主要实现对二进制可执行文件和shell文件的加载和执行，其中主要的函数是do_execve(),它是系统中断调用int 0x80的功能号__NR_execve()调用，是exec()函数的主要实现以下几点功能：
1.执行对参数和环境参数空间页面的初始化操作，初始化空间页面指针数组，根据执行文件名取执行对象的i节点，计算参数个数和环境变量个数，检查文件类型、执行权限
2.根据执行文件开始部分的头数据结构，对其中信息进行处理，根据被执行文件i节点读取文件头部信息，若是shell以#！开始，则分析shell程序名和其参数；根据幻数和段长度等信息判断是否可执行
3.对当前调用进程进行运行新文件前初始化操作，指向新执行文件的i节点，复位信号处理句柄，根据头结构信息设置局部描述符地址和段长；设置参数和环境参数页面指针；修改进程各执行字段内容
4.替换堆栈上原调用execve()程序的返回地址为新执行程序运行地址，运行新加载的程序
create_tables函数用于根据给定的当前堆栈指针值p以及参数个数argc和环境变量个数envc，在新的程序堆栈中创建环境和参数变量指针表，并返回此时的堆栈指针值sp
create_tables
用戶内存中解析环境变量和参数字符串，创建指针表，并将地址放到堆栈上，返回栈指针
/*
* MAX_ARG_PAGES defines the number of pages allocated for arguments
* and envelope for the new program. 32 should suffice, this gives
* a maximum env+arg of 128kB !
*/
#define MAX_ARG_PAGES 32
/*
* create_tables() parses the env- and arg-strings in new user
* memory and creates the pointer tables from them, and puts their
* addresses on the "stack", returning the new stack pointer value.
*/
static unsigned long * create_tables(char * p,int argc,int envc)
{
unsigned long *argv,*envp;
unsigned long * sp;
//堆栈指针是以4字节为边界寻址
sp = (unsigned long *) (0xfffffffc & (unsigned long) p);
//sp向下移动，空出环境参数占用空间个数，让环境参数指针envp指向该处
sp -= envc+1;
envp = sp;
//sp向下移动，空出环境参数占用的空间个数，并让环境参数指针envp指向该处
sp -= argc+1;
argv = sp;
//argc、argv、envp
put_fs_long((unsigned long)envp,--sp);
put_fs_long((unsigned long)argv,--sp);
put_fs_long((unsigned long)argc,--sp);
//argc
while (argc-->0) {
put_fs_long((unsigned long) p,argv++);
while (get_fs_byte(p++)) /* nothing */ ;
}
//最后放置NULL
put_fs_long(0,argv);
//envc
while (envc-->0) {
put_fs_long((unsigned long) p,envp++);
while (get_fs_byte(p++)) /* nothing */ ;
}
//最后放置NULL
put_fs_long(0,envp);
return sp;
}
count
计算参数个数
/*
* count() counts the number of arguments/envelopes
*/
static int count(char ** argv)
{
int i=0;
char ** tmp;
if ((tmp = argv))
//判断为NULL就退出
while (get_fs_long((unsigned long *) (tmp++)))
i++;
return i;
}
copy_string
函数从用户内存空间拷贝参数和环境字符串到内核空闲页面内存中，argc
/*
* 'copy_string()' copies argument/envelope strings from user
* memory to free pages in kernel mem. These are in a format ready
* to be put directly into the top of new user memory.
*
* Modified by TYT, 11/24/91 to add the from_kmem argument, which specifies
* whether the string and the string array are from user or kernel segments:
*
* from_kmem argv * argv **
* 0 user space user space
* 1 kernel space user space
* 2 kernel space kernel space
*
* We do this by playing games with the fs segment register. Since it
* it is expensive to load a segment register, we try to avoid calling
* set_fs() unless we absolutely have to.
*/
static unsigned long copy_strings(int argc,char ** argv,unsigned long *page,
unsigned long p, int from_kmem)
{
char *tmp, *pag=NULL;
int len, offset = 0;
unsigned long old_fs, new_fs;
if (!p)
return 0; /* bullet-proofing */
new_fs = get_ds();
old_fs = get_fs();
if (from_kmem==2)
set_fs(new_fs);
while (argc-- > 0) {
if (from_kmem == 1)
set_fs(new_fs);
if (!(tmp = (char *)get_fs_long(((unsigned long *)argv)+argc)))
panic("argc is wrong");
if (from_kmem == 1)
set_fs(old_fs);
len=0; /* remember zero-padding */
//计算字符串的长度
do {
len++;
} while (get_fs_byte(tmp++));
//字符串长度判断
if (p-len < 0) { /* this shouldn't happen - 128kB */
set_fs(old_fs);
return 0;
}
while (len) {
--p; --tmp; --len;
//首次复制字符串，offset复制为0
if (--offset < 0) {
//p指针在页内的偏移
offset = p % PAGE_SIZE;
//恢复fs段寄存器
if (from_kmem==2)
set_fs(old_fs);
//如果pag不为NULL，说明页面在之前的拷贝过程得到分配
//pag指针为NULL，说明该页面还没有分配内存空间
if (!(pag = (char *) page[p/PAGE_SIZE]) &&
!(pag = (char *) (page[p/PAGE_SIZE] =
(unsigned long *) get_free_page())))
return 0;
if (from_kmem==2)
set_fs(new_fs);
}
*(pag + offset) = get_fs_byte(tmp);
}
}
//还原fs
if (from_kmem==2)
set_fs(old_fs);
return p;
}修改局部描述符表中的描述符基址和段限长，并将参数和环境页面放置在数据段末端
参数:text_size 执行文件头部中a_text字段给出的代码段长度值，page参数和环境空间页面指针数组
static unsigned long change_ldt(unsigned long text_size,unsigned long * page)
{
unsigned long code_limit,data_limit,code_base,data_base;
int i;
//根据执行文件头部a_text的值，计算以页面长度为边界代码段限长，并设置数据段长度为64MB
code_limit = text_size+PAGE_SIZE -1;
code_limit &= 0xFFFFF000;
data_limit = 0x4000000;
//取当前进程中局部描述符代码段描述符中代码段基地址，代码段基址和数据段基址相同
code_base = get_base(current->ldt[1]);
data_base = code_base;
//重新设置局部表中代码段和数据描述符的基址和段限长
set_base(current->ldt[1],code_base);
set_limit(current->ldt[1],code_limit);
set_base(current->ldt[2],data_base);
set_limit(current->ldt[2],data_limit);
/* make sure fs points to the NEW data segment */
__asm__("pushl $0x17\n\tpop %%fs"::);
data_base += data_limit;
for (i=MAX_ARG_PAGES-1 ; i>=0 ; i--) {
data_base -= PAGE_SIZE;
if (page[i])
put_page(page[i],data_base);
}
return data_limit;
}
execve系统中断调用函数，加载并执行子进程
函数系统中断调用int 0x80功能号__NR_execve，
参数：eip指向堆栈中调用系统中断的程序代码指针eip，
tmp：_sys_execve时返回地址
filename 被执行程序文件名
argv命令行参数指针数组
envp环境变量指针数组
/*
* 'do_execve()' executes a new program.
*/
int do_execve(unsigned long * eip,long tmp,char * filename,
char ** argv, char ** envp)
{
struct m_inode * inode;
struct buffer_head * bh;
struct exec ex;
unsigned long page[MAX_ARG_PAGES];
int i,argc,envc;
int e_uid, e_gid;
int retval;
int sh_bang = 0;
unsigned long p=PAGE_SIZE*MAX_ARG_PAGES-4;
if ((0xffff & eip[1]) != 0x000f)
panic("execve called from supervisor mode");
for (i=0 ; i<MAX_ARG_PAGES ; i++) /* clear page-table */
page[i]=0;
if (!(inode=namei(filename))) /* get executables inode */
return -ENOENT;
argc = count(argv);
envc = count(envp);
restart_interp:
if (!S_ISREG(inode->i_mode)) { /* must be regular file */
retval = -EACCES;
goto exec_error2;
}
i = inode->i_mode;
e_uid = (i & S_ISUID) ? inode->i_uid : current->euid;
e_gid = (i & S_ISGID) ? inode->i_gid : current->egid;
if (current->euid == inode->i_uid)
i >>= 6;
else if (current->egid == inode->i_gid)
i >>= 3;
if (!(i & 1) &&
!((inode->i_mode & 0111) && suser())) {
retval = -ENOEXEC;
goto exec_error2;
}
if (!(bh = bread(inode->i_dev,inode->i_zone[0]))) {
retval = -EACCES;
goto exec_error2;
}
ex = *((struct exec *) bh->b_data); /* read exec-header */
if ((bh->b_data[0] == '#') && (bh->b_data[1] == '!') && (!sh_bang)) {
/*
* This section does the #! interpretation.
* Sorta complicated, but hopefully it will work. -TYT
*/
char buf[1023], *cp, *interp, *i_name, *i_arg;
unsigned long old_fs;
strncpy(buf, bh->b_data+2, 1022);
brelse(bh);
iput(inode);
buf[1022] = '\0';
if ((cp = strchr(buf, '\n'))) {
*cp = '\0';
for (cp = buf; (*cp == ' ') || (*cp == '\t'); cp++);
}
if (!cp || *cp == '\0') {
retval = -ENOEXEC; /* No interpreter name found */
goto exec_error1;
}
interp = i_name = cp;
i_arg = 0;
for ( ; *cp && (*cp != ' ') && (*cp != '\t'); cp++) {
if (*cp == '/')
i_name = cp+1;
}
if (*cp) {
*cp++ = '\0';
i_arg = cp;
}
/*
* OK, we've parsed out the interpreter name and
* (optional) argument.
*/
if (sh_bang++ == 0) {
p = copy_strings(envc, envp, page, p, 0);
p = copy_strings(--argc, argv+1, page, p, 0);
}
/*
* Splice in (1) the interpreter's name for argv[0]
* (2) (optional) argument to interpreter
* (3) filename of shell script
*
* This is done in reverse order, because of how the
* user environment and arguments are stored.
*/
p = copy_strings(1, &filename, page, p, 1);
argc++;
if (i_arg) {
p = copy_strings(1, &i_arg, page, p, 2);
argc++;
}
p = copy_strings(1, &i_name, page, p, 2);
argc++;
if (!p) {
retval = -ENOMEM;
goto exec_error1;
}
/*
* OK, now restart the process with the interpreter's inode.
*/
old_fs = get_fs();
set_fs(get_ds());
if (!(inode=namei(interp))) { /* get executables inode */
set_fs(old_fs);
retval = -ENOENT;
goto exec_error1;
}
set_fs(old_fs);
goto restart_interp;
}
brelse(bh);
if (N_MAGIC(ex) != ZMAGIC || ex.a_trsize || ex.a_drsize ||
ex.a_text+ex.a_data+ex.a_bss>0x3000000 ||
inode->i_size < ex.a_text+ex.a_data+ex.a_syms+N_TXTOFF(ex)) {
retval = -ENOEXEC;
goto exec_error2;
}
if (N_TXTOFF(ex) != BLOCK_SIZE) {
printk("%s: N_TXTOFF != BLOCK_SIZE. See a.out.h.", filename);
retval = -ENOEXEC;
goto exec_error2;
}
if (!sh_bang) {
p = copy_strings(envc,envp,page,p,0);
p = copy_strings(argc,argv,page,p,0);
if (!p) {
retval = -ENOMEM;
goto exec_error2;
}
}
/* OK, This is the point of no return */
if (current->executable)
iput(current->executable);
current->executable = inode;
for (i=0 ; i<32 ; i++)
current->sigaction[i].sa_handler = NULL;
for (i=0 ; i<NR_OPEN ; i++)
if ((current->close_on_exec>>i)&1)
sys_close(i);
current->close_on_exec = 0;
free_page_tables(get_base(current->ldt[1]),get_limit(0x0f));
free_page_tables(get_base(current->ldt[2]),get_limit(0x17));
if (last_task_used_math == current)
last_task_used_math = NULL;
current->used_math = 0;
p += change_ldt(ex.a_text,page)-MAX_ARG_PAGES*PAGE_SIZE;
p = (unsigned long) create_tables((char *)p,argc,envc);
current->brk = ex.a_bss +
(current->end_data = ex.a_data +
(current->end_code = ex.a_text));
current->start_stack = p & 0xfffff000;
current->euid = e_uid;
current->egid = e_gid;
i = ex.a_text+ex.a_data;
while (i&0xfff)
put_fs_byte(0,(char *) (i++));
eip[0] = ex.a_entry; /* eip, magic happens :-) */
eip[3] = p; /* stack pointer */
return 0;
exec_error2:
iput(inode);
exec_error1:
for (i=0 ; i<MAX_ARG_PAGES ; i++)
free_page(page[i]);
return(retval);
}
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