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ELF_UPDATE(3) Library Functions Manual ELF_UPDATE(3)
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).
Application Supplied Information
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_UNIMPL] Argument elf contained a section with an unsupported
ELF type.
[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)
NetBSD 10.99 April 22, 2019 NetBSD 10.99