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Full Discussion: Dynamic Downloading and executing of ELF files
Top Forums UNIX for Advanced & Expert Users Dynamic Downloading and executing of ELF files Post 302385219 by Corona688 on Thursday 7th of January 2010 11:33:09 AM
Old 01-07-2010
You can't just download an ELF file into memory and execute it because an ELF file needs processing to load in the first place. An ELF file has potentially many separate memory segments with different characteristics, not one glob that it just lives in.

Build a shared library, use dlopen, that's what they're there for. Not to mention that how libraries are handled can depend a lot on your architecture and configuration, doing that manually yourself would be very unportable(not to mention reinventing the wheel.)

It handles everything for you, from filename to symbol name in two easy steps. Here's an example:
Code:
// shared.c
// compile like:  gcc shared.c -shared -fPIC -o shared.so
#include <stdio.h>

int library_function(int argc, char *argv[])
{
        fprintf(stderr, "library_function called\n");
        return(0);
}

Code:
// main.c
// compile like:  gcc main.c -ldl -o main
#include <stdio.h>
#include <dlfcn.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

int main(int argc, char *argv[])
{
        void *h=dlopen("./shared.so", RTLD_NOW);
        int (*fn)(int argc, char *argv[]);
        pid_t pid;

        if(h == NULL)
        {
                fprintf(stderr, "Couldn't open shared.so\n");
                return(1);
        }

        fn=dlsym(h, "library_function");
        if(fn == NULL)
        {
                fprintf(stderr, "Couldn't find library_function\n");
        }

        pid=fork();  // Create new process
        if(pid == 0)    // child code
        {       return(fn(argc, argv));         }
        else // Wait for child process
        {
                int status;
                wait(&status);
                fprintf(stderr, "Child returned %d\n", WEXITSTATUS(status));
        }

        dlclose(h);

        return(0);
}

Last edited by Corona688; 01-07-2010 at 12:39 PM..
 

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LEARN ABOUT FREEBSD

elf_cntl

ELF_CNTL(3)						   BSD Library Functions Manual 					       ELF_CNTL(3)

NAME

     elf_cntl -- control an elf file descriptor

LIBRARY

     ELF Access Library (libelf, -lelf)

SYNOPSIS

     #include <libelf.h>

     int
     elf_cntl(Elf *elf, Elf_Cmd cmd);

DESCRIPTION

     Function elf_cntl() controls the ELF library's subsequent use of the file descriptor used to create ELF descriptor elf.

     Argument cmd informs the library of the action to be taken:

     ELF_C_FDDONE  This value instructs the ELF library not to perform any further I/O on the file descriptor associated with argument elf.  For
		   ELF descriptors opened with mode ELF_C_WRITE or ELF_C_RDWR subsequent elf_update() operations on the descriptor will fail.

     ELF_C_FDREAD  This value instructs the ELF library to read in all necessary data associated with ELF descriptor elf into memory so that the
		   underlying file descriptor can be safely closed with command ELF_C_FDDONE.

     Argument elf must be an ELF descriptor associated with a file system object (e.g., an ar(1) archive, an ELF file, or other data file).

IMPLEMENTATION NOTES

     Due to use of mmap(2) internally, this function is a no-op for ELF objects opened in ELF_C_READ mode.

RETURN VALUES

     Function elf_cntl() returns 0 on success, or -1 if an error was detected.

ERRORS

     [ELF_E_ARCHIVE]   Argument elf is a descriptor for an archive member.

     [ELF_E_ARGUMENT]  Argument elf was NULL.

     [ELF_E_ARGUMENT]  Argument cmd was not recognized.

     [ELF_E_MODE]      An ELF_C_FDREAD operation was requested on an ELF descriptor opened for writing.

SEE ALSO

     elf(3), elf_begin(3), elf_end(3), elf_next(3), elf_update(3), gelf(3)

BSD
								  August 9, 2006							       BSD
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