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warning::buildid - build-id verification failures


Because systemtap's script translation / execution stages may be executed at different times and places, it is sometimes necessary to verify certain invariants. One such invariant is that if a script was informed by translate-time analysis of executables, then those same executables need to be used at run time. This checking is done based upon the build-id, a binary hash that modern (post-2007) compilers/toolchains add as an NT_GNU_BUILD_ID ELF note to object files and executables. Use the readelf -n command to examine the build-ids of binaries, if you are interested.

Only scripts are sensitive to executables' build-ids: generally those that perform deep analysis of the binaries or their debuginfo. For example, scripts that place .function or .statement probes, or use stack backtrace-related tapset functions may be sensitive. Other scripts that rely only on process.mark or kernel.trace probes do not require debuginfo. See the DWARF DEBUGINFO section in the stapprobes(3stap) man page.

During translation, systemtap saves a copy of the relevant files' build-ids within the compiled modules. At run-time, the modules compare the saved ones to the actual run-time build-ids in memory. The warning message indicates that they did not match, so the module will decline placing a probe that was computed based upon obsolete data. This is important for safety, as placing them at an inappropriate address could crash the programs. However, this is not necessarily a fatal error, since probes unrelated to the mismatching binaries may operate.

A build-id mismatch could be caused by a few different situations. The main one is where the executable versions or architecture were different between the systemtap translation and execution times/places. For example, one may run a stap-server on a slightly different version of the OS distribution. Someone may have rebuilt a new kernel image, but preserved the previous version numbers. The kernel running on the workstation may be slightly different from the version being targeted - perhaps due to a pending kernel upgrade leaving different files on disk versus running in memory. If your OS distribution uses separate debuginfo packages, the split .debug files may not exactly match the main binaries.

To disable build-id verification warnings, if one is confident that they are an artefact of build accidents rather than a real mismatch, one might try the -DSTP_NO_BUILDID_CHECK option.