From 6158b58569fd099989ceba00c30a56d130faa837 Mon Sep 17 00:00:00 2001
From: Emery Hemingway <ehmry@posteo.net>
Date: Wed, 12 Oct 2022 22:01:01 -0500
Subject: [PATCH] readelferislinks utility for parsing ERIS ELF notes

---
 doc/deferred-linking.md                       |  2 +-
 overlay/default.nix                           |  3 ++
 packages/readelferislinks/default.nix         | 17 +++++++
 .../readelferislinks/src/readelferislinks.nim | 47 +++++++++++++++++++
 4 files changed, 68 insertions(+), 1 deletion(-)
 create mode 100644 packages/readelferislinks/default.nix
 create mode 100644 packages/readelferislinks/src/readelferislinks.nim

diff --git a/doc/deferred-linking.md b/doc/deferred-linking.md
index ebaf5f5..ec2c7dc 100644
--- a/doc/deferred-linking.md
+++ b/doc/deferred-linking.md
@@ -21,4 +21,4 @@ The format of the CBOR data within the `.note.eris-links` section is a the self-
 #6.55799({ * tstr => #6.276(bstr) })
 ```
 
-At this time of writing there is no utility for dumping the note into a human readable format.
+This repository contains a `readelferislinks` utility for extracting the note into a textual format.
diff --git a/overlay/default.nix b/overlay/default.nix
index 84f0869..3c103e8 100644
--- a/overlay/default.nix
+++ b/overlay/default.nix
@@ -214,6 +214,9 @@ in nullPkgs // {
     # Patch to fix a bug in rewriting the .dynstr section.
   ] prev.patchelf;
 
+  readelferislinks =
+    final.nimPackages.callPackage ../packages/readelferislinks { };
+
   rsync = overrideHost {
     enableACLs = false;
     popt = null;
diff --git a/packages/readelferislinks/default.nix b/packages/readelferislinks/default.nix
new file mode 100644
index 0000000..2372c92
--- /dev/null
+++ b/packages/readelferislinks/default.nix
@@ -0,0 +1,17 @@
+{ lib, buildNimPackage, cbor, eris }:
+
+buildNimPackage rec {
+  pname = "readelferislinks";
+  version = "20221012";
+  nimBinOnly = true;
+  src = ./src;
+  preConfigure = ''
+    cat << EOF > ${pname}.nimble
+    bin = @["${pname}"]
+    EOF
+  '';
+  buildInputs = [ cbor eris ];
+  meta = {
+    description = "Utility for parsing ERIS links within an ELF binary";
+  };
+}
diff --git a/packages/readelferislinks/src/readelferislinks.nim b/packages/readelferislinks/src/readelferislinks.nim
new file mode 100644
index 0000000..594615f
--- /dev/null
+++ b/packages/readelferislinks/src/readelferislinks.nim
@@ -0,0 +1,47 @@
+# SPDX-FileCopyrightText: ☭ 2022 Emery Hemingway
+# SPDX-License-Identifier: GPL-3.0-or-later.txt
+
+import std/[os, streams, tables]
+import cbor, eris
+
+proc atChar(strm: Stream; c: char): bool =
+  if strm.atEnd:
+    quit "reached end of stream without finding ELF note"
+  c == strm.readChar()
+
+proc scanUntil(strm: Stream; pat: string) =
+  while true:
+    block patLoop:
+      for c in pat:
+        if not strm.atChar(c): break patLoop
+      return
+
+proc parseStream(strm: Stream) =
+  # TODO: write an ELF parser.
+  strm.scanUntil("Sigil\x00\x00\x00\xD9\xD9\xF7")
+    # scan for the ELF tag owner and the self-describing CBOR tag
+  let node = strm.readCbor()
+  if node.kind == cborUnsigned and node.uint == 0: discard
+  elif node.kind != cborMap:
+    quit("unrecognized CBOR data: " & $node)
+  else:
+    for key, val in node.map:
+      if key.kind != cborText or val.kind != cborBytes or not val.hasTag(erisCborTag):
+        quit("Unrecognized note data " & $node)
+      let cap = parseCap(val.bytes)
+      stdout.writeLine cap, " ", key.text
+
+proc main =
+  var args = commandLineParams()
+  if args.len == 0:
+    args.add "-"
+  for arg in args:
+    var str =
+      if arg == "-":
+        newFileStream(stdin)
+      else:
+        openFileStream(arg)
+    parseStream(str)
+    close(str)
+
+main()