[Bug ld/32003] Specifying --package-metadata might not be possible and is too fragile

[hjl.tools at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=32003

--- Comment #2 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Benjamin Drung from comment #0)
> In Ubuntu we enabled setting ELF package metadata for the Debian package
> that we build starting from Ubuntu 24.10 (oracular) on. Specifying the
> linker flag --package-metadata is not possible in a robust way. All tried
> approaches are either not working or too fragile:
> 
> 1. The comma in the JSON value is used to split the -Wl parameter specified
> for gcc:
> 
> ```
> $ echo "void main() { }" > test.c
> $ gcc '-Wl,--package-metadata={"type":"deb","os":"ubuntu"}' test.c 
> /usr/bin/ld: cannot find "os":"ubuntu"}: No such file or directory
> collect2: error: ld returned 1 exit status
> ```

This is a real issue.

> 2. The quotation marks in the JSON value are eaten by configure scripts and
> Makefiles. Example:
> 
> ```
> $ echo "void main() { }" > test.c
> $ printf 'test:\n\tgcc $(CFLAGS) test.c\n' > Makefile
> $ env CFLAGS='-Wl,--package-metadata={"type":"deb"}' make
> gcc -Wl,--package-metadata={"type":"deb"} test.c
> /usr/bin/ld: warning: --package-metadata={type:deb} does not contain valid
> JSON, ignoring: string or '}' expected near 'type'
> ```

The linker testcase has

"--package-metadata='{\"foo\":\"bar\"}'"

which is how this option should be used.

> 3. Add `-specs=<spec-file>` to the gcc linker flags. Then this spec file
> could construct the package metadata parameter. Example spec file:
> 
> ```
> $ cat /usr/share/dpkg/elf-package-metadata.specs
> *link:
> +
> --package-metadata={\"type\":\"deb\",\"os\":\"%:
> getenv(DEB_BUILD_OS_RELEASE_ID \",\"name\":\"%:getenv(DEB_SOURCE
> \",\"version\":\"%:getenv(DEB_VERSION
> \",\"architecture\":\"%:getenv(DEB_HOST_ARCH \"}))))
> ```
> 
> Issue with that approach: It requires the spec file to be around and the
> environment variables to be set. This will be the case during the package
> build, but not at a later stage. See https://launchpad.net/bugs/2071468 for
> examples where this breaks.
> 
> ## Proposed solution
> 
> Add support for an `--escaped-package-metadata` parameter to the linkers
> that takes a percent encoded (RFC 3986) parameter. Example:
> ```
> -Wl,--encoded-package-metadata,%7B%22type%22:%22deb%22%2C%22os%22:
> %22ubuntu%22%2C%22name%22:%22dpkg%22%2C%22version%22:%221.22.
> 6ubuntu15%22%2C%22architecture%22:%22amd64%22%7D
> ```

It is very much unreadable.  The main issue is that compiler drivers eat
comma.  Can we update linker to support an escape for comma which won't be
eaten by compiler drivers?

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