Implement setgid and friends on Linux

[qmuntal]

What issue is this addressing?

#343

What type of issue is this addressing?

bug

What this PR does | solves

Implement and linkname the following functions:

//go:linkname _cgo_libc_setegid syscall.cgo_libc_setegid
//go:linkname _cgo_libc_seteuid syscall.cgo_libc_seteuid
//go:linkname _cgo_libc_setgid syscall.cgo_libc_setgid
//go:linkname _cgo_libc_setregid syscall.cgo_libc_setregid
//go:linkname _cgo_libc_setresgid syscall.cgo_libc_setresgid
//go:linkname _cgo_libc_setresuid syscall.cgo_libc_setresuid
//go:linkname _cgo_libc_setreuid syscall.cgo_libc_setreuid
//go:linkname _cgo_libc_setuid syscall.cgo_libc_setuid
//go:linkname _cgo_libc_setgroups syscall.cgo_libc_setgroups

Note that almost all new code is autogenerated. The generator has been extended to support os-specific functions, which has added a bit of complexity. A simpler alternative would be to implement the new functions for all supported OSes, and only linkname them on Linux.

[Copilot]

Pull Request Overview

This PR implements setgid and related system call wrappers on Linux to fix issue #343. It provides purego implementations for 9 Linux-specific syscalls that mirror glibc functions with POSIX semantics when CGO is disabled.

• Implements 9 new Linux-specific functions: setegid, seteuid, setgid, setregid, setresgid, setresuid, setreuid, setuid, and setgroups
• Extends the code generator to support OS-specific functions with proper linkname integration
• Adds comprehensive tests to verify the syscall wrappers work correctly

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
syscall_linux_test.go	Test file verifying syscall wrappers return expected EPERM errors for non-root users
internal/fakecgo/symbols_linux.go	Core implementation with syscall wrappers, trampolines, and linkname declarations
internal/fakecgo/linux.go	Linux-specific linkname bindings to syscall package and errno handling
internal/fakecgo/fakecgo.go	Common helper functions for argument handling moved from symbols.go
internal/fakecgo/gen.go	Enhanced code generator supporting OS-specific functions and trampoline generation
internal/fakecgo/ztrampolines_*.s	Generated assembly trampolines for amd64, arm64, and loong64 architectures
internal/fakecgo/symbols.go	Removed duplicate function declarations now in fakecgo.go
README.md	Updated documentation to reflect new linux.go file

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[hajimehoshi]

TestSetuidEtc/Setregid seems to get stuck.

[qmuntal]

TestSetuidEtc/Setregid seems to get stuck.

It does. Unfortunately I can't reproduce the failure locally. Will fix it (sorry for the noise).

[hajimehoshi]

It does. Unfortunately I can't reproduce the failure locally. Will fix it (sorry for the noise).

I think you can run the tests in your GitHub repository instead of here.