Aliases for standard C functions (memset/memcpy/memcmp)

[rmikielis]

JIRA: RTOS-1041

Description

GCC requires free standing environments to provide few standard C functions (ex. memset). These aliases redirect such calls to HAL library functions (hal_memset)

Motivation and Context

Required by GCC

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (refactoring, style fixes, git/CI config, submodule management, no code logic changes)

How Has This Been Tested?

• Already covered by automatic testing.
• New test added: (add PR link here).
• Tested by hand on: (ia32 qemu, arm qemu).

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work (list the PRs, preferably in merge-order).
• I will merge this PR by myself when appropriate.

[gemini-code-assist]

Hello @rmikielis, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

Summary of Changes

As gemini-code-assist, I've reviewed this pull request. The primary goal, as stated in the title and description, is to satisfy a requirement from GCC for freestanding environments to provide implementations of standard C functions like memset, memcpy, and memcmp. This pull request addresses this by adding linker flags to the build process that create symbol aliases. These aliases redirect calls to the standard function names (memset, memcpy, memcmp) to their corresponding implementations provided elsewhere, specifically hal_memset, hal_memcpy, and hal_memcmp. This change is categorized as a chore and has been hand-tested on ia32 and arm qemu.

Highlights

• Standard C Function Aliases: Introduces linker-level aliases for the standard C library functions memset, memcpy, and memcmp.
• GCC Freestanding Requirement: Satisfies a requirement from the GCC compiler when building for freestanding environments, ensuring necessary standard functions are available via expected names.
• Build System Change: Implements the aliasing by adding -Wl,--defsym linker flags within the Makefile.

Changelog

• Makefile
  • Added a line to the LDFLAGS variable to include linker flags (-Wl,--defsym=...) that create aliases for memcpy, memcmp, and memset, pointing them to hal_memcpy, hal_memcmp, and hal_memset respectively. This change is added around line 32.

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---

Symbols link the code,
Aliases point the way now,
Functions find their home.

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[agkaminski]

Nice solution! Does it work? 😄 Have you tried e.g. assigning struct to force use of memcpy?

[gemini-code-assist]

Code Review

This pull request introduces aliases for standard C functions (memset, memcpy, memcmp) to their HAL counterparts, which is required by GCC in freestanding environments. The use of LDFLAGS with -Wl,--defsym is an appropriate approach. However, there's a critical concern regarding the overlap safety of memcpy and the different hal_memcpy implementations across architectures.

Summary of Findings

• Critical Risk: memcpy Overlap Safety: The hal_memcpy implementation for ia32 (and potentially riscv64) isn't safe for overlapping memory regions. Aliasing memcpy to it can lead to data corruption. Either make all hal_memcpy instances overlap-safe or provide and alias a separate, overlap-safe memmove (and its hal_memmove implementation).

Merge Readiness

Due to the critical concern regarding memcpy's behavior with overlapping memory regions and the current hal_memcpy implementation on ia32 (and potentially other platforms), I recommend that this pull request not be merged until this issue is addressed. Resolving this is essential to prevent potential data corruption bugs.

I am not authorized to approve pull requests, but based on this review, changes are requested before this PR can be considered safe for merging. Please ensure further review and approval from authorized maintainers after addressing the concerns.

[gemini-code-assist]

While aliasing is necessary, there's a critical issue concerning memcpy's behavior with overlapping memory regions and the current hal_memcpy implementations.

• The C standard memcpy has undefined behavior for overlapping source/destination regions. memmove should be used when overlap is possible.
• The aarch64 hal_memcpy (in hal/aarch64/_memcpy.S) handles overlaps, acting like memmove. However, the ia32 hal_memcpy (in hal/ia32/string.c) uses a forward-only copy (rep movsl/movsb with cld), which is not safe for all overlap scenarios (e.g., dst > src). The riscv64 hal_memcpy also doesn't explicitly guarantee overlap safety.
• Aliasing memcpy to a hal_memcpy that isn't universally overlap-safe (especially on ia32) introduces a high risk of data corruption if the compiler generates memcpy calls or if developers use memcpy for operations that might overlap.

To address this, consider these options:

1. Option A (Make hal_memcpy universally overlap-safe):
   Update all hal_memcpy implementations (especially for ia32 and riscv64, if affected) to handle overlaps correctly, similar to memmove. If this is done, consider aliasing memmove to this now-safe hal_memcpy:
   LDFLAGS += -Wl,--defsym=memcpy=hal_memcpy,--defsym=memcmp=hal_memcmp,--defsym=memset=hal_memset,--defsym=memmove=hal_memcpy
   (Assuming hal_memcpy becomes fully overlap-safe on all platforms).

2. Option B (Provide a separate, safe hal_memmove - Recommended):
   Keep hal_memcpy non-overlap-safe. Provide a separate, overlap-safe hal_memmove function for all architectures, and alias memmove to it. This is crucial because code might rely on memmove for overlap safety. The change would be:
   LDFLAGS += -Wl,--defsym=memcpy=hal_memcpy,--defsym=memcmp=hal_memcmp,--defsym=memset=hal_memset,--defsym=memmove=hal_memmove
   (This requires hal_memmove to be implemented correctly and be overlap-safe across all platforms).

Clarify the intended strategy for memory operation safety and ensure a mechanism for safe overlapping memory copies (memmove semantics) is reliably available across all target architectures. Without this, the change poses a significant risk of data integrity issues.

LDFLAGS += -Wl,--defsym=memcpy=hal_memcpy,--defsym=memcmp=hal_memcmp,--defsym=memset=hal_memset,--defsym=memmove=hal_memmove

[github-actions]

Unit Test Results

7 958 tests  ±0   7 434 ✅ ±0   40m 48s ⏱️ +4s
  470 suites ±0     524 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit ce38458. ± Comparison against base commit 4584c03.

[rmikielis]

Nice solution! Does it work? 😄 Have you tried e.g. assigning struct to force use of memcpy?

I was able to force GCC to submit memset and memcpy, and aliases worked fine in final kernel .elf (redirections to hal_* can be seen). I added bit more under https://confluence.phoenix-rtos.com/display/RTOS/Standard+C+library+calls :)

[agkaminski]

@Darchiv @nalajcie I vote for merging this. Pls fix commit message though (and add JIRA ID)