Stream: git-wasmtime

Topic: wasmtime / issue #10680 LLDB cannot set breakpoint on exp...

Wasmtime GitHub notifications bot (Apr 27 2025 at 14:00):

sammyne opened issue #10680:

Environment

• wasmtime 32.0.0
• wasi-sdk 22.0

Source codes

fib-cc/lib.cc: C++ source codes

#include <cstdint>

__attribute__((__export_name__("fib")))
extern "C" uint32_t fib(uint32_t n) {
  uint32_t a = 1;
  uint32_t b = 1;

  for(int i=0; i<n; ++i) {
    auto t = a;
    a = b;
    b += t;
  }

  return b;
}

debugger/src/main.rs: embedded wasmtime

use wasmtime::*;

fn main() -> Result<()> {
    let path = match std::env::args().skip(1).next() {
        Some(v) => v,
        None => "target/wasm32-unknown-unknown/debug/fib.wasm".to_owned(),
    };

    // Load our previously compiled wasm file (built previously with Cargo) and
    // also ensure that we generate debuginfo so this executable can be
    // debugged in GDB.
    let engine = Engine::new(
        Config::new()
            .debug_info(true)
            .cranelift_opt_level(OptLevel::None),
    )?;
    let mut store = Store::new(&engine, ());

    //const PATH: &str = "target/wasm32-wasip1/debug/fib.wasm";
    //const PATH: &str = "target/wasm32-unknown-unknown/debug/fib.wasm";

    let module = Module::from_file(&engine, path)?;
    let instance = Instance::new(&mut store, &module, &[])?;

    // Invoke `fib` export
    let fib = instance.get_typed_func::<i32, i32>(&mut store, "fib")?;
    println!("fib(6) = {}", fib.call(&mut store, 6)?);
    Ok(())
}

Makefile

debugger = target/release/debugger
fibcc = fib-cc/build/fib-cc.wasm

.PHONY: all
all: $(debugger) $(fibcc)

$(debugger): debugger/src/main.rs
    cargo build -r -p debugger

$(fibcc): fib-cc/lib.cc
    $(MAKE) -C fib-cc

lldb-cc: $(debugger) $(fibcc)
    lldb -O 'settings set target.disable-aslr false' \
        -o 'breakpoint set -n main' \
        -o 'r' \
        -o "breakpoint set -n 'fib'" \
        -o 'c' \
        -o 'n' \
        -o 'p a' \
        -- $(debugger) $(fibcc)

.PHONY: clean
clean:
    cargo clean
    $(MAKE) -C fib-cc clean

Using export name as 'fib'

Running make lldb-cc with __attribute__((__export_name__("fib"))) in fib-cc/lib.cc gives me expected output as

root@eafb8747aaf8:/workspace/module# make lldb-cc
cargo build -r -p debugger
   Compiling debugger v0.1.0 (/workspace/module/debugger)
    Finished `release` profile [optimized] target(s) in 1.36s
lldb -O 'settings set target.disable-aslr false' \
        -o 'breakpoint set -n main' \
        -o 'r' \
        -o "breakpoint set -n 'fib'" \
        -o 'c' \
        -o 'n' \
        -o 'p a' \
        -- target/release/debugger fib-cc/build/fib-cc.wasm
(lldb) settings set target.disable-aslr false
(lldb) target create "target/release/debugger"
Current executable set to '/workspace/module/target/release/debugger' (x86_64).
(lldb) settings set -- target.run-args  "fib-cc/build/fib-cc.wasm"
(lldb) breakpoint set -n main
Breakpoint 1: where = debugger`main, address = 0x00000000000baac0
(lldb) r
Process 93176 launched: '/workspace/module/target/release/debugger' (x86_64)
Process 93176 stopped
* thread #1, name = 'debugger', stop reason = breakpoint 1.1
    frame #0: 0x0000559dedb3aac0 debugger`main
debugger`main:
->  0x559dedb3aac0 <+0>: pushq  %rax
    0x559dedb3aac1 <+1>: movq   %rsi, %rcx
    0x559dedb3aac4 <+4>: movslq %edi, %rdx
    0x559dedb3aac7 <+7>: leaq   -0x59e(%rip), %rax ; debugger::main::h4a2646090ba600e9
(lldb) breakpoint set -n 'fib'
Breakpoint 2: no locations (pending).
WARNING:  Unable to resolve breakpoint to any actual locations.
(lldb) c
1 location added to breakpoint 2
Process 93176 resuming
Process 93176 stopped
* thread #1, name = 'debugger', stop reason = breakpoint 2.1
    frame #0: 0x00007ff04c07b013 JIT(0x559def806040)`fib(n=6) at lib.cc:6:12
   3    // export_name 之后才能找到调试符号。
   4    __attribute__((__export_name__("fib")))
   5    extern "C" uint32_t fib(uint32_t n) {
-> 6      uint32_t a = 1;
   7      uint32_t b = 1;
   8
   9      for(int i=0; i<n; ++i) {
(lldb) n
Process 93176 stopped
* thread #1, name = 'debugger', stop reason = step over
    frame #0: 0x00007ff04c07b020 JIT(0x559def806040)`fib(n=6) at lib.cc:7:12
   4    __attribute__((__export_name__("fib")))
   5    extern "C" uint32_t fib(uint32_t n) {
   6      uint32_t a = 1;
-> 7      uint32_t b = 1;
   8
   9      for(int i=0; i<n; ++i) {
   10       auto t = a;
(lldb) p a
(uint32_t) 1

Problem: using export name as 'fib-cc' gives unexpected output

Running make lldb-cc with

• __attribute__((__export_name__("fib-cc"))) in fib-cc/lib.cc
• renaming target func as 'fib-cc' in debugger/src/main.rs
  gives me unexpected output as

root@eafb8747aaf8:/workspace/module# make lldb-cc
lldb -O 'settings set target.disable-aslr false' \
        -o 'breakpoint set -n main' \
        -o 'r' \
        -o "breakpoint set -n 'fib-cc'" \
        -o 'c' \
        -o 'n' \
        -o 'p a' \
        -- target/release/debugger fib-cc/build/fib-cc.wasm
(lldb) settings set target.disable-aslr false
(lldb) target create "target/release/debugger"
Current executable set to '/workspace/module/target/release/debugger' (x86_64).
(lldb) settings set -- target.run-args  "fib-cc/build/fib-cc.wasm"
(lldb) breakpoint set -n main
Breakpoint 1: where = debugger`main, address = 0x00000000000baac0
(lldb) r
Process 103930 launched: '/workspace/module/target/release/debugger' (x86_64)
Process 103930 stopped
* thread #1, name = 'debugger', stop reason = breakpoint 1.1
    frame #0: 0x000055557f202ac0 debugger`main
debugger`main:
->  0x55557f202ac0 <+0>: pushq  %rax
    0x55557f202ac1 <+1>: movq   %rsi, %rcx
    0x55557f202ac4 <+4>: movslq %edi, %rdx
    0x55557f202ac7 <+7>: leaq   -0x59e(%rip), %rax ; debugger::main::h4a2646090ba600e9
(lldb) breakpoint set -n 'fib-cc'
Breakpoint 2: no locations (pending).
WARNING:  Unable to resolve breakpoint to any actual locations.
(lldb) c
fib(6) = 21
Process 103930 resuming
Process 103930 exited with status = -1 (0xffffffff) lost connection
(lldb) n
error: Command requires a process which is currently stopped.

Why LLDB cannot find symbol exported name 'fib-cc'?

The full project is hosted in https://github.com/sammyne/deep-dive-wasm-debug/tree/main/module.

Wasmtime GitHub notifications bot (Apr 27 2025 at 14:04):

sammyne edited issue #10680:

Environment

• wasmtime 32.0.0
• wasi-sdk 22.0

Source codes

fib-cc/lib.cc: C++ source codes

#include <cstdint>

__attribute__((__export_name__("fib")))
extern "C" uint32_t fib(uint32_t n) {
  uint32_t a = 1;
  uint32_t b = 1;

  for(int i=0; i<n; ++i) {
    auto t = a;
    a = b;
    b += t;
  }

  return b;
}

debugger/src/main.rs: embedded wasmtime

use wasmtime::*;

fn main() -> Result<()> {
    let path = match std::env::args().skip(1).next() {
        Some(v) => v,
        None => "target/wasm32-unknown-unknown/debug/fib.wasm".to_owned(),
    };

    // Load our previously compiled wasm file (built previously with Cargo) and
    // also ensure that we generate debuginfo so this executable can be
    // debugged in GDB.
    let engine = Engine::new(
        Config::new()
            .debug_info(true)
            .cranelift_opt_level(OptLevel::None),
    )?;
    let mut store = Store::new(&engine, ());

    //const PATH: &str = "target/wasm32-wasip1/debug/fib.wasm";
    //const PATH: &str = "target/wasm32-unknown-unknown/debug/fib.wasm";

    let module = Module::from_file(&engine, path)?;
    let instance = Instance::new(&mut store, &module, &[])?;

    // Invoke `fib` export
    let fib = instance.get_typed_func::<i32, i32>(&mut store, "fib")?;
    println!("fib(6) = {}", fib.call(&mut store, 6)?);
    Ok(())
}

Makefile

debugger = target/release/debugger
fibcc = fib-cc/build/fib-cc.wasm

.PHONY: all
all: $(debugger) $(fibcc)

$(debugger): debugger/src/main.rs
    cargo build -r -p debugger

$(fibcc): fib-cc/lib.cc
    $(MAKE) -C fib-cc

lldb-cc: $(debugger) $(fibcc)
    lldb -O 'settings set target.disable-aslr false' \
        -o 'breakpoint set -n main' \
        -o 'r' \
        -o "breakpoint set -n 'fib'" \
        -o 'c' \
        -o 'n' \
        -o 'p a' \
        -- $(debugger) $(fibcc)

.PHONY: clean
clean:
    cargo clean
    $(MAKE) -C fib-cc clean

Using export name as 'fib'

Running make lldb-cc with __attribute__((__export_name__("fib"))) in fib-cc/lib.cc gives me expected output as

root@eafb8747aaf8:/workspace/module# make lldb-cc
cargo build -r -p debugger
   Compiling debugger v0.1.0 (/workspace/module/debugger)
    Finished `release` profile [optimized] target(s) in 1.36s
lldb -O 'settings set target.disable-aslr false' \
        -o 'breakpoint set -n main' \
        -o 'r' \
        -o "breakpoint set -n 'fib'" \
        -o 'c' \
        -o 'n' \
        -o 'p a' \
        -- target/release/debugger fib-cc/build/fib-cc.wasm
(lldb) settings set target.disable-aslr false
(lldb) target create "target/release/debugger"
Current executable set to '/workspace/module/target/release/debugger' (x86_64).
(lldb) settings set -- target.run-args  "fib-cc/build/fib-cc.wasm"
(lldb) breakpoint set -n main
Breakpoint 1: where = debugger`main, address = 0x00000000000baac0
(lldb) r
Process 93176 launched: '/workspace/module/target/release/debugger' (x86_64)
Process 93176 stopped
* thread #1, name = 'debugger', stop reason = breakpoint 1.1
    frame #0: 0x0000559dedb3aac0 debugger`main
debugger`main:
->  0x559dedb3aac0 <+0>: pushq  %rax
    0x559dedb3aac1 <+1>: movq   %rsi, %rcx
    0x559dedb3aac4 <+4>: movslq %edi, %rdx
    0x559dedb3aac7 <+7>: leaq   -0x59e(%rip), %rax ; debugger::main::h4a2646090ba600e9
(lldb) breakpoint set -n 'fib'
Breakpoint 2: no locations (pending).
WARNING:  Unable to resolve breakpoint to any actual locations.
(lldb) c
1 location added to breakpoint 2
Process 93176 resuming
Process 93176 stopped
* thread #1, name = 'debugger', stop reason = breakpoint 2.1
    frame #0: 0x00007ff04c07b013 JIT(0x559def806040)`fib(n=6) at lib.cc:6:12
   3    // export_name 之后才能找到调试符号。
   4    __attribute__((__export_name__("fib")))
   5    extern "C" uint32_t fib(uint32_t n) {
-> 6      uint32_t a = 1;
   7      uint32_t b = 1;
   8
   9      for(int i=0; i<n; ++i) {
(lldb) n
Process 93176 stopped
* thread #1, name = 'debugger', stop reason = step over
    frame #0: 0x00007ff04c07b020 JIT(0x559def806040)`fib(n=6) at lib.cc:7:12
   4    __attribute__((__export_name__("fib")))
   5    extern "C" uint32_t fib(uint32_t n) {
   6      uint32_t a = 1;
-> 7      uint32_t b = 1;
   8
   9      for(int i=0; i<n; ++i) {
   10       auto t = a;
(lldb) p a
(uint32_t) 1

Problem: using export name as 'fib-cc' gives unexpected output

Running make lldb-cc with

• __attribute__((__export_name__("fib-cc"))) in fib-cc/lib.cc
• renaming target func as 'fib-cc' in debugger/src/main.rs
  gives me unexpected output as

root@eafb8747aaf8:/workspace/module# make lldb-cc
lldb -O 'settings set target.disable-aslr false' \
        -o 'breakpoint set -n main' \
        -o 'r' \
        -o "breakpoint set -n 'fib-cc'" \
        -o 'c' \
        -o 'n' \
        -o 'p a' \
        -- target/release/debugger fib-cc/build/fib-cc.wasm
(lldb) settings set target.disable-aslr false
(lldb) target create "target/release/debugger"
Current executable set to '/workspace/module/target/release/debugger' (x86_64).
(lldb) settings set -- target.run-args  "fib-cc/build/fib-cc.wasm"
(lldb) breakpoint set -n main
Breakpoint 1: where = debugger`main, address = 0x00000000000baac0
(lldb) r
Process 103930 launched: '/workspace/module/target/release/debugger' (x86_64)
Process 103930 stopped
* thread #1, name = 'debugger', stop reason = breakpoint 1.1
    frame #0: 0x000055557f202ac0 debugger`main
debugger`main:
->  0x55557f202ac0 <+0>: pushq  %rax
    0x55557f202ac1 <+1>: movq   %rsi, %rcx
    0x55557f202ac4 <+4>: movslq %edi, %rdx
    0x55557f202ac7 <+7>: leaq   -0x59e(%rip), %rax ; debugger::main::h4a2646090ba600e9
(lldb) breakpoint set -n 'fib-cc'
Breakpoint 2: no locations (pending).
WARNING:  Unable to resolve breakpoint to any actual locations.
(lldb) c
fib(6) = 21
Process 103930 resuming
Process 103930 exited with status = -1 (0xffffffff) lost connection
(lldb) n
error: Command requires a process which is currently stopped.

Why LLDB cannot find symbol exported name 'fib-cc'?

The full project is hosted in https://github.com/sammyne/deep-dive-wasm-debug/tree/main/module.

Wasmtime GitHub notifications bot (Apr 27 2025 at 14:56):

sammyne edited issue #10680:

Environment

• wasmtime 32.0.0
• wasi-sdk 22.0
• LLDB 18.1.8

Source codes

fib-cc/lib.cc: C++ source codes

#include <cstdint>

__attribute__((__export_name__("fib")))
extern "C" uint32_t fib(uint32_t n) {
  uint32_t a = 1;
  uint32_t b = 1;

  for(int i=0; i<n; ++i) {
    auto t = a;
    a = b;
    b += t;
  }

  return b;
}

debugger/src/main.rs: embedded wasmtime

use wasmtime::*;

fn main() -> Result<()> {
    let path = match std::env::args().skip(1).next() {
        Some(v) => v,
        None => "target/wasm32-unknown-unknown/debug/fib.wasm".to_owned(),
    };

    // Load our previously compiled wasm file (built previously with Cargo) and
    // also ensure that we generate debuginfo so this executable can be
    // debugged in GDB.
    let engine = Engine::new(
        Config::new()
            .debug_info(true)
            .cranelift_opt_level(OptLevel::None),
    )?;
    let mut store = Store::new(&engine, ());

    //const PATH: &str = "target/wasm32-wasip1/debug/fib.wasm";
    //const PATH: &str = "target/wasm32-unknown-unknown/debug/fib.wasm";

    let module = Module::from_file(&engine, path)?;
    let instance = Instance::new(&mut store, &module, &[])?;

    // Invoke `fib` export
    let fib = instance.get_typed_func::<i32, i32>(&mut store, "fib")?;
    println!("fib(6) = {}", fib.call(&mut store, 6)?);
    Ok(())
}

Makefile

debugger = target/release/debugger
fibcc = fib-cc/build/fib-cc.wasm

.PHONY: all
all: $(debugger) $(fibcc)

$(debugger): debugger/src/main.rs
    cargo build -r -p debugger

$(fibcc): fib-cc/lib.cc
    $(MAKE) -C fib-cc

lldb-cc: $(debugger) $(fibcc)
    lldb -O 'settings set target.disable-aslr false' \
        -o 'breakpoint set -n main' \
        -o 'r' \
        -o "breakpoint set -n 'fib'" \
        -o 'c' \
        -o 'n' \
        -o 'p a' \
        -- $(debugger) $(fibcc)

.PHONY: clean
clean:
    cargo clean
    $(MAKE) -C fib-cc clean

Using export name as 'fib'

Running make lldb-cc with __attribute__((__export_name__("fib"))) in fib-cc/lib.cc gives me expected output as

root@eafb8747aaf8:/workspace/module# make lldb-cc
cargo build -r -p debugger
   Compiling debugger v0.1.0 (/workspace/module/debugger)
    Finished `release` profile [optimized] target(s) in 1.36s
lldb -O 'settings set target.disable-aslr false' \
        -o 'breakpoint set -n main' \
        -o 'r' \
        -o "breakpoint set -n 'fib'" \
        -o 'c' \
        -o 'n' \
        -o 'p a' \
        -- target/release/debugger fib-cc/build/fib-cc.wasm
(lldb) settings set target.disable-aslr false
(lldb) target create "target/release/debugger"
Current executable set to '/workspace/module/target/release/debugger' (x86_64).
(lldb) settings set -- target.run-args  "fib-cc/build/fib-cc.wasm"
(lldb) breakpoint set -n main
Breakpoint 1: where = debugger`main, address = 0x00000000000baac0
(lldb) r
Process 93176 launched: '/workspace/module/target/release/debugger' (x86_64)
Process 93176 stopped
* thread #1, name = 'debugger', stop reason = breakpoint 1.1
    frame #0: 0x0000559dedb3aac0 debugger`main
debugger`main:
->  0x559dedb3aac0 <+0>: pushq  %rax
    0x559dedb3aac1 <+1>: movq   %rsi, %rcx
    0x559dedb3aac4 <+4>: movslq %edi, %rdx
    0x559dedb3aac7 <+7>: leaq   -0x59e(%rip), %rax ; debugger::main::h4a2646090ba600e9
(lldb) breakpoint set -n 'fib'
Breakpoint 2: no locations (pending).
WARNING:  Unable to resolve breakpoint to any actual locations.
(lldb) c
1 location added to breakpoint 2
Process 93176 resuming
Process 93176 stopped
* thread #1, name = 'debugger', stop reason = breakpoint 2.1
    frame #0: 0x00007ff04c07b013 JIT(0x559def806040)`fib(n=6) at lib.cc:6:12
   3    // export_name 之后才能找到调试符号。
   4    __attribute__((__export_name__("fib")))
   5    extern "C" uint32_t fib(uint32_t n) {
-> 6      uint32_t a = 1;
   7      uint32_t b = 1;
   8
   9      for(int i=0; i<n; ++i) {
(lldb) n
Process 93176 stopped
* thread #1, name = 'debugger', stop reason = step over
    frame #0: 0x00007ff04c07b020 JIT(0x559def806040)`fib(n=6) at lib.cc:7:12
   4    __attribute__((__export_name__("fib")))
   5    extern "C" uint32_t fib(uint32_t n) {
   6      uint32_t a = 1;
-> 7      uint32_t b = 1;
   8
   9      for(int i=0; i<n; ++i) {
   10       auto t = a;
(lldb) p a
(uint32_t) 1

Problem: using export name as 'fib-cc' gives unexpected output

Running make lldb-cc with

• __attribute__((__export_name__("fib-cc"))) in fib-cc/lib.cc
• renaming target func as 'fib-cc' in debugger/src/main.rs
  gives me unexpected output as

root@eafb8747aaf8:/workspace/module# make lldb-cc
lldb -O 'settings set target.disable-aslr false' \
        -o 'breakpoint set -n main' \
        -o 'r' \
        -o "breakpoint set -n 'fib-cc'" \
        -o 'c' \
        -o 'n' \
        -o 'p a' \
        -- target/release/debugger fib-cc/build/fib-cc.wasm
(lldb) settings set target.disable-aslr false
(lldb) target create "target/release/debugger"
Current executable set to '/workspace/module/target/release/debugger' (x86_64).
(lldb) settings set -- target.run-args  "fib-cc/build/fib-cc.wasm"
(lldb) breakpoint set -n main
Breakpoint 1: where = debugger`main, address = 0x00000000000baac0
(lldb) r
Process 103930 launched: '/workspace/module/target/release/debugger' (x86_64)
Process 103930 stopped
* thread #1, name = 'debugger', stop reason = breakpoint 1.1
    frame #0: 0x000055557f202ac0 debugger`main
debugger`main:
->  0x55557f202ac0 <+0>: pushq  %rax
    0x55557f202ac1 <+1>: movq   %rsi, %rcx
    0x55557f202ac4 <+4>: movslq %edi, %rdx
    0x55557f202ac7 <+7>: leaq   -0x59e(%rip), %rax ; debugger::main::h4a2646090ba600e9
(lldb) breakpoint set -n 'fib-cc'
Breakpoint 2: no locations (pending).
WARNING:  Unable to resolve breakpoint to any actual locations.
(lldb) c
fib(6) = 21
Process 103930 resuming
Process 103930 exited with status = -1 (0xffffffff) lost connection
(lldb) n
error: Command requires a process which is currently stopped.

Why LLDB cannot find symbol exported name 'fib-cc'?

The full project is hosted in https://github.com/sammyne/deep-dive-wasm-debug/tree/main/module.

Wasmtime GitHub notifications bot (Apr 27 2025 at 15:11):

SingleAccretion commented on issue #10680:

The debug info produced by wasmtime and consumed by LLDB is based on the debug info that exists in the original WASM file. There is no C function named fib-cc (it would be an illegal C identifier) in the output, hence there is no such function in the output. In other words, DWARF debug info doesn't record export names.

In theory, we _could_ synthesize an alias symbol in wasmtime, so that fib now has two symbols associated with it (in fact it already has two, but one of them is broken, so doesn't usually show up), but I am not sure it would be worth it.

Wasmtime GitHub notifications bot (Apr 27 2025 at 15:11):

SingleAccretion edited a comment on issue #10680:

The debug info produced by wasmtime and consumed by LLDB is based on the debug info that exists in the original WASM file. There is no C function named fib-cc (it would be an illegal C identifier) in the input, hence there is no such function in the output. In other words, DWARF debug info doesn't record export names.

In theory, we _could_ synthesize an alias symbol in wasmtime, so that fib now has two symbols associated with it (in fact it already has two, but one of them is broken, so doesn't usually show up), but I am not sure it would be worth it.

Wasmtime GitHub notifications bot (Apr 27 2025 at 15:29):

sammyne commented on issue #10680:

Supporting export names would make debugging exported function name generated from wit possible.
I think it worths.

Wasmtime GitHub notifications bot (Apr 27 2025 at 15:33):

SingleAccretion commented on issue #10680:

Supporting export names would make debugging exported function name generated from wit possible.

I would have expected that there is always C function name you could break on. Or do simply mean that it is not ergonomic to break on those auto-generated names?

Wasmtime GitHub notifications bot (Apr 28 2025 at 02:08):

sammyne closed issue #10680:

Environment

• wasmtime 32.0.0
• wasi-sdk 22.0
• LLDB 18.1.8

Source codes

fib-cc/lib.cc: C++ source codes

#include <cstdint>

__attribute__((__export_name__("fib")))
extern "C" uint32_t fib(uint32_t n) {
  uint32_t a = 1;
  uint32_t b = 1;

  for(int i=0; i<n; ++i) {
    auto t = a;
    a = b;
    b += t;
  }

  return b;
}

debugger/src/main.rs: embedded wasmtime

use wasmtime::*;

fn main() -> Result<()> {
    let path = match std::env::args().skip(1).next() {
        Some(v) => v,
        None => "target/wasm32-unknown-unknown/debug/fib.wasm".to_owned(),
    };

    // Load our previously compiled wasm file (built previously with Cargo) and
    // also ensure that we generate debuginfo so this executable can be
    // debugged in GDB.
    let engine = Engine::new(
        Config::new()
            .debug_info(true)
            .cranelift_opt_level(OptLevel::None),
    )?;
    let mut store = Store::new(&engine, ());

    //const PATH: &str = "target/wasm32-wasip1/debug/fib.wasm";
    //const PATH: &str = "target/wasm32-unknown-unknown/debug/fib.wasm";

    let module = Module::from_file(&engine, path)?;
    let instance = Instance::new(&mut store, &module, &[])?;

    // Invoke `fib` export
    let fib = instance.get_typed_func::<i32, i32>(&mut store, "fib")?;
    println!("fib(6) = {}", fib.call(&mut store, 6)?);
    Ok(())
}

Makefile

debugger = target/release/debugger
fibcc = fib-cc/build/fib-cc.wasm

.PHONY: all
all: $(debugger) $(fibcc)

$(debugger): debugger/src/main.rs
    cargo build -r -p debugger

$(fibcc): fib-cc/lib.cc
    $(MAKE) -C fib-cc

lldb-cc: $(debugger) $(fibcc)
    lldb -O 'settings set target.disable-aslr false' \
        -o 'breakpoint set -n main' \
        -o 'r' \
        -o "breakpoint set -n 'fib'" \
        -o 'c' \
        -o 'n' \
        -o 'p a' \
        -- $(debugger) $(fibcc)

.PHONY: clean
clean:
    cargo clean
    $(MAKE) -C fib-cc clean

Using export name as 'fib'

Running make lldb-cc with __attribute__((__export_name__("fib"))) in fib-cc/lib.cc gives me expected output as

root@eafb8747aaf8:/workspace/module# make lldb-cc
cargo build -r -p debugger
   Compiling debugger v0.1.0 (/workspace/module/debugger)
    Finished `release` profile [optimized] target(s) in 1.36s
lldb -O 'settings set target.disable-aslr false' \
        -o 'breakpoint set -n main' \
        -o 'r' \
        -o "breakpoint set -n 'fib'" \
        -o 'c' \
        -o 'n' \
        -o 'p a' \
        -- target/release/debugger fib-cc/build/fib-cc.wasm
(lldb) settings set target.disable-aslr false
(lldb) target create "target/release/debugger"
Current executable set to '/workspace/module/target/release/debugger' (x86_64).
(lldb) settings set -- target.run-args  "fib-cc/build/fib-cc.wasm"
(lldb) breakpoint set -n main
Breakpoint 1: where = debugger`main, address = 0x00000000000baac0
(lldb) r
Process 93176 launched: '/workspace/module/target/release/debugger' (x86_64)
Process 93176 stopped
* thread #1, name = 'debugger', stop reason = breakpoint 1.1
    frame #0: 0x0000559dedb3aac0 debugger`main
debugger`main:
->  0x559dedb3aac0 <+0>: pushq  %rax
    0x559dedb3aac1 <+1>: movq   %rsi, %rcx
    0x559dedb3aac4 <+4>: movslq %edi, %rdx
    0x559dedb3aac7 <+7>: leaq   -0x59e(%rip), %rax ; debugger::main::h4a2646090ba600e9
(lldb) breakpoint set -n 'fib'
Breakpoint 2: no locations (pending).
WARNING:  Unable to resolve breakpoint to any actual locations.
(lldb) c
1 location added to breakpoint 2
Process 93176 resuming
Process 93176 stopped
* thread #1, name = 'debugger', stop reason = breakpoint 2.1
    frame #0: 0x00007ff04c07b013 JIT(0x559def806040)`fib(n=6) at lib.cc:6:12
   3    // export_name 之后才能找到调试符号。
   4    __attribute__((__export_name__("fib")))
   5    extern "C" uint32_t fib(uint32_t n) {
-> 6      uint32_t a = 1;
   7      uint32_t b = 1;
   8
   9      for(int i=0; i<n; ++i) {
(lldb) n
Process 93176 stopped
* thread #1, name = 'debugger', stop reason = step over
    frame #0: 0x00007ff04c07b020 JIT(0x559def806040)`fib(n=6) at lib.cc:7:12
   4    __attribute__((__export_name__("fib")))
   5    extern "C" uint32_t fib(uint32_t n) {
   6      uint32_t a = 1;
-> 7      uint32_t b = 1;
   8
   9      for(int i=0; i<n; ++i) {
   10       auto t = a;
(lldb) p a
(uint32_t) 1

Problem: using export name as 'fib-cc' gives unexpected output

Running make lldb-cc with

• __attribute__((__export_name__("fib-cc"))) in fib-cc/lib.cc
• renaming target func as 'fib-cc' in debugger/src/main.rs
  gives me unexpected output as

root@eafb8747aaf8:/workspace/module# make lldb-cc
lldb -O 'settings set target.disable-aslr false' \
        -o 'breakpoint set -n main' \
        -o 'r' \
        -o "breakpoint set -n 'fib-cc'" \
        -o 'c' \
        -o 'n' \
        -o 'p a' \
        -- target/release/debugger fib-cc/build/fib-cc.wasm
(lldb) settings set target.disable-aslr false
(lldb) target create "target/release/debugger"
Current executable set to '/workspace/module/target/release/debugger' (x86_64).
(lldb) settings set -- target.run-args  "fib-cc/build/fib-cc.wasm"
(lldb) breakpoint set -n main
Breakpoint 1: where = debugger`main, address = 0x00000000000baac0
(lldb) r
Process 103930 launched: '/workspace/module/target/release/debugger' (x86_64)
Process 103930 stopped
* thread #1, name = 'debugger', stop reason = breakpoint 1.1
    frame #0: 0x000055557f202ac0 debugger`main
debugger`main:
->  0x55557f202ac0 <+0>: pushq  %rax
    0x55557f202ac1 <+1>: movq   %rsi, %rcx
    0x55557f202ac4 <+4>: movslq %edi, %rdx
    0x55557f202ac7 <+7>: leaq   -0x59e(%rip), %rax ; debugger::main::h4a2646090ba600e9
(lldb) breakpoint set -n 'fib-cc'
Breakpoint 2: no locations (pending).
WARNING:  Unable to resolve breakpoint to any actual locations.
(lldb) c
fib(6) = 21
Process 103930 resuming
Process 103930 exited with status = -1 (0xffffffff) lost connection
(lldb) n
error: Command requires a process which is currently stopped.

Why LLDB cannot find symbol exported name 'fib-cc'?

The full project is hosted in https://github.com/sammyne/deep-dive-wasm-debug/tree/main/module.

Wasmtime GitHub notifications bot (Apr 28 2025 at 02:08):

sammyne commented on issue #10680:

Set breakpoint on the C function instead of export name works~

Last updated: Dec 06 2025 at 06:05 UTC