Warning: client switching stacks?  SP change: 0x5aaa068 --> 0x1ffefffd00
==302700==          to suppress, use: --max-stackframe=137327107224 or greater
==302700==
==302700== HEAP SUMMARY:
==302700==     in use at exit: 920 bytes in 7 blocks
==302700==   total heap usage: 659 allocs, 652 frees, 152,738 bytes allocated
==302700==
==302700== LEAK SUMMARY:
==302700==    definitely lost: 0 bytes in 0 blocks
==302700==    indirectly lost: 0 bytes in 0 blocks
==302700==      possibly lost: 0 bytes in 0 blocks
==302700==    still reachable: 920 bytes in 7 blocks
==302700==         suppressed: 0 bytes in 0 blocks
==302700== Rerun with --leak-check=full to see details of leaked memory
==302700==
==302700== For lists of detected and suppressed errors, rerun with: -s
==302700== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

Versions and Environment

Wasmtime version or commit: Commit c71ad9490

Operating system: Arch Linux

Architecture: x86_64

Last updated: Dec 23 2024 at 13:07 UTC