On Sat, Aug 2, 2008 at 3:24 AM, Asim <linkasim@xxxxxxxxx> wrote:
> Hi,
>
> I'm working on a project related to no-downtime recovery of device drivers.
>
> I wish to allocate a special section in module code and store a copy
> of the .data section that a module can potentially modify. I intend to
> use this section to store a copy of initial .data values that a driver
why must u do this? when u declare any global variables, or static
var, it is put into a section of its own, and then when GCC generate
the assembly, it will know the relative offset of the variables, and
dereference the memory easily. if u declare your own section, then
in your assembly u have to calculate the offset DYNAMICALLY.
Alternatively, u can do it STATICALLY, which is to write your own
ELF-generator, (it is called "packers", u can find many in the open
source - eg, UPX) - designing your own sections + offset etc...very
complex. In fact...I am talking about modifying UPX itself.
> may modify during the course of its operation. I want my section to be
> towards the end and page-aligned. I was wondering what would be my
why the end? many viruses do that, and if u put the executable portion
near the end of a section (eg, .text or .data section), McAfee
immediately flagged it as a virus.
> start and
> end addresses for this section.
>
layout_section() called this:
static long get_offset(unsigned long *size, Elf_Shdr *sechdr)
{
long ret;
ret = ALIGN(*size, sechdr->sh_addralign ?: 1);
*size = ret + sechdr->sh_size;
return ret;
}
>From header:
typedef struct {
Elf32_Word sh_name;
Elf32_Word sh_type;
Elf32_Word sh_flags;
Elf32_Addr sh_addr;
Elf32_Off sh_offset;
Elf32_Word sh_size;
Elf32_Word sh_link;
Elf32_Word sh_info;
Elf32_Word sh_addralign;
Elf32_Word sh_entsize;
} Elf32_Shdr;
typedef struct elf64_shdr {
Elf64_Word sh_name; /* Section name, index in string tbl */
Elf64_Word sh_type; /* Type of section */
Elf64_Xword sh_flags; /* Miscellaneous section attributes */
Elf64_Addr sh_addr; /* Section virtual addr at execution */
Elf64_Off sh_offset; /* Section file offset */
Elf64_Xword sh_size; /* Size of section in bytes */
Elf64_Word sh_link; /* Index of another section */
Elf64_Word sh_info; /* Additional section information */
Elf64_Xword sh_addralign; /* Section alignment */
Elf64_Xword sh_entsize; /* Entry size if section holds table */
} Elf64_Shdr;
Which I think is responsible for the alignment of the section in memory.
Two thing: memory offset and file offset - u have to distinguish
between the two. fileoffset no alignment, but memory yes. So in
loading process, some empty spaces have to inserted.
sechdr->sh_size==> this is physical file offset (from byte 0 of the file).
ret --> ****I THINK**** is trying to round up the current *size value
to the next alignment value, and then add this
> I looked into layout_sections being called from sys_init_module in
> module.c and I'm having trouble understanding it completely. I was
> wondering how I would go about adding a section
> here. Is there any space in the headers between 1 and hdr->e_shnum
not sure what u mean, but I know that the space between the header and
the .text section is limited.....and u cannot keep on increasing the
number of section because of that.
> (where hdr is ELF header)? Or will need to add beyond e_shnum. Kindly
> help.
>
> Regards,
> Asim
>
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>
--
Regards,
Peter Teoh
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