WIP Apply Base Relocs For Webcil in Wasm CoreCLR by adamperlin · Pull Request #126436 · dotnet/runtime

adamperlin · 2026-04-01T23:15:14Z

This PR is currently untested, but is a WIP implementation for a runtime base relocation scheme for Webcil.

The WasmObjectWriter has been updated to emit PE-style base relocs into a reloc webcil section (the last section in the webcil payload). The relocations use an encoding identical to that used by PE image base style relocs. New base relocation types have been added to represent "wasm function pointer table base" fixups for wasm32 and wasm64.

The Webcil loader path in CoreCLR has been updated to interpret these relocs. In particular ApplyBaseRelocations will now run in the Webcil case, and will handle applying both standard image base style relocations and the new table base relocs. The real table base used at load time is not yet being plumbed through; this will eventually be set as a field in the webcil header itself and is dependent on #126388.

…X, WASM_FUNCTION_INDEX type relocs

baseRelocsMap in WasmObjectWriter

…-object-writer-runtime-relocs

…name

… segment description after we've recorded all possible base relocs, and we must re-order when we can write the memory import as a result

…ldWebcilDataSegment()

…w Wasm table-based relocs

dotnet-policy-service · 2026-04-01T23:16:19Z

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src/coreclr/vm/peimagelayout.inl

Copilot

Pull request overview

Implements an initial WebAssembly/Webcil runtime base relocation scheme by emitting PE-style base relocation blocks in the Webcil payload and teaching CoreCLR’s Webcil loader path to apply those relocations (including new Wasm table-base fixups).

Changes:

Emit a PE-style .reloc-equivalent Webcil section and encode relocation entries (including new Wasm table-base relocation kinds).
Enable ApplyBaseRelocations for Webcil images and apply both image-base and table-base relocation deltas.
Extend Webcil decoding and documentation to expose base relocation presence and locate relocation data.

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
src/coreclr/vm/peimagelayout.inl	Adds decoder-dispatched relocation helpers (e.g., `SetRelocated`, `HasBaseRelocations`) for PE/Webcil.
src/coreclr/vm/peimagelayout.h	Adds temporary table-base offset plumbing and switches `SetRelocated` to decoder dispatch.
src/coreclr/vm/peimagelayout.cpp	Runs base relocations for Webcil and adds handling for Wasm table-base relocation types.
src/coreclr/utilcode/webcildecoder.cpp	Exposes Webcil base relocation presence and supports BASERELOC directory-style access.
src/coreclr/tools/Common/Compiler/ObjectWriter/WebcilEncoder.cs	Minor formatting adjustments in Webcil section header encoding size helper.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs	Emits Webcil relocation section data and records base relocation entries from certain relocation types.
src/coreclr/tools/Common/Compiler/DependencyAnalysis/Relocation.cs	Adds Webcil relocation kinds and maps Wasm table index relocations to file/base-reloc types.
src/coreclr/pal/inc/rt/ntimage.h	Defines Webcil-specific base relocation type codes for Wasm table-base fixups.
src/coreclr/inc/webcildecoder.h	Adds relocated-state tracking and a `HasBaseRelocations` API for Webcil.
docs/design/mono/webcil.md	Documents Reserved0 usage for base reloc section indexing and the relocation block/entry encoding.

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs

src/coreclr/utilcode/webcildecoder.cpp

docs/design/mono/webcil.md

src/coreclr/inc/webcildecoder.h

src/coreclr/vm/peimagelayout.cpp

…r.cs Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 9 out of 10 changed files in this pull request and generated 10 comments.

src/coreclr/vm/peimagelayout.inl

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs

src/coreclr/utilcode/webcildecoder.cpp

docs/design/mono/webcil.md

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs

…r.cs Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 9 out of 10 changed files in this pull request and generated 4 comments.

src/coreclr/vm/peimagelayout.inl

src/coreclr/utilcode/webcildecoder.cpp

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs

When emitting base relocations, webcil section addresses were not yet assigned, leading to a bug where base relocations were not properly offset by the section they apply to. This commit re-orders our base relocation construction/emission so that we construct our base relocations AFTER Webcil sections have been assigned addresses

….com:adamperlin/runtime into adamperlin/wasm-object-writer-runtime-relocs

Copilot

Pull request overview

Copilot reviewed 9 out of 10 changed files in this pull request and generated 5 comments.

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs

src/coreclr/vm/peimagelayout.cpp

- peimagelayout.inl: Use DECODER_DISPATCH for IsRelocated() so Webcil images correctly report relocated state instead of always FALSE - peimagelayout.cpp: Gate PAL_LOADMarkSectionAsNotNeeded with !TARGET_WASM to avoid operating on Webcil payload pointers - webcildecoder.cpp: Delegate HasDirectoryEntry(BASERELOC) to HasBaseRelocations() for consistent Reserved0 range validation - WasmObjectWriter.cs: Use SortedDictionary for _baseRelocMap to ensure deterministic reloc block emission order - WasmObjectWriter.cs: Replace redundant pattern match on GetFileRelocationType with direct assignment - WasmObjectWriter.cs: Use explicit cast instead of as-cast for WebcilSection, add Debug.Assert for non-negative offset Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 9 out of 10 changed files in this pull request and generated 3 comments.

src/coreclr/vm/peimagelayout.h

src/coreclr/utilcode/webcildecoder.cpp

src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs

davidwrighton

LGTM

davidwrighton · 2026-04-03T17:53:23Z

/ba-g Build analysis seems to have hung. Maybe this will unstick it.

This PR is currently testable only with future follow on work, but is an implementation for a runtime base relocation scheme for Webcil. The WasmObjectWriter has been updated to emit PE-style base relocs into a `reloc` webcil section (the last section in the webcil payload). The relocations use an encoding identical to that used by PE image base style relocs. New base relocation types have been added to represent "wasm function pointer table base" fixups for wasm32 and wasm64. The Webcil loader path in CoreCLR has been updated to interpret these relocs. In particular `ApplyBaseRelocations` will now run in the Webcil case, and will handle applying both standard image base style relocations and the new table base relocs. The real table base used at load time is not yet being plumbed through; this will eventually be set as a field in the webcil header itself and is dependent on dotnet#126388. --------- Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com> Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

adamperlin added 12 commits March 27, 2026 13:37

Implement relocation resolution for WASM_MEMORY_ADDR, WASM_TABLE_INDE…

bb98144

…X, WASM_FUNCTION_INDEX type relocs

WIP relocations for runtime function pointer fixup

473c46b

Webcil native relocations

cc8a1a8

Re-use IMAGE_REL_BASED style relocs for Wasm, and collect into

c33cd2f

baseRelocsMap in WasmObjectWriter

Merge branch 'main' of github.com:dotnet/runtime into adamperlin/wasm…

cfd4342

…-object-writer-runtime-relocs

Fix bug in EncodeSectionHeader, small cleanup of unused code+field re…

04c1a48

…name

Emit Webcil Reloc section. This means we need to construct the webcil…

f1bebc4

… segment description after we've recorded all possible base relocs, and we must re-order when we can write the memory import as a result

Refactor: move cor header/debug directory rva initialization into Bui…

9fc1308

…ldWebcilDataSegment()

Reuse Reserved0 field for index of image base relocs

2d81282

Add ApplyBaseRelocs functionality for webcil in CoreCLR, including ne…

d207de8

…w Wasm table-based relocs

Various fixes

b437a79

Update docs

a575fae

adamperlin requested review from Copilot and davidwrighton April 1, 2026 23:15

github-actions bot added the area-VM-coreclr label Apr 1, 2026

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adamperlin changed the title ~~WIP Apply Base Relocs in Wasm~~ WIP Apply Base Relocs in Wasm CoreCLR Apr 1, 2026

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adamperlin and others added 2 commits April 1, 2026 16:22

Fix markdown lint errors

3a341bd

Update src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWrite…

397f687

…r.cs Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Copilot AI review requested due to automatic review settings April 1, 2026 23:27

Update src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWrite…

8ec58e8

…r.cs Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Copilot started reviewing on behalf of adamperlin April 1, 2026 23:28 View session

adamperlin and others added 2 commits April 1, 2026 16:28

Update src/coreclr/utilcode/webcildecoder.cpp

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

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adamperlin changed the title ~~WIP Apply Base Relocs in Wasm CoreCLR~~ WIP Apply Base Relocs For Webcil in Wasm CoreCLR Apr 2, 2026

Update src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWrite…

43d6f6b

…r.cs Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Copilot AI review requested due to automatic review settings April 2, 2026 16:47

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adamperlin added 3 commits April 2, 2026 14:45

Merge branch 'adamperlin/wasm-object-writer-runtime-relocs' of github…

31308a9

….com:adamperlin/runtime into adamperlin/wasm-object-writer-runtime-relocs

Fix formatting

c67663e

Copilot AI review requested due to automatic review settings April 2, 2026 21:56

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adamperlin marked this pull request as ready for review April 2, 2026 23:28

adamperlin requested a review from MichalStrehovsky as a code owner April 2, 2026 23:28

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davidwrighton approved these changes Apr 3, 2026

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davidwrighton merged commit d5dc74a into dotnet:main Apr 3, 2026
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This was referenced Apr 3, 2026

[main] Source code updates from dotnet/runtime dotnet/dotnet#5880

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[main] Source code updates from dotnet/runtime dotnet/dotnet#5910

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github-actions bot mentioned this pull request Apr 9, 2026

[release-notes] .NET 11 Preview 3 jeffhandley/dotnet-core-release-notes#4

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