NAME
elf_update —
update an ELF
descriptor
LIBRARY
ELF Access Library (libelf, -lelf)
SYNOPSIS
#include <libelf.h>
off_t
elf_update(
Elf
*elf,
Elf_Cmd cmd);
DESCRIPTION
Function
elf_update() causes the library to recalculate the
structure of an ELF object and optionally write out the image of the object to
file.
Argument
elf should reference a valid ELF descriptor.
Argument
cmd can be one of the following values:
-
-
ELF_C_NULL
- The library will recalculate structural information
flagging modified structures with the
ELF_F_DIRTY
flag, but will not write data to the underlying file image.
-
-
ELF_C_WRITE
- The library will recalculate structural information and
will also write the new image to the underlying file. The ELF descriptor
referenced by argument elf should permit the
underlying ELF object to be written or updated (see
elf_begin(3)).
All pointers to
Elf_Scn and
Elf_Data
descriptors associated with descriptor
elf should be
considered invalid after a call to
elf_update().
Specifying Object Layout
The
ELF Access Library (libelf, -lelf) supports two
layout modes.
-
-
- Library Layout
- If the
ELF_F_LAYOUT
flag is not set
on the ELF descriptor, the ELF library will lay out the ELF object
according to the following scheme:
- EHDR
- The ELF executable header will be placed at the start
of the object.
- PHDR
- If the ELF descriptor contains a program header table,
it will be placed after the Executable Header.
- Section
Data
- ELF section data, if any, will be placed next, keeping
each section's alignment requirements in mind.
- SHDR
- The ELF section header table, if any, will be placed
last.
-
-
- Application Controlled
Layout
- The application can take full control of the layout of the
ELF object by setting the
ELF_F_LAYOUT
flag on the
ELF descriptor (see
elf_flagelf(3)). In
this case the library will lay out the ELF object using
application-supplied information as below:
- EHDR
- The ELF executable header will be placed at the start
of the object.
- PHDR
- The ELF program header table, if any, it will be placed
at the offset specified in the e_phoff field of
the ELF executable header.
- Section
Data
- The data for each ELF section will be placed at the
offset specified by the sh_offset field of the
section's header. The size of the section will be taken from the
sh_size field of the section header.
- SHDR
- The ELF section header table, if any, will be placed at
the offset specified by the e_shoff field of the
executable header.
Gaps in the coverage of the file's contents will be set to the fill value
specified by
elf_fill(3).
The application needs to set the following fields in the data structures
associated with the ELF descriptor prior to calling
elf_update().
-
-
- Executable Header
- The fields of the ELF executable header that need to be set
by the application are:
- e_entry
- To be set to the desired entry address for
executables.
- e_flags
- To be set to the desired processor specific flags.
- e_ident[EI_DATA]
- Must be set to one of
ELFDATA2LSB
or
ELFDATA2MSB
.
- e_ident[EI_OSABI]
- To be set to the OS ABI desired. For example, for
FreeBSD executables, this field should be set
to
ELFOSABI_FREEBSD
.
- e_machine
- To be set to the desired machine architecture, one of
the
EM_*
values in the header file
<elfdefinitions.h>.
- e_phoff
- If the application is managing the object's layout, it
must set this field to the file offset of the ELF program header
table.
- e_shoff
- If the application is managing the object's layout, it
must set this field to the file offset of the ELF section header
table.
- e_shstrndx
- To be set to the index of the string table containing
section names.
- e_type
- To be set to the type of the ELF object, one of the
ET_*
values in the header file
<elfdefinitions.h>.
- e_version
- To be set to the desired version of the ELF
object.
-
-
- Program Header
- All fields of the entries in the program header table need
to be set by the application.
-
-
- Section Header
- The fields of ELF section headers that need to be set by
the application are:
- sh_addr
- To be set to the memory address where the section
should reside.
- sh_addralign
- If the application is managing the file layout, it must
set this field to the desired alignment for the section's contents.
This value must be a power of two and must be at least as large as the
largest alignment needed by any Elf_Data
descriptor associated with the section.
- sh_entsize
- To be set to the size of each entry, for sections
containing fixed size elements, or set to zero for sections without
fixed size elements. If the application is not managing file layout,
it may leave this field as zero for those sections whose types are
known to the library.
- sh_flags
- To be set to the desired section flags.
- sh_info
- To be set as described in
elf(5).
- sh_link
- To be set as described in
elf(5).
- sh_name
- To be set to the index of the section's name in the
string table containing section names.
- sh_offset
- If the application is managing the file layout, it must
set this field to the file offset of the section's contents.
- sh_size
- If the application is managing the file layout, it must
set this field to the file size of the section's contents.
- sh_type
- To be set to the type of the section.
-
-
- Section Data
- The Elf_Data descriptors associated
with each section specify its contents (see
elf_getdata(3)). While
all the fields in these descriptors are under application control, the
following fields influence object layout:
- d_align
- To be set to the desired alignment, within the
containing section, of the descriptor's data.
- d_off
- If the application is managing object layout, it must
set this field to the file offset, within the section, at which the
descriptor's data should be placed.
- d_size
- To be set to the size in bytes of the memory
representation of the descriptor's data.
RETURN VALUES
Function
elf_update() returns the total size of the file image
if successful, or -1 if an error occurred.
ERRORS
This function may fail with the following errors:
-
-
- [
ELF_E_ARGUMENT
]
- Argument elf was null.
-
-
- [
ELF_E_ARGUMENT
]
- Argument cmd was not recognized.
-
-
- [
ELF_E_ARGUMENT
]
- The argument elf was not a descriptor
for an ELF object.
-
-
- [
ELF_E_CLASS
]
- The e_ident[EI_CLASS] field of the
executable header of argument elf did not match the
class of the file.
-
-
- [
ELF_E_DATA
]
- An Elf_Data descriptor contained in
argument elf specified an unsupported type.
-
-
- [
ELF_E_DATA
]
- An Elf_Data descriptor specified an
alignment that was zero or was not a power of two.
-
-
- [
ELF_E_HEADER
]
- The ELF header in argument elf
requested a different byte order from the byte order already associated
with the file.
-
-
- [
ELF_E_IO
]
- An I/O error was encountered.
-
-
- [
ELF_E_LAYOUT
]
- An Elf_Data descriptor contained in
argument elf specified an alignment incompatible
with its containing section.
-
-
- [
ELF_E_LAYOUT
]
- Argument elf contained section
descriptors that overlapped in extent.
-
-
- [
ELF_E_LAYOUT
]
- Argument elf contained section
descriptors that were incorrectly aligned or were too small for their
data.
-
-
- [
ELF_E_LAYOUT
]
- The flag
ELF_F_LAYOUT
was set on
the Elf descriptor and the executable header overlapped with the program
header table.
-
-
- [
ELF_E_LAYOUT
]
- The flag
ELF_F_LAYOUT
was set on
the Elf descriptor and the program header table was placed at a misaligned
file offset.
-
-
- [
ELF_E_LAYOUT
]
- The flag
ELF_F_LAYOUT
was set on
the Elf descriptor and the section header table overlapped an extent
mapped by a section descriptor.
-
-
- [
ELF_E_LAYOUT
]
- The
ELF_F_LAYOUT
flag was set on
the Elf descriptor, and the d_offset field in an
Elf_Data descriptor contained a value that was not a
multiple of the descriptor's specified alignment.
-
-
- [
ELF_E_MODE
]
- An
ELF_C_WRITE
operation was
requested with an ELF descriptor that was not opened for writing or
updating.
-
-
- [
ELF_E_SECTION
]
- Argument elf contained a section with
an unrecognized type.
-
-
- [
ELF_E_SECTION
]
- The section header at index
SHN_UNDEF
had an illegal section type.
-
-
- [
ELF_E_SEQUENCE
]
- An
ELF_C_WRITE
operation was
requested after a prior call to
elf_cntl(elf,
ELF_C_FDDONE) disassociated the ELF descriptor
elf from its underlying file.
-
-
- [
ELF_E_VERSION
]
- Argument elf had an unsupported
version or contained an Elf_Data descriptor with an
unsupported version.
SEE ALSO
elf(3),
elf32_getehdr(3),
elf32_getphdr(3),
elf32_newehdr(3),
elf32_newphdr(3),
elf64_getehdr(3),
elf64_getphdr(3),
elf64_newehdr(3),
elf64_newphdr(3),
elf_begin(3),
elf_cntl(3),
elf_fill(3),
elf_flagehdr(3),
elf_flagelf(3),
elf_getdata(3),
elf_getscn(3),
elf_newdata(3),
elf_newscn(3),
elf_rawdata(3),
gelf(3),
gelf_newehdr(3),
gelf_newphdr(3),
elf(5)