Absolutely feasible—you can definitely track down that shared ancestor commit (the "minimum common denominator" you’re targeting) using the Git tools you already have. This will let you cleanly separate the company’s custom changes from upstream updates made after the fork. Here’s a step-by-step breakdown of how to do it:

• Set up your upstream remote
  First, add the official Linux kernel repository as a remote in your local Git repo if you haven’t already:

  git remote add upstream https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
  git fetch upstream
  

  This pulls in all the upstream commit history so Git can compare it to your company’s repo.

• Identify the commit for your company's kernel snapshot
  If your snapshot is already part of your company’s Git history, grab its commit hash (let’s call it company-snap-hash). If you need to import the snapshot as a new commit, check out a clean branch, replace the files with the snapshot, then run:

  git add . && git commit -m "Import company kernel snapshot"
  

• Find the nearest common ancestor with git merge-base
  The core tool here is git merge-base, which finds the most recent commit present in both your company’s snapshot history and the upstream kernel tree. Run:

  git merge-base company-snap-hash upstream/master
  

  (Replace upstream/master with the appropriate upstream branch if you’re targeting a stable release, like upstream/linux-6.1.y.)
  The output will be a commit hash—this is your best candidate for the fork point.

• Validate the candidate commit
  To confirm this is the right divergence point:

  • List all commits the company made after the fork with:

    git log --oneline company-snap-hash ^<merge-base-hash>
    

  • List all upstream commits after the fork with:

    git log --oneline upstream/master ^<merge-base-hash>
    

  • Compare the snapshot directly to the ancestor commit to verify it contains only the company’s changes:

    git diff <merge-base-hash> company-snap-hash
    

• Handle edge cases (like repeated upstream merges)
  If the company periodically merged upstream changes into their fork, there might be multiple common ancestors. Use git merge-base --all to list all of them, and run:

  git log --graph --oneline --decorate company-snap-hash upstream/master
  

  to visualize the commit tree and pick the earliest fork point before any subsequent upstream merges.

This approach will give you a solid (often exact) estimate of the divergence point. Git’s built-in ancestry tracking makes this kind of comparison straightforward with the right commands.

内容的提问来源于stack exchange，提问作者ARF