CTF(5) FreeBSD File Formats Manual CTF(5)
NAME
ctf - Compact C Type Format
SYNOPSIS
#include <sys/ctf.h>
DESCRIPTION
ctf is designed to be a compact representation of the C programming
language's type information focused on serving the needs of dynamic
tracing, debuggers, and other in-situ and post-mortem introspection
tools. ctf data is generally included in ELF objects and is tagged as
SHT_PROGBITS to ensure that the data is accessible in a running process
and in subsequent core dumps, if generated.
The ctf data contained in each file has information about the layout and
sizes of C types, including intrinsic types, enumerations, structures,
typedefs, and unions, that are used by the corresponding ELF object. The
ctf data may also include information about the types of global objects
and the return type and arguments of functions in the symbol table.
Because a ctf file is often embedded inside a file, rather than being a
standalone file itself, it may also be referred to as a ctf container.
On FreeBSD systems, ctf data is consumed by dtrace(1). Programmatic
access to ctf data can be obtained through libctf.
The ctf file format is broken down into seven different sections. The
first two sections are the preamble and header, which describe the
version of the ctf file, the links it has to other ctf files, and the
sizes of the other sections. The next section is the label section,
which provides a way of identifying similar groups of ctf data across
multiple files. This is followed by the object information section,
which describes the types of global symbols. The subsequent section is
the function information section, which describes the return types and
arguments of functions. The next section is the type information
section, which describes the format and layout of the C types themselves,
and finally the last section is the string section, which contains the
names of types, enumerations, members, and labels.
While strictly speaking, only the preamble and header are required, to be
actually useful, both the type and string sections are necessary.
A ctf file may contain all of the type information that it requires, or
it may optionally refer to another ctf file which holds the remaining
types. When a ctf file refers to another file, it is called the child
and the file it refers to is called the parent. A given file may only
refer to one parent. This process is called uniquification because it
ensures each child only has type information that is unique to it. A
common example of this is that most kernel modules in illumos are
uniquified against the kernel module genunix and the type information
that comes from the IP module. This means that a module only has types
that are unique to itself and the most common types in the kernel are not
duplicated. Uniquification is not used when building kernel modules on
FreeBSD.
FILE FORMAT
This documents version three of the ctf file format. The ctfconvert(1)
and ctfmerge(1) utilities emit ctf version 3, and all other applications
and libraries can operate on versions 2 and 3.
The file format can be summarized with the following image, the following
sections will cover this in more detail.
+-------------+ 0t0
+--------| Preamble |
| +-------------+ 0t4
|+-------| Header |
|| +-------------+ 0t36 + cth_lbloff
||+------| Labels |
||| +-------------+ 0t36 + cth_objtoff
|||+-----| Objects |
|||| +-------------+ 0t36 + cth_funcoff
||||+----| Functions |
||||| +-------------+ 0t36 + cth_typeoff
|||||+---| Types |
|||||| +-------------+ 0t36 + cth_stroff
||||||+--| Strings |
||||||| +-------------+ 0t36 + cth_stroff + cth_strlen
|||||||
|||||||
|||||||
||||||| +-- magic - vers flags
||||||| | | | |
||||||| +------+------+------+------+
+---------| 0xcf | 0xf1 | 0x03 | 0x00 |
|||||| +------+------+------+------+
|||||| 0 1 2 3 4
||||||
|||||| + parent label + objects
|||||| | + parent name | + functions + strings
|||||| | | + label | | + types | + strlen
|||||| | | | | | | | |
|||||| +------+------+------+------+------+-------+-------+-------+
+--------| 0x00 | 0x00 | 0x00 | 0x08 | 0x36 | 0x110 | 0x5f4 | 0x611 |
||||| +------+------+------+------+------+-------+-------+-------+
||||| 0x04 0x08 0x0c 0x10 0x14 0x18 0x1c 0x20 0x24
|||||
||||| + Label name
||||| | + Label type
||||| | | + Next label
||||| | | |
||||| +-------+------+-----+
+-----------| 0x01 | 0x42 | ... |
|||| +-------+------+-----+
|||| cth_lbloff +0x4 +0x8 cth_objtoff
||||
||||
|||| Symidx 0t15 0t43 0t44
|||| +------+------+------+-----+
+----------| 0x00 | 0x42 | 0x36 | ... |
||| +------+------+------+-----+
||| cth_objtoff +0x4 +0x8 +0xc cth_funcoff
|||
||| + CTF_TYPE_INFO + CTF_TYPE_INFO
||| | + Return type |
||| | | + arg0 |
||| +--------+------+------+-----+
+---------| 0x2c10 | 0x08 | 0x0c | ... |
|| +--------+------+------+-----+
|| cth_funcff +0x4 +0x8 +0xc cth_typeoff
||
|| + ctf_stype_t for type 1
|| | integer + integer encoding
|| | | + ctf_stype_t for type 2
|| | | |
|| +--------------------+-----------+-----+
+--------| 0x19 * 0xc01 * 0x0 | 0x1000000 | ... |
| +--------------------+-----------+-----+
| cth_typeoff +0x0c +0x10 cth_stroff
|
| +--- str 0
| | +--- str 1 + str 2
| | | |
| v v v
| +----+---+---+---+----+---+---+---+---+---+----+
+---| \0 | i | n | t | \0 | f | o | o | _ | t | \0 |
+----+---+---+---+----+---+---+---+---+---+----+
0 1 2 3 4 5 6 7 8 9 10 11
Every ctf file begins with a preamble, followed by a header. The
preamble is defined as follows:
typedef struct ctf_preamble {
uint16_t ctp_magic; /* magic number (CTF_MAGIC) */
uint8_t ctp_version; /* data format version number (CTF_VERSION) */
uint8_t ctp_flags; /* flags (see below) */
} ctf_preamble_t;
The preamble is four bytes long and must be four byte aligned. This
preamble defines the version of the ctf file which defines the format of
the rest of the header. While the header may change in subsequent
versions, the preamble will not change across versions, though the
interpretation of its flags may change from version to version. The
ctp_magic member defines the magic number for the ctf file format. This
must always be 0xcff1. If another value is encountered, then the file
should not be treated as a ctf file. The ctp_version member defines the
version of the ctf file. The current version is 3. It is possible to
encounter an unsupported version. In that case, software should not try
to parse the format, as it may have changed. Finally, the ctp_flags
member describes aspects of the file which modify its interpretation.
The following flags are currently defined:
#define CTF_F_COMPRESS 0x01
The flag CTF_F_COMPRESS indicates that the body of the ctf file, all the
data following the header, has been compressed through the zlib library
and its deflate algorithm. If this flag is not present, then the body
has not been compressed and no special action is needed to interpret it.
All offsets into the data as described by header, always refer to the
uncompressed data.
In versions two and three of the ctf file format, the header denotes
whether or not this ctf file is the child of another ctf file and also
indicates the size of the remaining sections. The structure for the
header logically contains a copy of the preamble and the two have a
combined size of 36 bytes.
typedef struct ctf_header {
ctf_preamble_t cth_preamble;
uint32_t cth_parlabel; /* ref to name of parent lbl uniq'd against */
uint32_t cth_parname; /* ref to basename of parent */
uint32_t cth_lbloff; /* offset of label section */
uint32_t cth_objtoff; /* offset of object section */
uint32_t cth_funcoff; /* offset of function section */
uint32_t cth_typeoff; /* offset of type section */
uint32_t cth_stroff; /* offset of string section */
uint32_t cth_strlen; /* length of string section in bytes */
} ctf_header_t;
After the preamble, the next two members cth_parlabel and cth_parname,
are used to identify the parent. The value of both members are offsets
into the string section which point to the start of a null-terminated
string. For more information on the encoding of strings, see the
subsection on String Identifiers. If the value of either is zero, then
there is no entry for that member. If the member cth_parlabel is set,
then the ctf_parname member must be set, otherwise it will not be
possible to find the parent. If ctf_parname is set, it is not necessary
to define cth_parlabel, as the parent may not have a label. For more
information on labels and their interpretation, see The Label Section.
The remaining members (excepting cth_strlen) describe the beginning of
the corresponding sections. These offsets are relative to the end of the
header. Therefore, something with an offset of 0 is at an offset of
thirty-six bytes relative to the start of the ctf file. The difference
between members indicates the size of the section itself. Different
offsets have different alignment requirements. The start of the
cth_objtoff and cth_funcoff must be two byte aligned, while the sections
cth_lbloff and cth_typeoff must be four-byte aligned. The section
cth_stroff has no alignment requirements. To calculate the size of a
given section, excepting the string section, one should subtract the
offset of the section from the following one. For example, the size of
the types section can be calculated by subtracting cth_typeoff from
cth_stroff.
Finally, the member cth_strlen describes the length of the string section
itself. From it, you can also calculate the size of the entire ctf file
by adding together the size of the ctf_header_t, the offset of the string
section in cth_stroff, and the size of the string section in cth_srlen.
Type Identifiers
Through the ctf data, types are referred to by identifiers. A given ctf
file supports up to 2147483646 (0x7ffffffe) types. ctf version 2 had a
much smaller limit of 32767 types. The first valid type identifier is
0x1. When a given ctf file is a child, indicated by a non-zero entry for
the header's cth_parname, then the first valid type identifier is
0x80000000 and the last is 0xfffffffe. In this case, type identifiers
0x1 through 0x7ffffffe are references to the parent. 0x7fffffff and
0xffffffff are not treated as valid type identifiers so as to enable the
use of -1 as an error value.
The type identifier zero is a sentinel value used to indicate that there
is no type information available or it is an unknown type.
Throughout the file format, the identifier is stored in different sized
values; however, the minimum size to represent a given identifier is a
uint16_t. Other consumers of ctf information may use larger or opaque
identifiers.
String Identifiers
String identifiers are always encoded as four byte unsigned integers
which are an offset into a string table. The ctf format supports two
different string tables which have an identifier of zero or one. This
identifier is stored in the high-order bit of the unsigned four byte
offset. Therefore, the maximum supported offset into one of these tables
is 0x7ffffffff.
Table identifier zero, always refers to the string section in the CTF
file itself. String table identifier one refers to an external string
table which is the ELF string table for the ELF symbol table associated
with the ctf container.
Type Encoding
Every ctf type begins with metadata encoded into a uint32_t. This
encoded information tells us three different pieces of information:
• The kind of the type
• Whether this type is a root type or not
• The length of the variable data
The 32 bits that make up the encoding are broken down into six bits for
the kind (bits 26 to 31), one bit for the root type flag (bit 25), and 25
bits for the length of the variable data.
The current version of the file format defines 14 different kinds. The
interpretation of these different kinds will be discussed in the section
The Type Section. If a kind is encountered that is not listed below,
then it is not a valid ctf file. The kinds are defined as follows:
#define CTF_K_UNKNOWN 0
#define CTF_K_INTEGER 1
#define CTF_K_FLOAT 2
#define CTF_K_POINTER 3
#define CTF_K_ARRAY 4
#define CTF_K_FUNCTION 5
#define CTF_K_STRUCT 6
#define CTF_K_UNION 7
#define CTF_K_ENUM 8
#define CTF_K_FORWARD 9
#define CTF_K_TYPEDEF 10
#define CTF_K_VOLATILE 11
#define CTF_K_CONST 12
#define CTF_K_RESTRICT 13
Programs directly reference many types; however, other types are
referenced indirectly because they are part of some other structure.
These types that are referenced directly and used are called root types.
Other types may be used indirectly, for example, a program may reference
a structure directly, but not one of its members which has a type. That
type is not considered a root type. If a type is a root type, then it
will have bit 25 set.
The variable length section is specific to each kind and is discussed in
the section The Type Section.
The following macros are useful for constructing and deconstructing the
encoded type information:
#define CTF_V3_MAX_VLEN 0x00ffffff
#define CTF_V3_INFO_KIND(info) (((info) & 0xfc000000) >> 26)
#define CTF_V3_INFO_ISROOT(info) (((info) & 0x02000000) >> 25)
#define CTF_V3_INFO_VLEN(info) (((info) & CTF_V3_MAX_VLEN))
#define CTF_V3_TYPE_INFO(kind, isroot, vlen) \
(((kind) << 26) | (((isroot) ? 1 : 0) << 25) | ((vlen) & CTF_V3_MAX_VLEN))
The Label Section
When consuming ctf data, it is often useful to know whether two different
ctf containers come from the same source base and version. For example,
when building illumos, there are many kernel modules that are built
against a single collection of source code. A label is encoded into the
ctf files that corresponds with the particular build. This ensures that
if files on the system were to become mixed up from multiple releases,
that they are not used together by tools, particularly when a child needs
to refer to a type in the parent. Because they are linked using the type
identifiers, if the wrong parent is used then the wrong type will be
encountered. Note that this mechanism is not currently used on FreeBSD.
In particular, kernel modules built on FreeBSD each contain a complete
type graph.
Each label is encoded in the file format using the following eight byte
structure:
typedef struct ctf_lblent {
uint32_t ctl_label; /* ref to name of label */
uint32_t ctl_typeidx; /* last type associated with this label */
} ctf_lblent_t;
Each label has two different components, a name and a type identifier.
The name is encoded in the ctl_label member which is in the format
defined in the section String Identifiers. Generally, the names of all
labels are found in the internal string section.
The type identifier encoded in the member ctl_typeidx refers to the last
type identifier that a label refers to in the current file. Labels only
refer to types in the current file, if the ctf file is a child, then it
will have the same label as its parent; however, its label will only
refer to its types, not its parent's.
It is also possible, though rather uncommon, for a ctf file to have
multiple labels. Labels are placed one after another, every eight bytes.
When multiple labels are present, types may only belong to a single
label.
The Object Section
The object section provides a mapping from ELF symbols of type STT_OBJECT
in the symbol table to a type identifier. Every entry in this section is
a uint32_t which contains a type identifier as described in the section
Type Identifiers. If there is no information for an object, then the
type identifier 0x0 is stored for that entry.
To walk the object section, you need to have a corresponding symbol table
in the ELF object that contains the ctf data. Not every object is
included in this section. Specifically, when walking the symbol table,
an entry is skipped if it matches any of the following conditions:
• The symbol type is not STT_OBJECT
• The symbol's section index is SHN_UNDEF
• The symbol's name offset is zero
• The symbol's section index is SHN_ABS and the value of the
symbol is zero.
• The symbol's name is _START_ or _END_. These are skipped
because they are used for scoping local symbols in ELF.
The following sample code shows an example of iterating the object
section and skipping the correct symbols:
#include <gelf.h>
#include <stdio.h>
/*
* Given the start of the object section in a CTFv3 file, the number of symbols,
* and the ELF Data sections for the symbol table and the string table, this
* prints the type identifiers that correspond to objects. Note, a more robust
* implementation should ensure that they don't walk beyond the end of the CTF
* object section.
*
* An implementation that handles CTFv2 must take into account the fact that
* type identifiers are 16 bits wide rather than 32 bits wide.
*/
static int
walk_symbols(uint32_t *objtoff, Elf_Data *symdata, Elf_Data *strdata,
long nsyms)
{
long i;
uintptr_t strbase = strdata->d_buf;
for (i = 1; i < nsyms; i++, objftoff++) {
const char *name;
GElf_Sym sym;
if (gelf_getsym(symdata, i, &sym) == NULL)
return (1);
if (GELF_ST_TYPE(sym.st_info) != STT_OBJECT)
continue;
if (sym.st_shndx == SHN_UNDEF || sym.st_name == 0)
continue;
if (sym.st_shndx == SHN_ABS && sym.st_value == 0)
continue;
name = (const char *)(strbase + sym.st_name);)(strbase + sym.st_name);
if (strcmp(name, "_START_") == 0 || strcmp(name, "_END_") == 0)
continue;
(void) printf("Symbol %d has type %d0, i, *objtoff);
}
return (0);
}
The Function Section
The function section of the ctf file encodes the types of both the
function's arguments and the function's return value. Similar to The
Object Section, the function section encodes information for all symbols
of type STT_FUNCTION, excepting those that fit specific criteria. Unlike
with objects, because functions have a variable number of arguments, they
start with a type encoding as defined in Type Encoding, which is the size
of a uint32_t. For functions which have no type information available,
they are encoded as CTF_V3_TYPE_INFO(CTF_K_UNKNOWN, 0, 0). Functions
with arguments are encoded differently. Here, the variable length is
turned into the number of arguments in the function. If a function is a
varargs type function, then the number of arguments is increased by one.
Functions with type information are encoded as:
CTF_V3_TYPE_INFO(CTF_K_FUNCTION, 0, nargs).
For functions that have no type information, nothing else is encoded, and
the next function is encoded. For functions with type information, the
next uint32_t is encoded with the type identifier of the return type of
the function. It is followed by each of the type identifiers of the
arguments, if any exist, in the order that they appear in the function.
Therefore, argument 0 is the first type identifier and so on. When a
function has a final varargs argument, that is encoded with the type
identifier of zero.
Like The Object Section, the function section is encoded in the order of
the symbol table. It has similar, but slightly different considerations
from objects. While iterating the symbol table, if any of the following
conditions are true, then the entry is skipped and no corresponding entry
is written:
• The symbol type is not STT_FUNCTION
• The symbol's section index is SHN_UNDEF
• The symbol's name offset is zero
• The symbol's name is _START_ or _END_. These are skipped
because they are used for scoping local symbols in ELF.
The Type Section
The type section is the heart of the ctf data. It encodes all of the
information about the types themselves. The base of the type information
comes in two forms, a short form and a long form, each of which may be
followed by a variable number of arguments. The following definitions
describe the short and long forms:
#define CTF_V3_MAX_SIZE 0xfffffffe /* max size of a type in bytes */
#define CTF_V3_LSIZE_SENT 0xffffffff /* sentinel for ctt_size */
#define CTF_V3_MAX_LSIZE UINT64_MAX
struct ctf_stype_v3 {
uint32_t ctt_name; /* reference to name in string table */
uint32_t ctt_info; /* encoded kind, variant length */
union {
uint32_t _size; /* size of entire type in bytes */
uint32_t _type; /* reference to another type */
} _u;
};
struct ctf_type_v3 {
uint32_t ctt_name; /* reference to name in string table */
uint32_t ctt_info; /* encoded kind, variant length */
union {
uint32_t _size; /* always CTF_LSIZE_SENT */
uint32_t _type; /* do not use */
} _u;
uint32_t ctt_lsizehi; /* high 32 bits of type size in bytes */
uint32_t ctt_lsizelo; /* low 32 bits of type size in bytes */
};
#define ctt_size _u._size /* for fundamental types that have a size */
#define ctt_type _u._type /* for types that reference another type */
Type sizes are stored in bytes. The basic small form uses a uint32_t to
store the number of bytes. If the number of bytes in a structure would
exceed 0xfffffffe, then the alternate form, the struct ctf_type_v3, is
used instead. To indicate that the larger form is being used, the member
ctt_size is set to value of CTF_V3_LSIZE_SENT (0xffffffff). In general,
when going through the type section, consumers use the struct ctf_type_v3
structure, but pay attention to the value of the member ctt_size to
determine whether they should increment their scan by the size of struct
ctf_stype_v3 or struct ctf_type_v3. Not all kinds of types use ctt_size.
Those which do not, will always use the struct ctf_stype_v3 structure.
The individual sections for each kind have more information.
Types are written out in order. Therefore the first entry encountered
has a type id of 0x1, or 0x8000 if a child. The member ctt_name is
encoded as described in the section String Identifiers. The string that
it points to is the name of the type. If the identifier points to an
empty string (one that consists solely of a null terminator) then the
type does not have a name, this is common with anonymous structures and
unions that only have a typedef to name them, as well as pointers and
qualifiers.
The next member, the ctt_info, is encoded as described in the section
Type Encoding. The type's kind tells us how to interpret the remaining
data in the struct ctf_type_v3 and any variable length data that may
exist. The rest of this section will be broken down into the
interpretation of the various kinds.
Encoding of Integers
Integers, which are of type CTF_K_INTEGER, have no variable length
arguments. Instead, they are followed by a uint32_t which describes
their encoding. All integers must be encoded with a variable length of
zero. The ctt_size member describes the length of the integer in bytes.
In general, integer sizes will be rounded up to the closest power of two.
The integer encoding contains three different pieces of information:
• The encoding of the integer
• The offset in bits of the type
• The size in bits of the type
This encoding can be expressed through the following macros:
#define CTF_INT_ENCODING(data) (((data) & 0xff000000) >> 24)
#define CTF_INT_OFFSET(data) (((data) & 0x00ff0000) >> 16)
#define CTF_INT_BITS(data) (((data) & 0x0000ffff))
#define CTF_INT_DATA(encoding, offset, bits) \
(((encoding) << 24) | ((offset) << 16) | (bits))
The following flags are defined for the encoding at this time:
#define CTF_INT_SIGNED 0x01
#define CTF_INT_CHAR 0x02
#define CTF_INT_BOOL 0x04
#define CTF_INT_VARARGS 0x08
By default, an integer is considered to be unsigned, unless it has the
CTF_INT_SIGNED flag set. If the flag CTF_INT_CHAR is set, that indicates
that the integer is of a type that stores character data, for example the
intrinsic C type char would have the CTF_INT_CHAR flag set. If the flag
CTF_INT_BOOL is set, that indicates that the integer represents a boolean
type. For example, the intrinsic C type _Bool would have the
CTF_INT_BOOL flag set. Finally, the flag CTF_INT_VARARGS indicates that
the integer is used as part of a variable number of arguments. This
encoding is rather uncommon.
Encoding of Floats
Floats, which are of type CTF_K_FLOAT, are similar to their integer
counterparts. They have no variable length arguments and are followed by
a four byte encoding which describes the kind of float that exists. The
ctt_size member is the size, in bytes, of the float. The float encoding
has three different pieces of information inside of it:
• The specific kind of float that exists
• The offset in bits of the float
• The size in bits of the float
This encoding can be expressed through the following macros:
#define CTF_FP_ENCODING(data) (((data) & 0xff000000) >> 24)
#define CTF_FP_OFFSET(data) (((data) & 0x00ff0000) >> 16)
#define CTF_FP_BITS(data) (((data) & 0x0000ffff))
#define CTF_FP_DATA(encoding, offset, bits) \
(((encoding) << 24) | ((offset) << 16) | (bits))
Where as the encoding for integers is a series of flags, the encoding for
floats maps to a specific kind of float. It is not a flag-based value.
The kinds of floats correspond to both their size, and the encoding.
This covers all of the basic C intrinsic floating point types. The
following are the different kinds of floats represented in the encoding:
#define CTF_FP_SINGLE 1 /* IEEE 32-bit float encoding */
#define CTF_FP_DOUBLE 2 /* IEEE 64-bit float encoding */
#define CTF_FP_CPLX 3 /* Complex encoding */
#define CTF_FP_DCPLX 4 /* Double complex encoding */
#define CTF_FP_LDCPLX 5 /* Long double complex encoding */
#define CTF_FP_LDOUBLE 6 /* Long double encoding */
#define CTF_FP_INTRVL 7 /* Interval (2x32-bit) encoding */
#define CTF_FP_DINTRVL 8 /* Double interval (2x64-bit) encoding */
#define CTF_FP_LDINTRVL 9 /* Long double interval (2x128-bit) encoding */
#define CTF_FP_IMAGRY 10 /* Imaginary (32-bit) encoding */
#define CTF_FP_DIMAGRY 11 /* Long imaginary (64-bit) encoding */
#define CTF_FP_LDIMAGRY 12 /* Long double imaginary (128-bit) encoding */
Encoding of Arrays
Arrays, which are of type CTF_K_ARRAY, have no variable length arguments.
They are followed by a structure which describes the number of elements
in the array, the type identifier of the elements in the array, and the
type identifier of the index of the array. With arrays, the ctt_size
member is set to zero. The structure that follows an array is defined
as:
struct ctf_array_v3 {
uint32_t cta_contents; /* reference to type of array contents */
uint32_t cta_index; /* reference to type of array index */
uint32_t cta_nelems; /* number of elements */
};
The cta_contents and cta_index members of the struct ctf_array_v3 are
type identifiers which are encoded as per the section Type Identifiers.
The member cta_nelems is a simple four byte unsigned count of the number
of elements. This count may be zero when encountering C99's flexible
array members.
Encoding of Functions
Function types, which are of type CTF_K_FUNCTION, use the variable length
list to be the number of arguments in the function. When the function
has a final member which is a varargs, then the argument count is
incremented by one to account for the variable argument. Here, the
ctt_type member is encoded with the type identifier of the return type of
the function. Note that the ctt_size member is not used here.
The variable argument list contains the type identifiers for the
arguments of the function, if any. Each one is represented by a uint32_t
and encoded according to the Type Identifiers section. If the function's
last argument is of type varargs, then it is also written out, but the
type identifier is zero. This is included in the count of the function's
arguments. In ctf version 2, an extra type identifier may follow the
argument and return type identifiers in order to maintain four-byte
alignment for the following type definition. Such a type identifier is
not included in the argument count and has a value of zero. In ctf
version 3, four-byte alignment occurs naturally and no padding is used.
Encoding of Structures and Unions
Structures and Unions, which are encoded with CTF_K_STRUCT and
CTF_K_UNION respectively, are very similar constructs in C. The main
difference between them is the fact that members of a structure follow
one another, where as in a union, all members share the same memory.
They are also very similar in terms of their encoding in ctf. The
variable length argument for structures and unions represents the number
of members that they have. The value of the member ctt_size is the size
of the structure and union. There are two different structures which are
used to encode members in the variable list. When the size of a
structure or union is greater than or equal to the large member
threshold, 536870912, then a different structure is used to encode the
member; all members are encoded using the same structure. The structure
for members is as follows:
struct ctf_member_v3 {
uint32_t ctm_name; /* reference to name in string table */
uint32_t ctm_type; /* reference to type of member */
uint32_t ctm_offset; /* offset of this member in bits */
};
struct ctf_lmember_v3 {
uint32_t ctlm_name; /* reference to name in string table */
uint32_t ctlm_type; /* reference to type of member */
uint32_t ctlm_offsethi; /* high 32 bits of member offset in bits */
uint32_t ctlm_offsetlo; /* low 32 bits of member offset in bits */
};
Both the ctm_name and ctlm_name refer to the name of the member. The
name is encoded as an offset into the string table as described by the
section String Identifiers. The members ctm_type and ctlm_type both
refer to the type of the member. They are encoded as per the section
Type Identifiers.
The last piece of information that is present is the offset which
describes the offset in memory at which the member begins. For unions,
this value will always be zero because each member of a union has an
offset of zero. For structures, this is the offset in bits at which the
member begins. Note that a compiler may lay out a type with padding.
This means that the difference in offset between two consecutive members
may be larger than the size of the member. When the size of the overall
structure is strictly less than 536870912 bytes, the normal structure,
struct ctf_member_v3, is used and the offset in bits is stored in the
member ctm_offset. However, when the size of the structure is greater
than or equal to 536870912 bytes, then the number of bits is split into
two 32-bit quantities. One member, ctlm_offsethi, represents the upper
32 bits of the offset, while the other member, ctlm_offsetlo, represents
the lower 32 bits of the offset. These can be joined together to get a
64-bit sized offset in bits by shifting the member ctlm_offsethi to the
left by thirty two and then doing a binary or of ctlm_offsetlo.
Encoding of Enumerations
Enumerations, noted by the type CTF_K_ENUM, are similar to structures.
Enumerations use the variable list to note the number of values that the
enumeration contains, which we'll term enumerators. In C, an enumeration
is always equivalent to the intrinsic type int, thus the value of the
member ctt_size is always the size of an integer which is determined
based on the current model. For FreeBSD systems, this will always be 4,
as an integer is always defined to be 4 bytes large in both ILP32 and
LP64, regardless of the architecture. For further details, see arch(7).
The enumerators encoded in an enumeration have the following structure in
the variable list:
typedef struct ctf_enum {
uint32_t cte_name; /* reference to name in string table */
int32_t cte_value; /* value associated with this name */
} ctf_enum_t;
The member cte_name refers to the name of the enumerator's value, it is
encoded according to the rules in the section String Identifiers. The
member cte_value contains the integer value of this enumerator.
Encoding of Forward References
Forward references, types of kind CTF_K_FORWARD, in a ctf file refer to
types which may not have a definition at all, only a name. If the ctf
file is a child, then it may be that the forward is resolved to an actual
type in the parent, otherwise the definition may be in another ctf
container or may not be known at all. The only member of the struct
ctf_type_v3 that matters for a forward declaration is the ctt_name which
points to the name of the forward reference in the string table as
described earlier. There is no other information recorded for forward
references.
Encoding of Pointers, Typedefs, Volatile, Const, and Restrict
Pointers, typedefs, volatile, const, and restrict are all similar in ctf.
They all refer to another type. In the case of typedefs, they provide an
alternate name, while volatile, const, and restrict change how the type
is interpreted in the C programming language. This covers the ctf kinds
CTF_K_POINTER, CTF_K_TYPEDEF, CTF_K_VOLATILE, CTF_K_RESTRICT, and
CTF_K_CONST.
These types have no variable list entries and use the member ctt_type to
refer to the base type that they modify.
Encoding of Unknown Types
Types with the kind CTF_K_UNKNOWN are used to indicate gaps in the type
identifier space. These entries consume an identifier, but do not define
anything. Nothing should refer to these gap identifiers.
Dependencies Between Types
C types can be imagined as a directed, cyclic, graph. Structures and
unions may refer to each other in a way that creates a cyclic dependency.
In cases such as these, the entire type section must be read in and
processed. Consumers must not assume that every type can be laid out in
dependency order; they cannot.
The String Section
The last section of the ctf file is the string section. This section
encodes all of the strings that appear throughout the other sections. It
is laid out as a series of characters followed by a null terminator.
Generally, all names are written out in ASCII, as most C compilers do not
allow any characters to appear in identifiers outside of a subset of
ASCII. However, any extended characters sets should be written out as a
series of UTF-8 bytes.
The first entry in the section, at offset zero, is a single null
terminator to reference the empty string. Following that, each C string
should be written out, including the null terminator. Offsets that refer
to something in this section should refer to the first byte which begins
a string. Beyond the first byte in the section being the null
terminator, the order of strings is unimportant.
Data Encoding and ELF Considerations
ctf data is generally included in ELF objects which specify information
to identify the architecture and endianness of the file. A ctf container
inside such an object must match the endianness of the ELF object. Aside
from the question of the endian encoding of data, there should be no
other differences between architectures. While many of the types in this
document refer to non-fixed size C integral types, they are equivalent in
the models ILP32 and LP64. If any other model is being used with ctf
data that has different sizes, then it must not use the model's sizes for
those integral types and instead use the fixed size equivalents based on
an ILP32 environment.
When placing a ctf container inside of an ELF object, there are certain
conventions that are expected for the purposes of tooling being able to
find the ctf data. In particular, a given ELF object should only contain
a single ctf section. Multiple containers should be merged together into
a single one.
The ctf file should be included in its own ELF section. The section's
name must be `.SUNW_ctf'. The type of the section must be SHT_PROGBITS.
The section should have a link set to the symbol table and its address
alignment must be 4.
SEE ALSO
ctfconvert(1), ctfdump(1), ctfmerge(1), dtrace(1), elf(3), gelf(3),
a.out(5), elf(5), arch(7)
FreeBSD 13.1-RELEASE-p6 February 28, 2022 FreeBSD 13.1-RELEASE-p6
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