[lld][WebAssembly] using `--import-memory` cancels out `--page-size=N` options without telling user.

[anlavandier]

The current version of the Webassembly linker wasm-ld allows the argument --page-size=N with N being either 1 or 65536 since the merging of #128942. For some reason, when also passing the --import-memory flag, the page-size is completely disregarded without telling the caller. This results in .wasm files that try to import a memory with the default page-size even if --page-size=1 was given.

Edit: After realizing that I'm an idiot and that wabt's wasm2wat does support the custom-page-sizes proposal by simply passing a flag to it, I edited the issue accordingly to make it more understandable

How to reproduce

I don't know anything about linkers and discovered that while compiling rust code to wasm bytecode so it's what I'll describe.
Setup :

rustc -Vv
rustc 1.88.0 (6b00bc388 2025-06-23)
binary: rustc
commit-hash: 6b00bc3880198600130e1cf62b8f8a93494488cc
commit-date: 2025-06-23
host: x86_64-unknown-linux-gnu
release: 1.88.0
LLVM version: 20.1.5

wasm-ld --version
LLD 21.0.0 (https://github.com/llvm/llvm-project.git 1b7cbe1f871ebe572e383a2531257b88168eea1b) # Compiled from source on 2.07.25 

wasm2wat --version 
1.0.37 (git~1.0.37-27-ga55fb946)

cargo new --lib wasm-ld-bug-test
cd wasm-ld-bug-test

add

[lib]
crate-type=["cdylib", "rlib]

to the Cargo.toml. Then change src/lib.rs to

#![no_std]

pub fn add(left: u64, right: u64) -> u64 {
    left + right
}

#[panic_handler]
fn panic_handler(_: &core::panic::PanicInfo<'_>) -> ! {
    core::arch::wasm32::unreachable()
}

Now, if you compile this with

cargo rustc --target wasm32v1-none -- -Clinker=wasm-ld -Clink-arg=--page-size=1

You can see that the compiled uses the custom-page-size proposal by trying to convert it to .wat

wasm2wat --enable-custom-page-sizes target/wasm32v1/debug/wasm_ld_bug_test.wasm 

(module $wasm_ld_bug_test.wasm
  (table (;0;) 1 1 funcref)
  (memory (;0;) 1048576 (pagesize 1))
  (global $__stack_pointer (mut i32) (i32.const 1048576))
  (global (;1;) i32 (i32.const 1048576))
  (global (;2;) i32 (i32.const 1048576))
  (export "memory" (memory 0))
  (export "__data_end" (global 1))
  (export "__heap_base" (global 2)))

Then by recompiling it while also using --import-memory you get

cargo rustc --target wasm32v1-none -- -Clinker=wasm-ld -Clink-arg=--page-size=1 -Clink-arg=--import-memory
wasm2wat target/wasm32v1/debug/wasm_ld_bug_test.wasm
target/wasm32v1-none/debug/wasm_ld_bug_test.wasm:000001b: error: initial pages (1048576) must be <= (65536)

This happens because by default, the linker sets the stack size to 1048576 bytes and since the memory pages are of size 1 (at least before being actually written to the wasm files), it asks for 1048576 pages of memory. This didn't happen in the previous case despite the fact that the file does also ask for 1048576 pages.

By changing the stack-size to something more manageable we can see that the pagesize variable for the memory has simply been removed.

cargo rustc --target wasm32v1-none -- -Clinker=wasm-ld -Clink-arg=-zstack-size=1024 -Clink-arg=--page-size=1 -Clink-arg=-
-import-memory
wasm2wat -enable-custom-page-sizes target/wasm32v1/debug/wasm_ld_bug_test.wasm

(module $wasm_ld_bug_test.wasm
  (import "env" "memory" (memory (;0;) 1024))
  (table (;0;) 1 1 funcref)
  (global $__stack_pointer (mut i32) (i32.const 1024))
  (global (;1;) i32 (i32.const 1024))
  (global (;2;) i32 (i32.const 1024))
  (export "__data_end" (global 1))
  (export "__heap_base" (global 2)))

[llvmbot]

@llvm/issue-subscribers-lld-wasm

Author: None (anlavandier)

The current version of the Webassembly linker `wasm-ld` allows the argument `--page-size=N` with N being either `1` or `65536` since the merging of #128942. For some reason, when also passing the `--import-memory` flag, the `page-size` is completely disregarded without telling the caller. This results in `.wasm` files that try to import a memory with the default page-size even if `--page-size=1` was given.

How to reproduce

I don't know anything about linkers and discovered that while compiling rust code to wasm bytecode so it's what I'll describe.
Setup :

rustc -Vv
rustc 1.88.0 (6b00bc388 2025-06-23)
binary: rustc
commit-hash: 6b00bc3880198600130e1cf62b8f8a93494488cc
commit-date: 2025-06-23
host: x86_64-unknown-linux-gnu
release: 1.88.0
LLVM version: 20.1.5

wasm-ld --version
LLD 21.0.0 (https://github.com/llvm/llvm-project.git 1b7cbe1f871ebe572e383a2531257b88168eea1b) # Compiled from source on 2.07.25 

cargo new --lib wasm-ld-bug-test
cd wasm-ld-bug-test

add

[lib]
crate-type=["cdylib", "rlib]

to the Cargo.toml

[anlavandier]

Sorry, I pressed enter at the wrong time and now the @llvmbot description is wrong.
In any case, seeing as they're the person that contributed the PR that introduced this, I assume that @fitzgen probably knows what's at play here.

[fitzgen]

The behavior described is definitely not what I intended.

[anlavandier]

From the errors I see, it seems to me like the --import-memory command makes the linker write the bytecode corresponding to

(import "env" "memory" (memory (;0;) XX))

regardless of the page-size asked by the user. But wam-ld does take the page-size into account when checking the number of pages it needs to ask and thus what XX should be. This would explain why when leaving the initial stack-size at 1MiB wasm2wat complains about the number of pages written in the file.

[anlavandier]

Hello, I realized that I wasm2wat supports the custom-page-size proposal and reworded my issue a bit to make it less complicated. Here are some quotes of what was changed

Now, if you compile this with

cargo rustc --target wasm32v1-none -- -Clinker=wasm-ld -Clink-arg=--page-size=1

You can see that the compiled uses the custom-page-size proposal by trying to convert it to .wat

wasm2wat --enable-custom-page-sizes target/wasm32v1/debug/wasm_ld_bug_test.wasm

(module $wasm_ld_bug_test.wasm
(table (;0;) 1 1 funcref)
(memory (;0;) 1048576 (pagesize 1))
(global $__stack_pointer (mut i32) (i32.const 1048576))
(global (;1;) i32 (i32.const 1048576))
(global (;2;) i32 (i32.const 1048576))
(export "memory" (memory 0))
(export "__data_end" (global 1))
(export "__heap_base" (global 2)))

By changing the stack-size to something more manageable we can see that the pagesize variable for the memory has simply been removed.

cargo rustc --target wasm32v1-none -- -Clinker=wasm-ld -Clink-arg=-zstack-size=1024 -Clink-arg=--page-size=1 -Clink-arg=-
-import-memory
wasm2wat -enable-custom-page-sizes target/wasm32v1/debug/wasm_ld_bug_test.wasm

(module $wasm_ld_bug_test.wasm
(import "env" "memory" (memory (;0;) 1024))
(table (;0;) 1 1 funcref)
(global $__stack_pointer (mut i32) (i32.const 1024))
(global (;1;) i32 (i32.const 1024))
(global (;2;) i32 (i32.const 1024))
(export "__data_end" (global 1))
(export "__heap_base" (global 2)))

[fitzgen]

Looks like I just forgot to check for and set the custom page size when emitting the memory import. Small patch, just need to write a test.

[fitzgen]

Fix in #146916