wasmtime / Issue #1896 Problem with debugging wasm module... · git-wasmtime

Stream: git-wasmtime

Topic: wasmtime / Issue #1896 Problem with debugging wasm module...

Wasmtime GitHub notifications bot (Jun 18 2020 at 05:05):

lei-april opened Issue #1896:

Hi,

I'm trying to use wasmtime's debug support. I followed the simple demo from https://github.com/bytecodealliance/wasmtime/issues/1613, and it works as expected. But when I try to debug a slightly more complex C++ program, there's some problem.

Here's the C++ snippet:
#include <string>

std::string hello(int x) {
    const char* s1 = "Hello";
    std::string s2 = "World";
    return s1 + s2 + std::to_string(x);
}

int main() {
    auto s = hello(12345);
    return s.length();
}
I compiled it with a freshly built clang (commit), coupled with prebuilt wasi-sysroot from wasi-sdk:
$ clang++ --target=wasm32-wasi --sysroot ./wasi-sysroot -fno-exceptions -g t.cc -o t.wasm
Then I use lldb (8.0.1) and a freshly built wasmtime (commit) to debug it:
$ lldb -- ./wasmtime/target/release/wasmtime -g t.wasm
Setting breakpoint and stepping thru statements all work fine. But somehow none of the local variables are printed as expected:
Process 26379 stopped
* thread #1, name = 'wasmtime', stop reason = step over
    frame #0: 0x00007ffff7fcf5af JIT(0x5555562ec490)`hello(x=0) at t.cc:6:12
   3    std::string hello(int x) {
   4        const char* s1 = "Hello";
   5        std::string s2 = "World";
-> 6        return s1 + s2 + std::to_string(x);
   7    }
   8
   9    int main() {
(lldb) p x
(int) $0 = 0
(lldb) p s1
(WebAssemblyPtrWrapper<const char>) $1 = (__ptr = 0)
(lldb) p s2
(std::__2::string) $2 = {}
Any hints?

Wasmtime GitHub notifications bot (Jun 18 2020 at 06:07):

lei-april edited Issue #1896:

Hi,

I'm trying to use wasmtime's debug support. I followed the simple demo from https://github.com/bytecodealliance/wasmtime/issues/1613, and it works as expected. But when I try to debug a slightly more complex C++ program, there's some problem.

Here's the C++ snippet:
#include <string>

std::string hello(int x) {
    const char* s1 = "Hello";
    std::string s2 = "World";
    return s1 + s2 + std::to_string(x);
}

int main() {
    auto s = hello(12345);
    return s.length();
}
I compiled it with a freshly built clang (commit), coupled with prebuilt wasi-sysroot from wasi-sdk:
$ clang++ --target=wasm32-wasi --sysroot ./wasi-sysroot -fno-exceptions -g t.cc -o t.wasm
Then I use lldb (8.0.1) and a freshly built wasmtime (commit) to debug it:
$ lldb -- ./wasmtime/target/release/wasmtime -g t.wasm
Setting breakpoint and stepping thru statements all work fine. But somehow none of the local variables are printed as expected:
Process 26379 stopped
* thread #1, name = 'wasmtime', stop reason = step over
    frame #0: 0x00007ffff7fcf5af JIT(0x5555562ec490)`hello(x=0) at t.cc:6:12
   3    std::string hello(int x) {
   4        const char* s1 = "Hello";
   5        std::string s2 = "World";
-> 6        return s1 + s2 + std::to_string(x);
   7    }
   8
   9    int main() {
(lldb) p x
(int) $0 = 0
(lldb) p s1
(WebAssemblyPtrWrapper<const char>) $1 = (__ptr = 0)
(lldb) p s2
(std::__2::string) $2 = {}
Any hints?

P.S. I'm doing all the experiments on Linux.

Wasmtime GitHub notifications bot (Jun 18 2020 at 23:04):

yurydelendik commented on Issue #1896:

hen I use lldb (8.0.1) and a freshly built wasmtime (commit) to debug it.

At this moment recommended debuggers lldb-10 and gdb for various reasons/issues.

(WebAssemblyPtrWrapper<const char>) $1 = (__ptr = 0)

The wasm pointer is not really a pointer in computer memory that can be accessed. You may check https://github.com/bytecodealliance/wasmtime/pull/1482 on how to use helpers to unwrap wasm pointers in a debugger.

(WebAssemblyPtrWrapper<const char>) $1 = (__ptr = 0)

Not sure why s1 points to the memory offset 0. Did you try debugging non-optimized -O0 wasm?

Wasmtime GitHub notifications bot (Jun 18 2020 at 23:05):

yurydelendik edited a comment on Issue #1896:

hen I use lldb (8.0.1) and a freshly built wasmtime (commit) to debug it.

At this moment recommended debuggers lldb-10 and gdb for various reasons/issues.

(WebAssemblyPtrWrapper<const char>) $1 = (__ptr = 0)

The wasm pointer is not really a pointer in computer memory that can be accessed. You may check https://github.com/bytecodealliance/wasmtime/pull/1482 on how to use helpers to unwrap wasm pointers in a debugger.

Not sure why s1 points to the memory offset 0. Did you try debugging non-optimized -O0 wasm?

Wasmtime GitHub notifications bot (Jun 18 2020 at 23:21):

yurydelendik commented on Issue #1896:

Okay, I just check that with or without optimization -- the issue is the same.

Wasmtime GitHub notifications bot (Jun 18 2020 at 23:21):

yurydelendik assigned Issue #1896:

Hi,

I'm trying to use wasmtime's debug support. I followed the simple demo from https://github.com/bytecodealliance/wasmtime/issues/1613, and it works as expected. But when I try to debug a slightly more complex C++ program, there's some problem.

Here's the C++ snippet:
#include <string>

std::string hello(int x) {
    const char* s1 = "Hello";
    std::string s2 = "World";
    return s1 + s2 + std::to_string(x);
}

int main() {
    auto s = hello(12345);
    return s.length();
}
I compiled it with a freshly built clang (commit), coupled with prebuilt wasi-sysroot from wasi-sdk:
$ clang++ --target=wasm32-wasi --sysroot ./wasi-sysroot -fno-exceptions -g t.cc -o t.wasm
Then I use lldb (8.0.1) and a freshly built wasmtime (commit) to debug it:
$ lldb -- ./wasmtime/target/release/wasmtime -g t.wasm
Setting breakpoint and stepping thru statements all work fine. But somehow none of the local variables are printed as expected:
Process 26379 stopped
* thread #1, name = 'wasmtime', stop reason = step over
    frame #0: 0x00007ffff7fcf5af JIT(0x5555562ec490)`hello(x=0) at t.cc:6:12
   3    std::string hello(int x) {
   4        const char* s1 = "Hello";
   5        std::string s2 = "World";
-> 6        return s1 + s2 + std::to_string(x);
   7    }
   8
   9    int main() {
(lldb) p x
(int) $0 = 0
(lldb) p s1
(WebAssemblyPtrWrapper<const char>) $1 = (__ptr = 0)
(lldb) p s2
(std::__2::string) $2 = {}
Any hints?

P.S. I'm doing all the experiments on Linux.

Wasmtime GitHub notifications bot (Jun 19 2020 at 02:15):

lei-april commented on Issue #1896:

Thanks for you reply :) @yurydelendik

At this moment recommended debuggers lldb-10 and gdb for various reasons/issues.

I retried the previous experiment with lldb 11.0.0 (freshly built) and gdb 8.2, respectively. Here are the results:

lldb
Process 101273 stopped
* thread #1, name = 'wasmtime', stop reason = step over
    frame #0: 0x00007ffff7fcf5af JIT(0x5555562ef450)`hello(x=0) at t.cc:6:12
   3    std::string hello(int x) {
   4        const char* s1 = "Hello";
   5        std::string s2 = "World";
-> 6        return s1 + s2 + std::to_string(x);
   7    }
   8
   9    int main() {
(lldb) p x
(int) $0 = 0
(lldb) p s1
(WebAssemblyPtrWrapper<const char>) $1 = (__ptr = 0)
(lldb) p s2
(std::string) $2 = Summary Unavailable
gdb
Thread 1 "wasmtime" hit Breakpoint 1, hello (x=0) at t.cc:4
4       const char* s1 = "Hello";
(gdb) n
5       std::string s2 = "World";
(gdb) n
6       return s1 + s2 + std::to_string(x);
(gdb) p x
$1 = 0
(gdb) p s1
$2 = {__ptr = 0}
(gdb) p s2
$3 = <incomplete type>
It seems switching debugger version doesn't help much.

The wasm pointer is not really a pointer in computer memory that can be accessed. You may check #1482 on how to use helpers to unwrap wasm pointers in a debugger.

Cool. I'll give it a try.

Wasmtime GitHub notifications bot (Jun 23 2020 at 16:04):

yurydelendik commented on Issue #1896:

I looked at the issue. There are at least two problems:

[ ] TODO related to reference types (specific to C++) at https://github.com/bytecodealliance/wasmtime/blob/master/crates/debug/src/transform/unit.rs#L398

[ ] There is an issue with CL value tracking, which is a cause for wrong location in DWARF

@lei-april you can also turn off option at CL via --opt-level 0

I will try to address the above issues soon.

Wasmtime GitHub notifications bot (Jun 24 2020 at 04:02):

lei-april commented on Issue #1896:

you can also turn off option at CL via --opt-level 0

Thanks. This does help :)

The result looks better now (with lldb 11.0.0):

Process 76848 stopped
* thread #1, name = 'wasmtime', stop reason = step over
    frame #0: 0x00007ffff7fc369e JIT(0x55555639a690)`hello(x=12345) at t.cc:6:12
   3    std::string hello(int x) {
   4        const char* s1 = "Hello";
   5        std::string s2 = "World";
-> 6        return s1 + s2 + std::to_string(x);
   7    }
   8
   9    int main() {
(lldb) p x
(int) $0 = 12345
(lldb) p s1
(WebAssemblyPtrWrapper<const char>) $1 = (__ptr = 1024)
(lldb) p s2
(std::string) $2 = Summary Unavailable

Wasmtime GitHub notifications bot (Jun 24 2020 at 13:37):

yurydelendik commented on Issue #1896:

(std::string) $2 = Summary Unavailable

C++ types with reference types in them will likely fail due to TODO mentioned above.

Wasmtime GitHub notifications bot (Jun 25 2020 at 13:01):

yurydelendik labeled Issue #1896 (assigned to yurydelendik):

Hi,

I'm trying to use wasmtime's debug support. I followed the simple demo from https://github.com/bytecodealliance/wasmtime/issues/1613, and it works as expected. But when I try to debug a slightly more complex C++ program, there's some problem.

Here's the C++ snippet:
#include <string>

std::string hello(int x) {
    const char* s1 = "Hello";
    std::string s2 = "World";
    return s1 + s2 + std::to_string(x);
}

int main() {
    auto s = hello(12345);
    return s.length();
}
I compiled it with a freshly built clang (commit), coupled with prebuilt wasi-sysroot from wasi-sdk:
$ clang++ --target=wasm32-wasi --sysroot ./wasi-sysroot -fno-exceptions -g t.cc -o t.wasm
Then I use lldb (8.0.1) and a freshly built wasmtime (commit) to debug it:
$ lldb -- ./wasmtime/target/release/wasmtime -g t.wasm
Setting breakpoint and stepping thru statements all work fine. But somehow none of the local variables are printed as expected:
Process 26379 stopped
* thread #1, name = 'wasmtime', stop reason = step over
    frame #0: 0x00007ffff7fcf5af JIT(0x5555562ec490)`hello(x=0) at t.cc:6:12
   3    std::string hello(int x) {
   4        const char* s1 = "Hello";
   5        std::string s2 = "World";
-> 6        return s1 + s2 + std::to_string(x);
   7    }
   8
   9    int main() {
(lldb) p x
(int) $0 = 0
(lldb) p s1
(WebAssemblyPtrWrapper<const char>) $1 = (__ptr = 0)
(lldb) p s2
(std::__2::string) $2 = {}
Any hints?

P.S. I'm doing all the experiments on Linux.

Last updated: Dec 23 2024 at 12:05 UTC