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invalid flag in #cgo CFLAGS
This page describes the background for build errors like
invalid flag in #cgo CFLAGS and what you can do about them.
CVE-2018-6574 described a potential security violation in the go tool: running
go get downloads and builds Go code from the Internet, Go code that uses cgo can specify options to pass to the compiler, so careful use of
-fplugin can cause
go get to execute arbitrary code. While it is difficult to block every possible way that the compiler might be attacked, we have chosen to block the obvious ones.
As described at issue 23672, this is done by using a whitelist of compiler/linker options that are permitted during
go build, and friends. When cgo code tries to use to pass an option that is not on the whitelist, the go tool will report an error
invalid flag in #cgo CFLAGS (or
pkg-config --ldflags, and so forth).
This whitelist is new in releases 1.8.7, 1.9.4, and 1.10, and all subsequent releases.
What can I do?
If this happens to you, and the option is benign, you should do two things:
- Set the environment variable
CGO_CXXFLAGS_ALLOW, and so forth) to a regexp that matches the option.
- File a bug requesting that the option be added to the whitelist. Be sure to include the complete error message and, if possible, a description of the code you are building.
Why not use a blacklist?
Because if some new unsafe option is added to a compiler, all existing Go releases will become immediately vulnerable.
Why not get a complete list of compiler options and whitelist all of them?
Because there are hundreds of options, and there is no clear way to get a complete list. Many compiler and linker options are target dependent, and thus only reported on specific platforms or in specific configurations. The documentation is known to be incomplete.