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fix: Implement plugin callsbacks required for thin LTO #1902

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Merged
davidlattimore merged 1 commit into from
May 8, 2026
Merged

fix: Implement plugin callsbacks required for thin LTO #1902

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40 changes: 30 additions & 10 deletions libwild/src/linker_plugins.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -124,6 +124,10 @@ struct FileHandle<'data> {

/// This isn't known initially because we allocate file IDs later.
file_id: AtomicCell<Option<FileId>>,

fd: RawFd,
offset: u64,
name: &'data CStr,
}

#[derive(Default)]
Expand Down Expand Up @@ -249,14 +253,22 @@ impl<'data> LinkerPlugin<'data> {
wrap_symbols: self.wrap_symbols,
};

let name = CString::new(input_ref.file.filename.as_os_str().as_encoded_bytes())?;
let name = CStr::from_bytes_with_nul(
self.herd.get().alloc_slice_copy(name.as_bytes_with_nul()),
)
.unwrap();

let handle = FileHandle {
data,
name,
fd,
offset,
file_id: AtomicCell::new(None),
};

let handle = self.herd.get().alloc(handle);

let name = CString::new(input_ref.file.filename.as_os_str().as_encoded_bytes())?;
let file = LdPluginInputFile {
name: name.as_ptr(),
Comment on lines -259 to 273

@mati865 mati865 May 8, 2026

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Wasn't that use-after-free previously?
CString created allocation and unless I'm missing something due to viewing this on my phone, we only used pointer to it. So the name would be dropped when going out of scope.

With the new code name is stored within the struct, so the allocation is kept alive.

Borrow checker doesn't catch such issues because it's legal and safe to create dangling pointers. The part requiring unsafe code is reading from the pointers.

@davidlattimore davidlattimore May 8, 2026

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The pointer to name would previously have been valid for the duration of the call into the plugin's claim-file hook. But if the claim-file hook stored the pointer and used it later, then yes, it would have been a use-after-free. AFAIK though it doesn't, but the linker plugin API docs are pretty absent of any mentions of required lifetimes, so who knows.

@mati865 mati865 May 8, 2026

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Ah, this function returns LtoInputInfo, not LdPluginInputFile. Line wrapping in the mobile view tricked me.

fd,
Expand Down Expand Up @@ -913,18 +925,26 @@ fn get_symbol_resolution<'data>(
}
}

// We don't currently implement this. The LLVM plugin gives an error if we don't define it, but then
// it doesn't appear to actually call it. Or maybe we just haven't found a test case that causes it
// to be called.
extern "C" fn get_input_file(
_handle: *const libc::c_void,
_file: *mut LdPluginInputFile,
) -> Status {
Status::Err
extern "C" fn get_input_file(handle: *const libc::c_void, file: *mut LdPluginInputFile) -> Status {
catch_panics(|| {
if handle.is_null() || file.is_null() {
return Status::Err;
}
let handle = unsafe { &*(handle as *const FileHandle) };
let file = unsafe { &mut *file };

file.fd = handle.fd;
file.offset = handle.offset as i64;
file.file_size = handle.data.len() as i64;
file.name = handle.name.as_ptr();

Status::Ok
})
}

extern "C" fn release_input_file(_handle: *const libc::c_void) -> Status {
Status::Err
// We don't allocate in `get_input_file`, so there's nothing to free here.
Status::Ok
}

extern "C" fn get_view(
Expand Down
9 changes: 9 additions & 0 deletions wild/tests/sources/elf/linker-plugin-lto/linker-plugin-lto.c
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Original file line number Diff line number Diff line change
Expand Up @@ -22,6 +22,15 @@
//#Object:linker-plugin-lto-2.c
//#DiffIgnore:section.eh_frame.type

//#Config:clang-thin:default
//#Compiler:clang
//#CompArgs:-flto=thin
//#LinkerDriver:clang
//#LinkArgs:-Wl,-znow -flto=thin -nostdlib -O0
//#Object:runtime.c
//#Object:linker-plugin-lto-2.c
//#DiffIgnore:section.eh_frame.type

//#Config:clang-link-gcc:error
//#Compiler:clang
//#CompArgs:-flto
Expand Down
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