From c4772d30bfbed6cfbfdf92066990b5c6dc4065bb Mon Sep 17 00:00:00 2001 From: Cherry Mui Date: Tue, 14 May 2024 00:01:49 -0400 Subject: cmd/link: disallow pull-only linknames As mentioned in CL 584598, linkname is a mechanism that, when abused, can break API integrity and even safety of Go programs. CL 584598 is a first step to restrict the use of linknames, by implementing a blocklist. This CL takes a step further, tightening up the restriction by allowing linkname references ("pull") only when the definition side explicitly opts into it, by having a linkname on the definition (possibly to itself). This way, it is at least clear on the definition side that the symbol, despite being unexported, is accessed outside of the package. Unexported symbols without linkname can now be actually private. This is similar to the symbol visibility rule used by gccgo for years (which defines unexported non-linknamed symbols as C static symbols). As there can be pull-only linknames in the wild that may be broken by this change, we currently only enforce this rule for symbols defined in the standard library. Push linknames are added in the standard library to allow things build. Linkname references to external (non-Go) symbols are still allowed, as their visibility is controlled by the C symbol visibility rules and enforced by the C (static or dynamic) linker. Assembly symbols are treated similar to linknamed symbols. This is controlled by -checklinkname linker flag, currently not enabled by default. A follow-up CL will enable it by default. Change-Id: I07344f5c7a02124dbbef0fbc8fec3b666a4b2b0e Reviewed-on: https://go-review.googlesource.com/c/go/+/585358 LUCI-TryBot-Result: Go LUCI Reviewed-by: Than McIntosh Reviewed-by: Russ Cox --- src/internal/cpu/cpu.go | 10 ++++++++++ 1 file changed, 10 insertions(+) (limited to 'src/internal/cpu/cpu.go') diff --git a/src/internal/cpu/cpu.go b/src/internal/cpu/cpu.go index d794e53cee..9be280c6ba 100644 --- a/src/internal/cpu/cpu.go +++ b/src/internal/cpu/cpu.go @@ -6,6 +6,8 @@ // used by the Go standard library. package cpu +import _ "unsafe" // for linkname + // DebugOptions is set to true by the runtime if the OS supports reading // GODEBUG early in runtime startup. // This should not be changed after it is initialized. @@ -121,6 +123,14 @@ var S390X struct { _ CacheLinePad } +// CPU feature variables are accessed by assembly code in various packages. +//go:linkname X86 +//go:linkname ARM +//go:linkname ARM64 +//go:linkname MIPS64X +//go:linkname PPC64 +//go:linkname S390X + // Initialize examines the processor and sets the relevant variables above. // This is called by the runtime package early in program initialization, // before normal init functions are run. env is set by runtime if the OS supports -- cgit v1.3