Validate archive entries and stage inside the pack root

- Reject archive members that are absolute, escape via .., are special files, or link outside the destination, before anything is written

- Stage under the pack root so the swap is a same-filesystem rename, with an EXDEV copy fallback; propagate registry auth errors and validate dir_name

tests/fs_spec.lua

@@ -63,6 +63,85 @@ describe('swap', function()
     assert_match('cannot move new install into place', err)
     assert_eq({ 'old' }, vim.fn.readfile(root .. '/target/marker'))
   end)
+
+  -- Forced EXDEV: deterministic everywhere, no second mount required.
+  local function with_exdev_on(path, fn)
+    local uv = vim.uv or vim.loop
+    local real = uv.fs_rename
+    uv.fs_rename = function(from, to)
+      if from == path then
+        return nil, 'EXDEV: cross-device link not permitted: ' .. from .. ' -> ' .. to, 'EXDEV'
+      end
+      return real(from, to)
+    end
+    local ok, err = pcall(fn)
+    uv.fs_rename = real
+    if not ok then
+      error(err, 0)
+    end
+  end
+
+  it('falls back to a copy when staged and target are on different filesystems', function()
+    local root = tmpdir()
+    vim.fn.mkdir(root .. '/target', 'p')
+    vim.fn.writefile({ 'old' }, root .. '/target/marker')
+    vim.fn.mkdir(root .. '/staged/lua', 'p')
+    vim.fn.writefile({ 'new' }, root .. '/staged/lua/plug.lua')
+
+    with_exdev_on(root .. '/staged', function()
+      assert_true(fsmod.swap(root .. '/staged', root .. '/target'))
+    end)
+    assert_eq({ 'new' }, vim.fn.readfile(root .. '/target/lua/plug.lua'))
+    assert_true(not fsmod.exists(root .. '/target/marker'))
+    assert_true(not fsmod.exists(root .. '/target.vimpack-old'))
+    assert_true(not fsmod.exists(root .. '/target.vimpack-new'))
+    assert_true(not fsmod.exists(root .. '/staged'))
+  end)
+
+  it('restores the previous install when the cross-filesystem copy fails', function()
+    local root = tmpdir()
+    vim.fn.mkdir(root .. '/target', 'p')
+    vim.fn.writefile({ 'old' }, root .. '/target/marker')
+
+    local ok, err
+    with_exdev_on(root .. '/absent', function()
+      ok, err = fsmod.swap(root .. '/absent', root .. '/target')
+    end)
+    assert_true(not ok)
+    assert_match('cannot move new install into place', err)
+    assert_eq({ 'old' }, vim.fn.readfile(root .. '/target/marker'))
+    assert_true(not fsmod.exists(root .. '/target.vimpack-new'))
+  end)
+
+  it('copies a tree across a real filesystem boundary', function()
+    local root = tmpdir()
+    local other = otherfs_dir(root)
+    if not other then
+      return
+    end
+    vim.fn.mkdir(root .. '/staged/lua', 'p')
+    vim.fn.writefile({ 'new' }, root .. '/staged/lua/plug.lua')
+    vim.fn.mkdir(root .. '/staged/empty', 'p')
+
+    assert_true(fsmod.copy_tree(root .. '/staged', other .. '/copy'))
+    assert_eq({ 'new' }, vim.fn.readfile(other .. '/copy/lua/plug.lua'))
+    assert_true(fsmod.exists(other .. '/copy/empty'))
+    fsmod.rm_rf(other)
+  end)
+end)
+
+describe('safe_relative', function()
+  it('accepts a plain relative path', function()
+    assert_eq('lua/plug.lua', fsmod.safe_relative('./lua/plug.lua'))
+    assert_eq('lua', fsmod.safe_relative('lua/sub/..'))
+  end)
+
+  it('rejects absolute paths and escapes', function()
+    assert_nil(fsmod.safe_relative('/etc/passwd'))
+    assert_nil(fsmod.safe_relative('../evil'))
+    assert_nil(fsmod.safe_relative('a/../../evil'))
+    assert_nil(fsmod.safe_relative(''))
+  end)
 end)
 
 describe('extract', function()
@@ -102,4 +181,91 @@ describe('extract', function()
     assert_true(not result.ok)
     assert_match('extraction failed', result.err)
   end)
+
+  it('unpacks a zip', function()
+    local root = tmpdir()
+    vim.fn.mkdir(root .. '/src/plug/lua', 'p')
+    vim.fn.writefile({ 'return {}' }, root .. '/src/plug/lua/plug.lua')
+    vim.fn.system({ 'sh', '-c', 'cd ' .. vim.fn.shellescape(root .. '/src') .. ' && zip -qr ../plug.zip plug' })
+    if vim.v.shell_error ~= 0 then
+      return
+    end
+
+    local result = wait_extract(root .. '/plug.zip', root .. '/dest')
+    assert_true(result.ok, tostring(result.err))
+    assert_true(fsmod.exists(root .. '/dest/plug/lua/plug.lua'))
+  end)
+
+  it('refuses a tarball that climbs out of the destination', function()
+    local root = tmpdir()
+    vim.fn.mkdir(root .. '/src/plug', 'p')
+    vim.fn.writefile({ 'pwned' }, root .. '/src/plug/payload')
+    vim.fn.system({
+      'tar',
+      '-czf',
+      root .. '/evil.tar.gz',
+      '-C',
+      root .. '/src',
+      '--transform',
+      's|^plug|../evil|',
+      'plug/payload',
+    })
+
+    local result = wait_extract(root .. '/evil.tar.gz', root .. '/dest')
+    assert_true(not result.ok)
+    assert_match('refusing to extract', result.err)
+    assert_match('../evil/payload', result.err)
+    assert_true(not fsmod.exists(root .. '/evil'))
+    assert_true(not fsmod.exists(root .. '/dest/payload'))
+  end)
+
+  it('refuses a tarball holding an absolute path', function()
+    local root = tmpdir()
+    local victim = root .. '/abs-evil'
+    vim.fn.writefile({ 'pwned' }, victim)
+    vim.fn.system({ 'tar', '-cPzf', root .. '/abs.tar.gz', victim })
+
+    local result = wait_extract(root .. '/abs.tar.gz', root .. '/dest')
+    assert_true(not result.ok)
+    assert_match('refusing to extract', result.err)
+    assert_match('is absolute or escapes the destination directory', result.err)
+    assert_true(not fsmod.exists(root .. '/dest'))
+  end)
+
+  it('refuses a tarball whose symlink points outside the destination', function()
+    local root = tmpdir()
+    vim.fn.mkdir(root .. '/src/plug', 'p')
+    vim.fn.system({ 'ln', '-s', '/etc/passwd', root .. '/src/plug/escape' })
+    vim.fn.system({ 'tar', '-czf', root .. '/link.tar.gz', '-C', root .. '/src', 'plug' })
+
+    local result = wait_extract(root .. '/link.tar.gz', root .. '/dest')
+    assert_true(not result.ok)
+    assert_match('refusing to extract', result.err)
+    assert_match('plug/escape', result.err)
+    assert_true(not fsmod.exists(root .. '/dest/plug'))
+  end)
+
+  it('refuses a tarball whose symlink climbs out with ..', function()
+    local root = tmpdir()
+    vim.fn.mkdir(root .. '/src/plug/lua', 'p')
+    vim.fn.system({ 'ln', '-s', '../../../outside', root .. '/src/plug/lua/escape' })
+    vim.fn.system({ 'tar', '-czf', root .. '/rel.tar.gz', '-C', root .. '/src', 'plug' })
+
+    local result = wait_extract(root .. '/rel.tar.gz', root .. '/dest')
+    assert_true(not result.ok)
+    assert_match('refusing to extract', result.err)
+    assert_match('plug/lua/escape', result.err)
+  end)
+
+  it('allows a symlink that stays inside the destination', function()
+    local root = tmpdir()
+    vim.fn.mkdir(root .. '/src/plug/lua', 'p')
+    vim.fn.writefile({ 'return {}' }, root .. '/src/plug/lua/plug.lua')
+    vim.fn.system({ 'ln', '-s', 'lua/plug.lua', root .. '/src/plug/alias' })
+    vim.fn.system({ 'tar', '-czf', root .. '/ok.tar.gz', '-C', root .. '/src', 'plug' })
+
+    local result = wait_extract(root .. '/ok.tar.gz', root .. '/dest')
+    assert_true(result.ok, tostring(result.err))
+    assert_true(fsmod.exists(root .. '/dest/plug/alias'))
+  end)
 end)