feat!: stack-based format buffer for `wdk` crate by leon-xd · Pull Request #611 · microsoft/windows-drivers-rs

leon-xd · 2026-02-13T00:10:03Z

Adds and integrates wdk::fmt::FormatBuffer.

Summary

Introduced crates/wdk/src/fmt.rs with a public wdk::fmt module containing two types:
- FormatBuffer<N> — a fixed-size, stack-allocated formatting buffer implementing fmt::Write
- FlushableFormatBuffer<F, N> — a wrapper that auto-flushes via closure on overflow and on drop
Swapped print.rs WDM/KMDF path to use FlushableFormatBuffer instead of DbgPrintBufWriter; removed that module and its tests.
Turns the alloc feature from the wdk crate into a no-op — it had no remaining uses after the stack-buffer migration. Avoids removal of feature to prevent a breaking change
Bumps MSRV from 1.85 to 1.91 due to use of str::char_floor_boundary

Details

FormatBuffer<N> stores a zero-initialized [u8; N] and tracks used bytes. The last byte is reserved for a NUL terminator, so usable capacity is N - 1. N must be at least 2; smaller values will not compile (const { assert!(N >= 2) }).
All buffer mutations go through a single append_bytes helper that copies data and sets buffer[used] = 0, centralizing the NUL terminator invariant.
as_str() returns an infallible UTF-8 view — only valid UTF-8 enters via write_str (both FormatBuffer::write_str and FlushableFormatBuffer::write_str copy from &str sources).
as_c_str() returns a &CStr view up to the first NUL. Infallible in practice since the NUL invariant is always maintained.
fmt::Write clamps overflow writes at a UTF-8 char boundary via floor_char_boundary and signals truncation via fmt::Error.
FlushableFormatBuffer<F, N> wraps FormatBuffer and auto-flushes via a closure when the buffer fills. Remaining content is flushed on drop. This allows arbitrarily long formatted output to be processed in fixed-size chunks.
print.rs WDM/KMDF path now formats into FlushableFormatBuffer<_, 512> and calls DbgPrint via %s using the resulting CStr.
Removed the alloc feature gate from the WDM/KMDF print path — print!/println! no longer require heap allocation and work at any IRQL where DbgPrint is valid.
Custom Debug impl on FormatBuffer shows only the used content, not stale bytes after clear().

Testing

29 tests covering:

basic write, as_str(), as_c_str() usage
overflow, multi-write, and exact-fit scenarios
empty writes and min-sized buffers (N = 2)
clear-then-shorter-write regression (NUL terminator correctness)
multi-byte UTF-8 char boundary handling in both buffer types
explicit flush() mid-stream and flush-on-drop
interior NUL truncation via as_c_str()
compile_fail doctest for N < 2

cargo test --package wdk --lib --all-features -- fmt --nocapture

Notes / Follow-ups

_print ignores fmt::write errors — partial output is acceptable for debug printing. Errors from individual Display impls are silently dropped.
The old DbgPrintBufWriter stripped interior NUL bytes and continued printing. The new WDM/KMDF path truncates each chunk at the first NUL (via as_c_str). UMDF still strips NULs and prints the remainder.

Copilot

Pull request overview

This PR introduces a new fixed-size, stack-friendly formatting buffer (WdkFormatBuffer) to support heap-free formatting in driver contexts, and integrates it into the kernel print!/println! path by replacing the previous DbgPrintBufWriter implementation.

Changes:

Added crates/wdk/src/fmt.rs implementing WdkFormatBuffer<const T: usize = 512> with fmt::Write, plus as_str() / as_cstr() helpers and tests.
Updated crates/wdk/src/print.rs (WDM/KMDF path) to format into WdkFormatBuffer and call DbgPrint directly; removed the old buffering/flushing module.
Adjusted the KMDF sample driver to add an explicit type annotation for the WdfDriverCreate output handle pointer.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 7 comments.

File	Description
examples/sample-kmdf-driver/src/lib.rs	Tweaks the `WdfDriverCreate` out-handle variable typing.
crates/wdk/src/print.rs	Switches kernel printing to `WdkFormatBuffer` + direct `DbgPrint`.
crates/wdk/src/lib.rs	Adds the new `fmt` module and re-exports `WdkFormatBuffer`.
crates/wdk/src/fmt.rs	New formatting buffer type, CStr/str views, and unit tests.

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codecov-commenter · 2026-02-13T00:38:05Z

Codecov Report

❌ Patch coverage is 91.78470% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.44%. Comparing base (2940ef8) to head (bfc706d).

Files with missing lines	Patch %	Lines
crates/wdk/src/fmt.rs	91.78%	26 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #611      +/-   ##
==========================================
+ Coverage   77.47%   78.44%   +0.97%     
==========================================
  Files          24       25       +1     
  Lines        4848     5201     +353     
  Branches     4848     5201     +353     
==========================================
+ Hits         3756     4080     +324     
- Misses        975     1001      +26     
- Partials      117      120       +3

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Copilot

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Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

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Adds and integrates `wdk::fmt::FormatBuffer`. ## Summary - Introduced `crates/wdk/src/fmt.rs` with a public `wdk::fmt` module containing two types: - `FormatBuffer<N>` — a fixed-size, stack-allocated formatting buffer implementing `fmt::Write` - `FlushableFormatBuffer<F, N>` — a wrapper that auto-flushes via closure on overflow and on drop - Swapped `print.rs` WDM/KMDF path to use `FlushableFormatBuffer` instead of `DbgPrintBufWriter`; removed that module and its tests. - Turns the `alloc` feature from the `wdk` crate into a no-op — it had no remaining uses after the stack-buffer migration. Avoids removal of feature to prevent a breaking change - Bumps MSRV from 1.85 to 1.91 due to use of [`str::char_floor_boundary`](https://doc.rust-lang.org/core/primitive.str.html#method.floor_char_boundary) ## Details - `FormatBuffer<N>` stores a zero-initialized `[u8; N]` and tracks `used` bytes. The last byte is reserved for a NUL terminator, so usable capacity is `N - 1`. `N` must be at least 2; smaller values will not compile (`const { assert!(N >= 2) }`). - All buffer mutations go through a single `append_bytes` helper that copies data and sets `buffer[used] = 0`, centralizing the NUL terminator invariant. - `as_str()` returns an infallible UTF-8 view — only valid UTF-8 enters via `write_str` (both `FormatBuffer::write_str` and `FlushableFormatBuffer::write_str` copy from `&str` sources). - `as_c_str()` returns a `&CStr` view up to the first NUL. Infallible in practice since the NUL invariant is always maintained. - `fmt::Write` clamps overflow writes at a UTF-8 char boundary via `floor_char_boundary` and signals truncation via `fmt::Error`. - `FlushableFormatBuffer<F, N>` wraps `FormatBuffer` and auto-flushes via a closure when the buffer fills. Remaining content is flushed on drop. This allows arbitrarily long formatted output to be processed in fixed-size chunks. - `print.rs` WDM/KMDF path now formats into `FlushableFormatBuffer<_, 512>` and calls `DbgPrint` via `%s` using the resulting `CStr`. - Removed the `alloc` feature gate from the WDM/KMDF print path — `print!`/`println!` no longer require heap allocation and work at any IRQL where `DbgPrint` is valid. - Custom `Debug` impl on `FormatBuffer` shows only the used content, not stale bytes after `clear()`. ## Testing 29 tests covering: - basic write, `as_str()`, `as_c_str()` usage - overflow, multi-write, and exact-fit scenarios - empty writes and min-sized buffers (`N = 2`) - clear-then-shorter-write regression (NUL terminator correctness) - multi-byte UTF-8 char boundary handling in both buffer types - explicit `flush()` mid-stream and flush-on-drop - interior NUL truncation via `as_c_str()` - `compile_fail` doctest for `N < 2` ```pwsh cargo test --package wdk --lib --all-features -- fmt --nocapture ``` ## Notes / Follow-ups - `_print` ignores `fmt::write` errors — partial output is acceptable for debug printing. Errors from individual `Display` impls are silently dropped. - The old `DbgPrintBufWriter` stripped interior NUL bytes and continued printing. The new WDM/KMDF path truncates each chunk at the first NUL (via `as_c_str`). UMDF still strips NULs and prints the remainder. --------- Signed-off-by: Leon Durrenberger <leon.durrenberger@gmail.com> Co-authored-by: leon-xd <leondu@microsoft.com> Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Copilot AI review requested due to automatic review settings February 13, 2026 00:10

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leon-xd changed the title ~~feat!: stack-based allocator for WDK crate~~ feat!: stack-based allocator for wdk crate Feb 13, 2026

Copilot AI review requested due to automatic review settings February 19, 2026 19:04

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Comment thread crates/wdk/src/lib.rs Outdated

Comment thread crates/wdk/src/print.rs Outdated

Comment thread crates/wdk/src/fmt.rs

leon-xd added 7 commits February 19, 2026 11:09

merged changes from accidental copilot branch

fb1c4ef

passing cargo fmt

2c1c9a3

passing cargo clippy stable

e065e50

added gated compilation for fmt

704fc5b

removed redundant import

bc15583

fix print!/println! docs to match stack-buffer implementation

8e5f834

fix doctest path to use public re-export

74a4e88

leon-xd force-pushed the generic-stack-alloc branch from 8050685 to 74a4e88 Compare February 19, 2026 19:10

use full path for WdkFormatBuffer in _print

b96937d

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leon-xd marked this pull request as ready for review February 19, 2026 22:29

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Addressed PR comments

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Comment thread crates/wdk/src/fmt.rs

make as_cstr infallible, enforce N>0 at compile time

7a64ee9

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Comment thread crates/wdk/src/fmt.rs

wmmc88 previously approved these changes Apr 9, 2026

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Comment thread crates/wdk/Cargo.toml

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Comment thread crates/wdk/src/fmt.rs

Comment thread crates/wdk/src/fmt.rs

Comment thread examples/sample-kmdf-driver/src/lib.rs

leon-xd and others added 2 commits April 15, 2026 12:50

Merge branch 'main' into generic-stack-alloc

940410e

address PR review comments from wmmc88

0ac4bc9

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leon-xd added 2 commits April 16, 2026 10:41

bump MSRV to 1.87, reuse self.flush() in write_str

9d9fb65

use derive(Clone) for FormatBuffer

37ee393

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leon-xd added 2 commits April 16, 2026 10:54

fix MSRV to 1.91 (floor_char_boundary)

12636e6

Merge microsoft/main into generic-stack-alloc

bfc706d

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leon-xd enabled auto-merge April 17, 2026 21:30

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leon-xd added this pull request to the merge queue Apr 17, 2026

Merged via the queue into microsoft:main with commit 5748ea2 Apr 17, 2026
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leon-xd deleted the generic-stack-alloc branch April 17, 2026 23:55

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