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
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
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.
const char *xref.file
const char *xref.func
Source code location of the xref.
<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.
Unique identifier, see below.
const char *xrefdata.hashstr
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 xref_logmsg *.
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
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
0-9, A-Z except I,L,O,U and
G-Z except I,L,O,U (i.e. the
identifiers always start with a letter.) When reading identifiers from user
L should be replaced with
O should be
0. There are 49 bits of entropy in this identifier.
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
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
Using these, both a constructor to run at startup as well as an ELF note are
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
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.
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.
The FRR source contains a matching tool to extract xref data from compiled ELF
python/xrelfo.py. This tool uses CPython extensions
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
ELF files from any operating system, CPU architecture and endianness can be
processed on any host. Any issues with this are bugs in
(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
.xreffilename extension is used.
Performs extra checks on log message format strings, particularly checks for
\ncharacters (which should not be used in log messages).
Generates cleanup hints for format string arguments where
printfrr()extensions could be used, e.g. replacing
Runs the Python profiler to identify hotspots in the
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.