Created attachment 36989 [details] allow reading OLD argument to -fdebug-prefix-map from environment Work on the reproducible-builds project [0] has identified that build paths are one cause of output variation between builds. This changeset allows users to avoid this variation when building C objects with debug symbols, while leaving the default behavior unchanged. Background ---------- gcc includes the build path in any generated DWARF debugging symbols, specifically in DW_AT_comp_dir, but allows the embedded path to be changed via -fdebug-prefix-map. When -fdebug-prefix-map is used with the current build path, it removes the build path from DW_AT_comp_dir but places it instead in DW_AT_producer, so the reproducibility problem isn't resolved. When building software for binary redistribution, the actual build path on the build machine is irrelevant, and doesn't need to be exposed in the debug symbols. Resolution ---------- This patch extends the first argument to -fdebug-prefix-map ("old") to be able to read from the environment, which allows a packager to avoid embedded build paths in the debugging symbols with something like: export SOURCE_BUILD_DIR="$(pwd)" gcc -fdebug-prefix-map=\$SOURCE_BUILD_DIR=/usr/src Details ------- Specifically, if the first character of the "old" argument is a literal $, then gcc will treat it as an environment variable name, and use the value of the env var for prefix mapping. As a result, DW_AT_producer contains the literal envvar name, DW_AT_comp_dir contains the transformed build path, and the actual build path is not at all present in the generated object file. This has been tested successfully on amd64 machines, and i see no reason why it would be platform-specific. More discussion of alternate approaches considered and discarded in the development of this change can be found at [1] for those interested. Feedback welcome! [0] https://reproducible-builds.org [1] https://lists.alioth.debian.org/pipermail/reproducible-builds/Week-of-Mon-20151130/004051.html https://gcc.gnu.org/ml/gcc-patches/2015-12/msg01168.html
Created attachment 37001 [details] allow reading OLD argument to -fdebug-prefix-map from environment (using ENV: prefix) I'm attaching an updated patch that uses an "ENV:" prefix instead of a literal "$", because the "$" is messy to pass through a complex build chain without a lot of escaping. So the reproducible use case would be something like: export SOURCE_BUILD_DIR="$(pwd)" gcc -fdebug-prefix-map=ENV:SOURCE_BUILD_DIR=/usr/src
Created attachment 37003 [details] allow reading OLD argument to -fdebug-prefix-map from environment (using ENV: prefix) the v3 patch fixes coding conventions (thanks, Bernd Schmidt!)
Created attachment 37004 [details] allow reading OLD argument to -fdebug-prefix-map from environment (using ENV: prefix)
Created attachment 37005 [details] allow reading OLD argument to -fdebug-prefix-map from environment (using ENV: prefix)
Created attachment 37007 [details] ignore -fdebug-prefix-map when generating DW_AT_producer Here is an alternate approach (suggested by Bernd Schmidt): just avoid writing -fdebug-prefix-map to the DW_AT_producer field entirely.