Introspection (xrefs)

The FRR library provides an introspection facility called “xrefs.” The intent is to provide structured access to annotated entities in the compiled binary, such as log messages and thread scheduling calls.

Enabling and use

Support for emitting an xref is included in the macros for the specific entities, e.g. zlog_info() contains the relevant statements. The only requirement for the system to work is a GNU compatible linker that supports section start/end symbols. (The only known linker on any system FRR supports that does not do this is the Solaris linker.)

To verify xrefs have been included in a binary or dynamic library, run readelf -n binary. For individual object files, it’s readelf -S object.o | grep xref_array instead.

Structure and contents

As a slight improvement to security and fault detection, xrefs are divided into a const struct xref * and an optional struct xrefdata *. The required const part contains:

enum xref_type xref.type: Identifies what kind of object the xref points to.

int line

const char *xref.file

const char *xref.func: Source code location of the xref. func will be <global> for xrefs outside of a function.

struct xrefdata *xref.xrefdata: The optional writable part of the xref. NULL if no non-const part exists.

The optional non-const part has:

const struct xref *xrefdata.xref: Pointer back to the constant part. Since circular pointers are close to impossible to emit from inside a function body’s static variables, this is initialized at startup.

char xrefdata.uid[16]: Unique identifier, see below.

const char *xrefdata.hashstr

uint32_t xrefdata.hashu32[2]: Input to unique identifier calculation. These should encompass all details needed to make an xref unique. If more than one string should be considered, use string concatenation for the initializer.

Both structures can be extended by embedding them in a larger type-specific struct, e.g. struct xref_logmsg *.

Unique identifiers

All xrefs that have a writable struct xrefdata * part are assigned an unique identifier, which is formed as base32 (crockford) SHA256 on:

the source filename
the hashstr field
the hashu32 fields

Note

Function names and line numbers are intentionally not included to allow moving items within a file without affecting the identifier.

For running executables, this hash is calculated once at startup. When directly reading from an ELF file with external tooling, the value must be calculated when necessary.

The identifiers have the form AXXXX-XXXXX where X is 0-9, A-Z except I,L,O,U and A is G-Z except I,L,O,U (i.e. the identifiers always start with a letter.) When reading identifiers from user input, I and L should be replaced with 1 and O should be replaced with 0. There are 49 bits of entropy in this identifier.

Underlying machinery

Xrefs are nothing other than global variables with some extra glue to make them possible to find from the outside by looking at the binary. The first non-obvious part is that they can occur inside of functions, since they’re defined as static. They don’t have a visible name – they don’t need one.

To make finding these variables possible, another global variable, a pointer to the first one, is created in the same way. However, it is put in a special ELF section through __attribute__((section("xref_array"))). This is the section you can see with readelf.

Finally, on the level of a whole executable or library, the linker will stuff the individual pointers consecutive to each other since they’re in the same section — hence the array. Start and end of this array is given by the linker-autogenerated __start_xref_array and __stop_xref_array symbols. Using these, both a constructor to run at startup as well as an ELF note are created.

The ELF note is the entrypoint for externally retrieving xrefs from a binary without having to run it. It can be found by walking through the ELF data structures even if the binary has been fully stripped of debug and section information. SystemTap’s SDT probes & LTTng’s trace points work in the same way (though they emit 1 note for each probe, while xrefs only emit one note in total which refers to the array.) Using xrefs does not impact SystemTap or LTTng, the notes have identifiers they can be distinguished by.

The ELF structure of a linked binary (library or executable) will look like this:

$ readelf --wide -l -n lib/.libs/libfrr.so

Elf file type is DYN (Shared object file)
Entry point 0x67d21
There are 12 program headers, starting at offset 64

Program Headers:
  Type           Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
  PHDR           0x000040 0x0000000000000040 0x0000000000000040 0x0002a0 0x0002a0 R   0x8
  INTERP         0x125560 0x0000000000125560 0x0000000000125560 0x00001c 0x00001c R   0x10
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  LOAD           0x000000 0x0000000000000000 0x0000000000000000 0x02aff0 0x02aff0 R   0x1000
  LOAD           0x02b000 0x000000000002b000 0x000000000002b000 0x0b2889 0x0b2889 R E 0x1000
  LOAD           0x0de000 0x00000000000de000 0x00000000000de000 0x070048 0x070048 R   0x1000
  LOAD           0x14e428 0x000000000014f428 0x000000000014f428 0x00fb70 0x01a2b8 RW  0x1000
  DYNAMIC        0x157a40 0x0000000000158a40 0x0000000000158a40 0x000270 0x000270 RW  0x8
  NOTE           0x0002e0 0x00000000000002e0 0x00000000000002e0 0x00004c 0x00004c R   0x4
  TLS            0x14e428 0x000000000014f428 0x000000000014f428 0x000000 0x000008 R   0x8
  GNU_EH_FRAME   0x12557c 0x000000000012557c 0x000000000012557c 0x00819c 0x00819c R   0x4
  GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RW  0x10
  GNU_RELRO      0x14e428 0x000000000014f428 0x000000000014f428 0x009bd8 0x009bd8 R   0x1

(...)

Displaying notes found in: .note.gnu.build-id
  Owner                Data size      Description
  GNU                  0x00000014     NT_GNU_BUILD_ID (unique build ID bitstring)         Build ID: 6a1f66be38b523095ebd6ec13cc15820cede903d

Displaying notes found in: .note.FRR
  Owner                Data size      Description
  FRRouting            0x00000010     Unknown note type: (0x46455258)    description data: 6c eb 15 00 00 00 00 00 74 ec 15 00 00 00 00 00

Where 0x15eb6c…0x15ec74 are the offsets (relative to the note itself) where the xref array is in the file. Also note the owner is clearly marked as “FRRouting” and the type is “XREF” in hex.

For SystemTap’s use of ELF notes, refer to https://libstapsdt.readthedocs.io/en/latest/how-it-works/internals.html as an entry point.

Note

Due to GCC bug 41091, the “xref_array” section is not correctly generated for C++ code when compiled by GCC. A workaround is present for runtime functionality, but to extract the xrefs from a C++ source file, it needs to be built with clang (or a future fixed version of GCC) instead.

Extraction tool

The FRR source contains a matching tool to extract xref data from compiled ELF binaries in python/xrelfo.py. This tool uses CPython extensions implemented in clippy and must therefore be executed with that.

xrelfo.py processes input from one or more ELF file (.o, .so, executable), libtool object (.lo, .la, executable wrapper script) or JSON (output from xrelfo.py) and generates an output JSON file. During standard FRR build, it is invoked on all binaries and libraries and the result is combined into frr.json.

ELF files from any operating system, CPU architecture and endianness can be processed on any host. Any issues with this are bugs in xrelfo.py (or clippy’s ELF code.)

xrelfo.py also performs some sanity checking, particularly on log messages. The following options are available:

-o OUTPUT: Filename to write JSON output to. As a convention, a .xref filename extension is used.

-Wlog-format: Performs extra checks on log message format strings, particularly checks for \t and \n characters (which should not be used in log messages).

-Wlog-args: Generates cleanup hints for format string arguments where printfrr() extensions could be used, e.g. replacing inet_ntoa with %pI4.

--profile: Runs the Python profiler to identify hotspots in the xrelfo.py code.

xrelfo.py uses information about C structure definitions saved in python/xrefstructs.json. This file is included with the FRR sources and only needs to be regenerated when some of the struct xref_* definitions are changed (which should be almost never). The file is written by python/tiabwarfo.py, which uses pahole to extract the necessary data from DWARF information.