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debugger: expose shared memories to gdbstub #14087

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2 changes: 1 addition & 1 deletion crates/debugger/Cargo.toml
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Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,7 @@ rust-version.workspace = true
workspace = true

[dependencies]
wasmtime = { workspace = true, features = ["debug", "std", "async", "component-model", "gc", "gc-drc"] }
wasmtime = { workspace = true, features = ["debug", "std", "async", "component-model", "gc", "gc-drc", "threads"] }
wasmtime-wasi = { workspace = true }
wasmtime-wasi-io = { workspace = true }
tokio = { workspace = true, features = ["rt", "sync", "macros"] }
Expand Down
138 changes: 121 additions & 17 deletions crates/debugger/src/host/api.rs
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Expand Up @@ -7,11 +7,32 @@ use std::pin::Pin;
use std::sync::Arc;
use std::sync::atomic::{AtomicBool, Ordering};
use wasmtime::{
Engine, ExnRef, FrameHandle, Func, Global, Instance, Memory, Module, OwnedRooted, Result,
Table, Tag, Val, component::Resource, component::ResourceTable,
Engine, ExnRef, FrameHandle, Func, Global, Instance, Module, OwnedRooted, Result, Table, Tag,
Val, component::Resource, component::ResourceTable,
};
use wasmtime_wasi::p2::{DynPollable, Pollable, subscribe};

/// A memory exposed through the debugger API.
///
/// The component-level resource is deliberately shared by both kinds of
/// Wasmtime linear memory. Shared memories use their own host API and do not
/// belong to a `Store`, unlike ordinary memories.
#[derive(Clone)]
pub enum Memory {
Unshared(wasmtime::Memory),
Shared(wasmtime::SharedMemory),
}

impl Memory {
fn unique_id(&self) -> u64 {
match self {
Memory::Unshared(memory) => memory.debug_index_in_store(),
// A shared memory's base address is stable for its lifetime.
Memory::Shared(memory) => memory.data().as_ptr() as usize as u64,

@cfallin cfallin Aug 7, 2026

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I'd prefer not to expose an internal pointer to the debug component like this -- it feels like an encapsulation violation. Instead we should add debug_index_in_store to SharedMemory as we did for Memory.

}
}
}

/// Representation of one debuggee: a store with debugged code inside,
/// under the control of the debugger.
pub struct Debuggee {
Expand Down Expand Up @@ -446,7 +467,7 @@ impl wit::HostModule for ResourceTable {

impl wit::HostMemory for ResourceTable {
async fn size_bytes(&mut self, self_: Resource<Memory>, d: Resource<Debuggee>) -> Result<u64> {
let memory = *self.get(&self_)?;
let memory = self.get(&self_)?.clone();
let d = debugger(self, &d)?;
d.memory_size_bytes(memory).await
}
Expand All @@ -456,7 +477,7 @@ impl wit::HostMemory for ResourceTable {
self_: Resource<Memory>,
d: Resource<Debuggee>,
) -> Result<u64> {
let memory = *self.get(&self_)?;
let memory = self.get(&self_)?.clone();
let d = debugger(self, &d)?;
d.memory_page_size(memory).await
}
Expand All @@ -467,7 +488,7 @@ impl wit::HostMemory for ResourceTable {
d: Resource<Debuggee>,
delta_bytes: u64,
) -> Result<u64> {
let memory = *self.get(&self_)?;
let memory = self.get(&self_)?.clone();
let d = debugger(self, &d)?;
d.memory_grow(memory, delta_bytes).await
}
Expand All @@ -479,7 +500,7 @@ impl wit::HostMemory for ResourceTable {
addr: u64,
len: u64,
) -> Result<Vec<u8>> {
let memory = *self.get(&self_)?;
let memory = self.get(&self_)?.clone();
let d = debugger(self, &d)?;
Ok(d.memory_read_bytes(memory, addr, len)
.await?
Expand All @@ -493,7 +514,7 @@ impl wit::HostMemory for ResourceTable {
addr: u64,
bytes: Vec<u8>,
) -> Result<()> {
let memory = *self.get(&self_)?;
let memory = self.get(&self_)?.clone();
let d = debugger(self, &d)?;
d.memory_write_bytes(memory, addr, bytes)
.await?
Expand All @@ -507,7 +528,7 @@ impl wit::HostMemory for ResourceTable {
d: Resource<Debuggee>,
addr: u64,
) -> Result<u8> {
let memory = *self.get(&self_)?;
let memory = self.get(&self_)?.clone();
let d = debugger(self, &d)?;
Ok(d.memory_read_u8(memory, addr)
.await?
Expand All @@ -520,7 +541,7 @@ impl wit::HostMemory for ResourceTable {
d: Resource<Debuggee>,
addr: u64,
) -> Result<u16> {
let memory = *self.get(&self_)?;
let memory = self.get(&self_)?.clone();
let d = debugger(self, &d)?;
Ok(d.memory_read_u16(memory, addr)
.await?
Expand All @@ -533,7 +554,7 @@ impl wit::HostMemory for ResourceTable {
d: Resource<Debuggee>,
addr: u64,
) -> Result<u32> {
let memory = *self.get(&self_)?;
let memory = self.get(&self_)?.clone();
let d = debugger(self, &d)?;
Ok(d.memory_read_u32(memory, addr)
.await?
Expand All @@ -546,7 +567,7 @@ impl wit::HostMemory for ResourceTable {
d: Resource<Debuggee>,
addr: u64,
) -> Result<u64> {
let memory = *self.get(&self_)?;
let memory = self.get(&self_)?.clone();
let d = debugger(self, &d)?;
Ok(d.memory_read_u64(memory, addr)
.await?
Expand All @@ -560,7 +581,7 @@ impl wit::HostMemory for ResourceTable {
addr: u64,
value: u8,
) -> Result<()> {
let memory = *self.get(&self_)?;
let memory = self.get(&self_)?.clone();
let d = debugger(self, &d)?;
d.memory_write_u8(memory, addr, value)
.await?
Expand All @@ -575,7 +596,7 @@ impl wit::HostMemory for ResourceTable {
addr: u64,
value: u16,
) -> Result<()> {
let memory = *self.get(&self_)?;
let memory = self.get(&self_)?.clone();
let d = debugger(self, &d)?;
d.memory_write_u16(memory, addr, value)
.await?
Expand All @@ -590,7 +611,7 @@ impl wit::HostMemory for ResourceTable {
addr: u64,
value: u32,
) -> Result<()> {
let memory = *self.get(&self_)?;
let memory = self.get(&self_)?.clone();
let d = debugger(self, &d)?;
d.memory_write_u32(memory, addr, value)
.await?
Expand All @@ -605,7 +626,7 @@ impl wit::HostMemory for ResourceTable {
addr: u64,
value: u64,
) -> Result<()> {
let memory = *self.get(&self_)?;
let memory = self.get(&self_)?.clone();
let d = debugger(self, &d)?;
d.memory_write_u64(memory, addr, value)
.await?
Expand All @@ -614,12 +635,12 @@ impl wit::HostMemory for ResourceTable {
}

async fn clone(&mut self, self_: Resource<Memory>) -> Result<Resource<Memory>> {
let memory = *self.get(&self_)?;
let memory = self.get(&self_)?.clone();
Ok(self.push(memory)?)
}

async fn unique_id(&mut self, self_: Resource<Memory>) -> Result<u64> {
Ok(self.get(&self_)?.debug_index_in_store())
Ok(self.get(&self_)?.unique_id())
}

async fn drop(&mut self, rep: Resource<Memory>) -> Result<()> {
Expand Down Expand Up @@ -1049,3 +1070,86 @@ impl wit::Host for ResourceTable {
err.downcast()
}
}

#[cfg(test)]
mod tests {

@cfallin cfallin Aug 7, 2026

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This is the wrong level at which to test; it's extremely verbose and a lot of machinery to poke inside resource-tables and such.

We have an integration test that starts up Wasmtime with the gdbstub component and runs LLDB against it. Perhaps you can write a test there with a custom-in-WAT debuggee that has a shared memory, and then show that you can print its memory contents with LLDB?

@cfallin cfallin Aug 7, 2026

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(On reading further, I see that you did that -- so, please just delete this whole unit test)

use super::*;
use crate::host::add_debuggee;
use wasmtime::{Config, Engine, Instance, Module, Store};

#[cfg_attr(miri, ignore)]
#[cfg(target_pointer_width = "64")]
#[tokio::test]
async fn shared_memory_is_exposed_to_debugger() -> wasmtime::Result<()> {
let mut config = Config::new();
config.wasm_threads(true);
config.shared_memory(true);
config.guest_debug(true);
let engine = Engine::new(&config)?;
let module = Module::new(&engine, r#"(module (memory (export "memory") 1 2 shared))"#)?;
let mut store = Store::new(&engine, ());
let instance = Instance::new_async(&mut store, &module, &[]).await?;

let debuggee = crate::Debuggee::new(store, |_store| Box::pin(async { Ok(()) }));
let mut table = ResourceTable::new();
let debuggee = add_debuggee(&mut table, debuggee)?;
let instance = table.push(instance)?;
let memory = <ResourceTable as wit::HostInstance>::get_memory(
&mut table,
instance,
Resource::new_borrow(debuggee.rep()),
0,
)
.await?;
let size = <ResourceTable as wit::HostMemory>::size_bytes(
&mut table,
Resource::new_borrow(memory.rep()),
Resource::new_borrow(debuggee.rep()),
)
.await?;
assert_eq!(size, 65536);

let page_size = <ResourceTable as wit::HostMemory>::page_size_bytes(
&mut table,
Resource::new_borrow(memory.rep()),
Resource::new_borrow(debuggee.rep()),
)
.await?;
assert_eq!(page_size, 65536);
let old_size = <ResourceTable as wit::HostMemory>::grow_to_bytes(
&mut table,
Resource::new_borrow(memory.rep()),
Resource::new_borrow(debuggee.rep()),
page_size,
)
.await?;
assert_eq!(old_size, size);
let size = <ResourceTable as wit::HostMemory>::size_bytes(
&mut table,
Resource::new_borrow(memory.rep()),
Resource::new_borrow(debuggee.rep()),
)
.await?;
assert_eq!(size, 2 * page_size);

<ResourceTable as wit::HostMemory>::set_u32(
&mut table,
Resource::new_borrow(memory.rep()),
Resource::new_borrow(debuggee.rep()),
0,
0x1234_5678,
)
.await?;
let value = <ResourceTable as wit::HostMemory>::get_u32(
&mut table,
Resource::new_borrow(memory.rep()),
Resource::new_borrow(debuggee.rep()),
0,
)
.await?;
assert_eq!(value, 0x1234_5678);

<ResourceTable as wit::HostDebuggee>::drop(&mut table, debuggee).await?;
Ok(())
}
}
2 changes: 1 addition & 1 deletion crates/debugger/src/host/bindings.rs
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Expand Up @@ -23,7 +23,7 @@ wasmtime::component::bindgen!({
"bytecodealliance:wasmtime/debuggee.module": wasmtime::Module,
"bytecodealliance:wasmtime/debuggee.table": wasmtime::Table,
"bytecodealliance:wasmtime/debuggee.global": wasmtime::Global,
"bytecodealliance:wasmtime/debuggee.memory": wasmtime::Memory,
"bytecodealliance:wasmtime/debuggee.memory": super::api::Memory,
"bytecodealliance:wasmtime/debuggee.wasm-tag": wasmtime::Tag,
"bytecodealliance:wasmtime/debuggee.wasm-func": wasmtime::Func,
"bytecodealliance:wasmtime/debuggee.wasm-exception": super::api::WasmException,
Expand Down
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