[linux-6.6.y]kbuild: Avoid weak external linkage where possible

[AaronDot]

This patchset is backported from upstream to avoid weak external linkage where possible.

Summary by Sourcery

Replace weak external linkage for linker-provided kernel symbols with stronger definitions and corresponding preliminary linker script symbols, and adjust related BTF and notes handling accordingly.

Enhancements:

• Remove __weak annotations from kallsyms, BTF, and .notes section extern symbol declarations and rely on proper linker-provided definitions instead.
• Introduce preliminary symbol definitions in vmlinux.lds.h using PROVIDE() and hook them into the RO data section layout to satisfy the first link stage without weak linkage.
• Drop redundant runtime BUG_ON checks in kallsyms that assumed missing symbol tables could occur only on misbuilt kernels and are now guaranteed by the link.
• Tighten BTF handling by gating vmlinux BTF parsing on CONFIG_DEBUG_INFO_BTF and simplifying zero-length BTF checks in sysfs exposure.

[sourcery-ai]

Reviewer's Guide

Backports an upstream change to avoid weak external linkage by replacing __weak extern declarations with strong externs and providing preliminary symbol definitions in the linker script, plus minor behavioral adjustments where weak linkage was previously relied upon.

Sequence diagram for reading vmlinux BTF via sysfs using strong __start_BTF/__stop_BTF

sequenceDiagram
  actor User
  participant Sysfs as sysfs_btf_vmlinux
  participant Kernel as kernel_btf

  User->>Sysfs: open /sys/kernel/btf/vmlinux
  Sysfs->>Kernel: btf_vmlinux_init()
  Kernel->>Kernel: size = __stop_BTF - __start_BTF
  Kernel->>Kernel: if size == 0 return 0
  Kernel->>Kernel: create kobject and bin_attribute
  Kernel-->>Sysfs: initialization result

  User->>Sysfs: read()
  Sysfs->>Kernel: btf_vmlinux_read(file, kobj, attr, buf, off, count)
  Kernel->>Kernel: compute offset in [__start_BTF, __stop_BTF)
  Kernel-->>Sysfs: copy BTF bytes into buf
  Sysfs-->>User: return BTF data

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Flow diagram for btf_parse_vmlinux with CONFIG_DEBUG_INFO_BTF gating

flowchart TD
  start([Start btf_parse_vmlinux])
  checkConfig{CONFIG_DEBUG_INFO_BTF enabled?}
  returnNoEnt["Return ERR_PTR(-ENOENT)"]
  allocEnv["kzalloc env"]
  allocFail{env is NULL?}
  returnNoMem["Return ERR_PTR(-ENOMEM)"]
  parseBTF["Parse vmlinux BTF using __start_BTF and __stop_BTF"]
  done([Return btf or error])

  start --> checkConfig
  checkConfig -->|No| returnNoEnt
  checkConfig -->|Yes| allocEnv
  allocEnv --> allocFail
  allocFail -->|Yes| returnNoMem
  allocFail -->|No| parseBTF
  parseBTF --> done

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File-Level Changes

Change	Details	Files
Replace weak kallsyms extern declarations with strong externs and rely on linker script PROVIDE() symbols for the first link stage.	Remove __weak and section attributes from kallsyms-related extern declarations in kallsyms_internal.h Introduce PRELIMINARY_SYMBOL_DEFINITIONS macro in vmlinux.lds.h using PROVIDE() to create placeholder kallsyms symbols for the first link pass Invoke PRELIMINARY_SYMBOL_DEFINITIONS at the start of the RO_DATA section so preliminary kallsyms symbols are always available during initial linking	kernel/kallsyms_internal.h include/asm-generic/vmlinux.lds.h
Stop using weak BTF section boundary symbols and adjust logic to no longer depend on them being NULL when absent.	Change __start_BTF and __stop_BTF declarations from weak to strong externs in BPF BTF-related code Guard btf_parse_vmlinux with a CONFIG_DEBUG_INFO_BTF check that returns -ENOENT when BTF is disabled, instead of relying on weak symbols Simplify sysfs BTF init logic to only check for zero-sized BTF data rather than testing __start_BTF for NULL	kernel/bpf/btf.c kernel/bpf/sysfs_btf.c
Stop using weak notes section boundary symbols, using strong externs instead, in both ksysfs and build-id initialization.	Change __start_notes and __stop_notes from weak to strong externs in ksysfs notes export Change __start_notes and __stop_notes from weak to strong externs in init_vmlinux_build_id Retain build-id and /sys/kernel/notes behavior while no longer depending on weak external linkage for notes section symbols	kernel/ksysfs.c lib/buildid.c
Remove now-unnecessary runtime BUG_ON checks that validated presence of kallsyms tables at boot.	Delete BUG_ON checks in get_symbol_pos that verified kallsyms_addresses or kallsyms_offsets were non-NULL, since the symbols are now always defined by the linker	kernel/kallsyms.c

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[deepin-ci-robot]

Hi @AaronDot. Thanks for your PR.

I'm waiting for a deepin-community member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[sourcery-ai]

Hey there - I've reviewed your changes - here's some feedback:

• Consider centralizing the __start_notes/__stop_notes declarations in a common header instead of repeating extern declarations in both ksysfs.c and buildid.c to keep these linker-provided symbols consistent across users.
• The PRELIMINARY_SYMBOL_DEFINITIONS macro is injected into RO_DATA(align); if any of these symbols ever need a different placement or alignment from generic rodata, it might be cleaner to hook them into a dedicated linker script block rather than piggybacking on the rodata section.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider centralizing the `__start_notes`/`__stop_notes` declarations in a common header instead of repeating `extern` declarations in both ksysfs.c and buildid.c to keep these linker-provided symbols consistent across users.
- The `PRELIMINARY_SYMBOL_DEFINITIONS` macro is injected into `RO_DATA(align)`; if any of these symbols ever need a different placement or alignment from generic rodata, it might be cleaner to hook them into a dedicated linker script block rather than piggybacking on the rodata section.

---

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[Copilot]

Pull request overview

This PR backports upstream changes to eliminate weak external linkage for linker-provided kernel symbols, replacing it with a more robust approach using preliminary symbol definitions. The motivation is to avoid relying on weak external linkage, which requires a Global Offset Table (GOT) when used in position-independent code, improving both code efficiency and reliability.

Key Changes

• Introduces PRELIMINARY_SYMBOL_DEFINITIONS macro in the linker script that uses PROVIDE() to define kallsyms symbols for the first link pass
• Removes __weak annotations from kallsyms, BTF, and .notes section symbol declarations across multiple files
• Removes redundant runtime BUG_ON checks in kallsyms that are no longer needed since symbols are guaranteed to exist

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
include/asm-generic/vmlinux.lds.h	Adds PRELIMINARY_SYMBOL_DEFINITIONS macro with PROVIDE() directives for kallsyms symbols and integrates it into RO_DATA section
kernel/kallsyms_internal.h	Removes __weak annotations from all kallsyms extern declarations and updates comments to explain the new PROVIDE() approach
kernel/kallsyms.c	Removes BUG_ON checks for kallsyms_addresses and kallsyms_offsets, now guaranteed to exist by linker script
kernel/bpf/btf.c	Removes __weak from __start_BTF/__stop_BTF and adds CONFIG_DEBUG_INFO_BTF guard to btf_parse_vmlinux()
kernel/bpf/sysfs_btf.c	Removes __weak from __start_BTF/__stop_BTF and simplifies null check to only test size (symbols guaranteed when CONFIG_DEBUG_INFO_BTF is set)
lib/buildid.c	Removes __weak annotations from __start_notes/__stop_notes declarations
kernel/ksysfs.c	Removes __weak annotations from __start_notes/__stop_notes declarations

---

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[Avenger-285714]

/approve

[Avenger-285714]

/ok-to-test

[deepin-ci-robot]

deepin pr auto review

这是一个关于Linux内核链接脚本和符号处理的改进补丁。让我从多个角度分析这个变更：

1. 语法逻辑分析：

• 移除了大量的__weak属性声明，改为使用PROVIDE()指令在链接脚本中提供符号的预定义值
• 新增的PRELIMINARY_SYMBOL_DEFINITIONS宏为kallsyms相关符号提供了初始定义
• 变更后的代码逻辑更加清晰，避免了弱符号可能带来的不确定性

2. 代码质量改进：

• 代码结构更加清晰，将符号定义集中管理
• 注释更加完善，解释了变更的原因和目的
• 统一了符号声明方式，提高了代码一致性

3. 性能优化：

• 移除弱符号(__weak)可以避免运行时的符号解析开销
• 使用PROVIDE()指令可以在链接时确定符号值，而不是运行时
• 减少了对GOT(Global Offset Table)的依赖，特别是在位置无关代码中

4. 安全性提升：

• 避免了弱符号可能带来的符号劫持风险
• 通过链接阶段的符号定义，确保了符号值的确定性
• 减少了运行时的动态符号解析，降低了攻击面

建议和改进意见：

1. 文档完善：

/*
 * PRELIMINARY_SYMBOL_DEFINITIONS - 为kallsyms相关符号提供预定义值
 * 
 * 这些符号在第一次链接时可能还不存在，但在最终链接时保证存在。
 * 使用PROVIDE()提供预定义值可以避免使用弱外部符号，特别是在
 * 位置无关代码中可以避免对GOT的依赖。
 */

2. 错误处理增强：

#ifdef CONFIG_DEBUG_INFO_BTF
if (!__start_BTF || bin_attr_btf_vmlinux.size == 0) {
    pr_warn("BTF information not available\n");
    return -EINVAL;
}
#endif

3. 类型安全改进：

/* 使用更具体的类型定义 */
extern const unsigned long kallsyms_addresses[] __attribute__((aligned(sizeof(unsigned long))));

4. 编译时检查：

/* 添加编译时检查确保符号对齐 */
#ifdef CONFIG_DEBUG_INFO_BTF
_Static_assert(__alignof__(__start_BTF) >= 8, "BTF symbols must be 8-byte aligned");
#endif

5. 性能优化建议：

/* 考虑使用likely/unlikely提示优化热点路径 */
if (likely(bin_attr_btf_vmlinux.size > 0)) {
    /* 处理正常情况 */
}

这些变更总体上是积极的，提高了代码的可靠性、性能和安全性。建议在后续版本中考虑上述改进建议，进一步提升代码质量。

[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Avenger-285714, opsiff

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