autoPatchelfHook: fix detection under crossSystem

In #84415, autoPatchelfHook was taught to use the correct path to the
readelf binary when a crossSystem is specified. Unfortunately, the
remainder of the functionality in the script depended on ldd, which only
reads ELF files of its own architecture. It has the further unfortunate
quality of not reporting any useful error, but rather that the file is
not a dynamic executable.

This change uses patchelf to directly analyze the DT_NEEDED tags in the
target files instead, which correctly works across architectures. It
also updates the use of objdump to be prefix-aware $OBJDUMP (which would
have been required in the PR mentioned above, but we never made it that
far into the script execution).

(cherry picked from commit b79483d2b72c8e349a8fa8a6e67e8061d82d6027)

pkgs/build-support/setup-hooks/auto-patchelf.sh

···
 declare -Ag autoPatchelfCachedDepsAssoc
 declare -ag autoPatchelfCachedDeps
 
-
 addToDepCache() {
     if [[ ${autoPatchelfCachedDepsAssoc[$1]+f} ]]; then return; fi
 
···
 declare -gi doneRecursiveSearch=0
 declare -g foundDependency
 
-getDepsFromSo() {
-    ldd "$1" 2> /dev/null | sed -n -e 's/[^=]*=> *\(.\+\) \+([^)]*)$/\1/p'
+getDepsFromElfBinary() {
+    # NOTE: This does not use runPatchelf because it may encounter non-ELF
+    # files. Caller is expected to check the return code if needed.
+    patchelf --print-needed "$1" 2> /dev/null
 }
 
-populateCacheWithRecursiveDeps() {
-    local so found foundso
-    for so in "${autoPatchelfCachedDeps[@]}"; do
-        for found in $(getDepsFromSo "$so"); do
-            local base="${found##*/}"
-            local soname="${base%.so*}"
-            for foundso in "${found%/*}/$soname".so*; do
+getRpathFromElfBinary() {
+    # NOTE: This does not use runPatchelf because it may encounter non-ELF
+    # files. Caller is expected to check the return code if needed.
+    local rpath
+    rpath="$(patchelf --print-rpath "$1" 2> /dev/null)" || return $?
+
+    local IFS=':'
+    printf "%s\n" $rpath
+}
+
+populateCacheForDep() {
+    local so="$1"
+    local rpath found
+    rpath="$(getRpathFromElfBinary "$so")" || return 1
+
+    for found in $(getDepsFromElfBinary "$so"); do
+        local rpathElem
+        for rpathElem in $rpath; do
+            # Ignore empty element or $ORIGIN magic variable which should be
+            # deterministically resolved by adding this package's library
+            # files early anyway.
+            #
+            # shellcheck disable=SC2016
+            # (Expressions don't expand in single quotes, use double quotes for
+            # that.)
+            if [[ -z "$rpathElem" || "$rpathElem" == *'$ORIGIN'* ]]; then
+                continue
+            fi
+
+            local soname="${found%.so*}"
+            local foundso=
+            for foundso in "$rpathElem/$soname".so*; do
                 addToDepCache "$foundso"
             done
+
+            # Found in this element of the rpath, no need to check others.
+            if [ -n "$foundso" ]; then
+                break
+            fi
         done
     done
+
+    # Not found in any rpath element.
+    return 1
+}
+
+populateCacheWithRecursiveDeps() {
+    # Dependencies may add more to the end of this array, so we use a counter
+    # with while instead of a regular for loop here.
+    local -i i=0
+    while [ $i -lt ${#autoPatchelfCachedDeps[@]} ]; do
+        populateCacheForDep "${autoPatchelfCachedDeps[$i]}"
+        i=$i+1
+    done
 }
 
 getSoArch() {
-    objdump -f "$1" | sed -ne 's/^architecture: *\([^,]\+\).*/\1/p'
+    $OBJDUMP -f "$1" | sed -ne 's/^architecture: *\([^,]\+\).*/\1/p'
 }
 
 # NOTE: If you want to use this function outside of the autoPatchelf function,
···
         fi
     fi
 
+    local libcLib
+    libcLib="$(< "$NIX_CC/nix-support/orig-libc")/lib"
+
     echo "searching for dependencies of $toPatch" >&2
 
-    # We're going to find all dependencies based on ldd output, so we need to
-    # clear the RPATH first.
-    runPatchelf --remove-rpath "$toPatch"
-
-    # If the file is not a dynamic executable, ldd/sed will fail,
-    # in which case we return, since there is nothing left to do.
     local missing
-    missing="$(
-        ldd "$toPatch" 2> /dev/null | \
-            sed -n -e 's/^[\t ]*\([^ ]\+\) => not found.*/\1/p'
-    )" || return 0
+    missing="$(getDepsFromElfBinary "$toPatch")" || return 0
 
     # This ensures that we get the output of all missing dependencies instead
     # of failing at the first one, because it's more useful when working on a
     # new package where you don't yet know its dependencies.
 
     for dep in $missing; do
+        # Check whether this library exists in libc. If so, we don't need to do
+        # any futher searching -- it will be resolved correctly by the linker.
+        if [ -f "$libcLib/$dep" ]; then
+            continue
+        fi
+
         echo -n "  $dep -> " >&2
         if findDependency "$dep" "$(getSoArch "$toPatch")"; then
             rpath="$rpath${rpath:+:}${foundDependency%/*}"
···
     done
 
     while IFS= read -r -d '' file; do
-    addToDepCache "$file"
+        addToDepCache "$file"
     done <  <(find "$@" "${findOpts[@]}" \! -type d \
             \( -name '*.so' -o -name '*.so.*' \) -print0)
 }
···
       segmentHeaders="$(LANG=C $READELF -l "$file")"
       # Skip if the ELF file doesn't have segment headers (eg. object files).
       # not using grep -q, because it can cause Broken pipe
-      [ -n "$(echo "$segmentHeaders" | grep '^Program Headers:')" ] || continue
+      grep -q '^Program Headers:' <<<"$segmentHeaders" || continue
       if isExecutable "$file"; then
           # Skip if the executable is statically linked.
-          [ -n "$(echo "$segmentHeaders" | grep "^ *INTERP\\>")" ] || continue
+          grep -q "^ *INTERP\\>" <<<"$segmentHeaders" || continue
       fi
       # Jump file if patchelf is unable to parse it
       # Some programs contain binary blobs for testing,
···
 # So what we do here is basically run in postFixup and emulate the same
 # behaviour as fixupOutputHooks because the setup hook for patchelf is run in
 # fixupOutput and the postFixup hook runs later.
+#
+# shellcheck disable=SC2016
+# (Expressions don't expand in single quotes, use double quotes for that.)
 postFixupHooks+=('
     if [ -z "${dontAutoPatchelf-}" ]; then
         autoPatchelf -- $(for output in $outputs; do