loadelf.c

/*
 * Copyright (c) 2000, 2001, 2002, 2003, 2004, 2005, 2008, 2009
 *      The President and Fellows of Harvard College.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE UNIVERSITY AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE UNIVERSITY OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */


/*
 * Code to load an ELF-format executable into the current address space.
 *
 * It makes the following address space calls:
 *    - first, as_define_region once for each segment of the program;
 *    - then, as_prepare_load;
 *    - then it loads each chunk of the program;
 *    - finally, as_complete_load.
 *
 * This gives the VM code enough flexibility to deal with even grossly
 * mis-linked executables if that proves desirable. Under normal
 * circumstances, as_prepare_load and as_complete_load probably don't
 * need to do anything.
 *
 * If you wanted to support memory-mapped executables you would need
 * to rearrange this to map each segment.
 *
 * To support dynamically linked executables with shared libraries
 * you'd need to change this to load the "ELF interpreter" (dynamic
 * linker). And you'd have to write a dynamic linker...
 */

#include <types.h>
#include <kern/errno.h>
#include <lib.h>
#include <uio.h>
#include <proc.h>
#include <current.h>
#include <addrspace.h>
#include <vnode.h>
#include <elf.h>

/*
 * Load a segment at virtual address VADDR. The segment in memory
 * extends from VADDR up to (but not including) VADDR+MEMSIZE. The
 * segment on disk is located at file offset OFFSET and has length
 * FILESIZE.
 *
 * FILESIZE may be less than MEMSIZE; if so the remaining portion of
 * the in-memory segment should be zero-filled.
 *
 * Note that uiomove will catch it if someone tries to load an
 * executable whose load address is in kernel space. If you should
 * change this code to not use uiomove, be sure to check for this case
 * explicitly.
 */
static
int
load_segment(struct addrspace *as, struct vnode *v,
             off_t offset, vaddr_t vaddr, 
             size_t memsize, size_t filesize,
             int is_executable)
{
        struct iovec iov;
        struct uio u;
        int result;

        if (filesize > memsize) {
                kprintf("ELF: warning: segment filesize > segment memsize\n");
                filesize = memsize;
        }

        DEBUG(DB_EXEC, "ELF: Loading %lu bytes to 0x%lx\n", 
              (unsigned long) filesize, (unsigned long) vaddr);

        iov.iov_ubase = (userptr_t)vaddr;
        iov.iov_len = memsize;           // length of the memory space
        u.uio_iov = &iov;
        u.uio_iovcnt = 1;
        u.uio_resid = filesize;          // amount to read from the file
        u.uio_offset = offset;
        u.uio_segflg = is_executable ? UIO_USERISPACE : UIO_USERSPACE;
        u.uio_rw = UIO_READ;
        u.uio_space = as;

        result = VOP_READ(v, &u);
        if (result) {
                return result;
        }

        if (u.uio_resid != 0) {
                /* short read; problem with executable? */
                kprintf("ELF: short read on segment - file truncated?\n");
                return ENOEXEC;
        }

        /*
         * If memsize > filesize, the remaining space should be
         * zero-filled. There is no need to do this explicitly,
         * because the VM system should provide pages that do not
         * contain other processes' data, i.e., are already zeroed.
         *
         * During development of your VM system, it may have bugs that
         * cause it to (maybe only sometimes) not provide zero-filled
         * pages, which can cause user programs to fail in strange
         * ways. Explicitly zeroing program BSS may help identify such
         * bugs, so the following disabled code is provided as a
         * diagnostic tool. Note that it must be disabled again before
         * you submit your code for grading.
         */
#if 0
        {
                size_t fillamt;

                fillamt = memsize - filesize;
                if (fillamt > 0) {
                        DEBUG(DB_EXEC, "ELF: Zero-filling %lu more bytes\n", 
                              (unsigned long) fillamt);
                        u.uio_resid += fillamt;
                        result = uiomovezeros(fillamt, &u);
                }
        }
#endif
        
        return result;
}

/*
 * Load an ELF executable user program into the current address space.
 *
 * Returns the entry point (initial PC) for the program in ENTRYPOINT.
 */
int
load_elf(struct vnode *v, vaddr_t *entrypoint)
{
        Elf_Ehdr eh;   /* Executable header */
        Elf_Phdr ph;   /* "Program header" = segment header */
        int result, i;
        struct iovec iov;
        struct uio ku;
        struct addrspace *as;

        as = curproc_getas();

        /*
         * Read the executable header from offset 0 in the file.
         */

        uio_kinit(&iov, &ku, &eh, sizeof(eh), 0, UIO_READ);
        result = VOP_READ(v, &ku);
        if (result) {
                return result;
        }

        if (ku.uio_resid != 0) {
                /* short read; problem with executable? */
                kprintf("ELF: short read on header - file truncated?\n");
                return ENOEXEC;
        }

        /*
         * Check to make sure it's a 32-bit ELF-version-1 executable
         * for our processor type. If it's not, we can't run it.
         *
         * Ignore EI_OSABI and EI_ABIVERSION - properly, we should
         * define our own, but that would require tinkering with the
         * linker to have it emit our magic numbers instead of the
         * default ones. (If the linker even supports these fields,
         * which were not in the original elf spec.)
         */

        if (eh.e_ident[EI_MAG0] != ELFMAG0 ||
            eh.e_ident[EI_MAG1] != ELFMAG1 ||
            eh.e_ident[EI_MAG2] != ELFMAG2 ||
            eh.e_ident[EI_MAG3] != ELFMAG3 ||
            eh.e_ident[EI_CLASS] != ELFCLASS32 ||
            eh.e_ident[EI_DATA] != ELFDATA2MSB ||
            eh.e_ident[EI_VERSION] != EV_CURRENT ||
            eh.e_version != EV_CURRENT ||
            eh.e_type!=ET_EXEC ||
            eh.e_machine!=EM_MACHINE) {
                return ENOEXEC;
        }

        /*
         * Go through the list of segments and set up the address space.
         *
         * Ordinarily there will be one code segment, one read-only
         * data segment, and one data/bss segment, but there might
         * conceivably be more. You don't need to support such files
         * if it's unduly awkward to do so.
         *
         * Note that the expression eh.e_phoff + i*eh.e_phentsize is 
         * mandated by the ELF standard - we use sizeof(ph) to load,
         * because that's the structure we know, but the file on disk
         * might have a larger structure, so we must use e_phentsize
         * to find where the phdr starts.
         */

        for (i=0; i<eh.e_phnum; i++) {
                off_t offset = eh.e_phoff + i*eh.e_phentsize;
                uio_kinit(&iov, &ku, &ph, sizeof(ph), offset, UIO_READ);

                result = VOP_READ(v, &ku);
                if (result) {
                        return result;
                }

                if (ku.uio_resid != 0) {
                        /* short read; problem with executable? */
                        kprintf("ELF: short read on phdr - file truncated?\n");
                        return ENOEXEC;
                }

                switch (ph.p_type) {
                    case PT_NULL: /* skip */ continue;
                    case PT_PHDR: /* skip */ continue;
                    case PT_MIPS_REGINFO: /* skip */ continue;
                    case PT_LOAD: break;
                    default:
                        kprintf("loadelf: unknown segment type %d\n", 
                                ph.p_type);
                        return ENOEXEC;
                }

                result = as_define_region(as,
                                          ph.p_vaddr, ph.p_memsz,
                                          ph.p_flags & PF_R,
                                          ph.p_flags & PF_W,
                                          ph.p_flags & PF_X);
                if (result) {
                        return result;
                }
        }

        result = as_prepare_load(as);
        if (result) {
                return result;
        }

        /*
         * Now actually load each segment.
         */

        for (i=0; i<eh.e_phnum; i++) {
                off_t offset = eh.e_phoff + i*eh.e_phentsize;
                uio_kinit(&iov, &ku, &ph, sizeof(ph), offset, UIO_READ);

                result = VOP_READ(v, &ku);
                if (result) {
                        return result;
                }

                if (ku.uio_resid != 0) {
                        /* short read; problem with executable? */
                        kprintf("ELF: short read on phdr - file truncated?\n");
                        return ENOEXEC;
                }

                switch (ph.p_type) {
                    case PT_NULL: /* skip */ continue;
                    case PT_PHDR: /* skip */ continue;
                    case PT_MIPS_REGINFO: /* skip */ continue;
                    case PT_LOAD: break;
                    default:
                        kprintf("loadelf: unknown segment type %d\n", 
                                ph.p_type);
                        return ENOEXEC;
                }

                result = load_segment(as, v, ph.p_offset, ph.p_vaddr, 
                                      ph.p_memsz, ph.p_filesz,
                                      ph.p_flags & PF_X);
                if (result) {
                        return result;
                }
        }

        result = as_complete_load(as);
        if (result) {
                return result;
        }

        *entrypoint = eh.e_entry;

        return 0;
}