RPATH: Support CMAKE_SYSROOT value of /

Fixes: #27882

Source/cmComputeLinkInformation.cxx

@@ -2266,7 +2266,7 @@ void cmComputeLinkInformation::GetRPath(std::vector<std::string>& runtimeDirs,
       // support or if using the link path as an rpath.
       if (use_build_rpath) {
         std::string d = ri;
-        if (!rootPath.empty() && cmHasPrefix(d, rootPath)) {
+        if (!rootPath.empty() && rootPath != "/" && cmHasPrefix(d, rootPath)) {
           d.erase(0, rootPath.size());
         } else if (cmNonempty(stagePath) && cmHasPrefix(d, *stagePath)) {
           d.erase(0, (*stagePath).size());
@@ -2297,7 +2297,8 @@ void cmComputeLinkInformation::GetRPath(std::vector<std::string>& runtimeDirs,
             !cmSystemTools::IsSubDirectory(ri, topSourceDir) &&
             !cmSystemTools::IsSubDirectory(ri, topBinaryDir)) {
           std::string d = ri;
-          if (!rootPath.empty() && cmHasPrefix(d, rootPath)) {
+          if (!rootPath.empty() && rootPath != "/" &&
+              cmHasPrefix(d, rootPath)) {
             d.erase(0, rootPath.size());
           } else if (cmNonempty(stagePath) && cmHasPrefix(d, *stagePath)) {
             d.erase(0, (*stagePath).size());

Tests/RunCMake/RuntimePath/RunCMakeTest.cmake

@@ -31,6 +31,8 @@ if(CMAKE_EXECUTABLE_FORMAT STREQUAL "ELF")
   run_RuntimePath(Genex)
   run_cmake_command(GenexCheck
     ${CMAKE_COMMAND} -Ddir=${RunCMake_BINARY_DIR}/Genex-build -P ${RunCMake_SOURCE_DIR}/GenexCheck.cmake)
+
+  run_RuntimePath(SysrootSlash)
 endif()
 
 block()

Tests/RunCMake/RuntimePath/SysrootSlash.cmake

@@ -0,0 +1,19 @@
+enable_language(C)
+
+# Setting CMAKE_SYSROOT to "/" must not strip the leading "/" from RPATH entries.
+set(CMAKE_SYSROOT "/")
+
+function(CheckRpath target rpath)
+  add_custom_command(
+    TARGET ${target}
+    POST_BUILD
+    COMMAND ${CMAKE_COMMAND} -Dfile=$<TARGET_FILE:${target}> -Drpath=${rpath}
+            -P "${CMAKE_CURRENT_SOURCE_DIR}/RelativeCheck.cmake"
+    VERBATIM
+  )
+endfunction()
+
+add_library(sysroot-slash-lib SHARED A.c)
+add_executable(sysroot-slash-exe main.c)
+target_link_libraries(sysroot-slash-exe sysroot-slash-lib)
+CheckRpath(sysroot-slash-exe "${CMAKE_CURRENT_BINARY_DIR}${cfg_dir}")