Home > Article > Backend Development > Gdb method traverses EG (symbol_table) hash table key_PHP tutorial
Gdb method traverses EG (symbol_table) hash table key_PHP tutorial
- WBOYOriginal
- 2016-07-13 17:47:301219browse
Sara Golemon wrote an article in which she mentioned: "Is there a special place where you can find the GLOBALS array?" The answer is "there", which is the EG (symbol_table)-Executor Globals structure. She also gave specific examples of where to find it. As follows
PHP_FUNCTION(confirm_getGlobal_compiled) {
char *varname;
int varname_len;
zval **varvalue;
If (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "s", &varname, &varname_len) == FAILURE) {
RETURN_NULL();
}
If (zend_hash_find(&EG(symbol_table), varname, varname_len + 1, (void**)&varvalue) == FAILURE) {
php_error_docref(NULL TSRMLS_CC, E_NOTICE, "Undefined variable: %s", varname);
RETURN_NULL();
}
*return_value = **varvalue;
zval_copy_ctor(return_value);
}
After compiling it into so and loading it, write the php test code
$abc = 'string';
$def = 'string2';
var_dump(confirm_getGlobal_compiled('abc'));
Execution results
string(6) "string"
You may feel strange why an extra def variable is written. This is what we will do next. Let’s take a look at the EG hashtable
gdb --args bin/php -c php.ini a.php
The debugging code is as follows
(gdb) b renzhi.c: 301 //Add a breakpoint in the written extension
No source file named renzhi.c.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (renzhi.c : 301) pending.
(gdb) r //Run to breakpoint
Starting program: /root/php-src-5.3/bin/php -c php.ini ceshi.php
warning: .dynamic section for "/lib/libc.so.6" is not at the expected address
warning: difference appears to be caused by prelink, adjusting expectations
[Thread debugging using libthread_db enabled]
Breakpoint 1, zif_confirm_getGlobal_compiled (ht=1, return_value=0x837a43c, return_value_ptr=0x0, this_ptr=0x0, return_value_used=1)
at /root/php-src-5.3/ext/renzhi/renzhi.c:305
305 if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "s", &varname, &varname_len) == FAILURE) {
(gdb) n
309 if (zend_hash_find(&EG(symbol_table), varname, varname_len + 1, (void**)&varvalue) == FAILURE) {
(gdb) step //Enter zend_hash_find hash search function
zend_hash_find (ht=0x82e3250, arKey=0x837a42c "abc", nKeyLength=4, pData=0xbfffc484) at /root/php-src-5.3/Zend/zend_hash.c:872
Check out the key below
(gdb) p *ht
$9 = {nTableSize = 64, nTableMask = 63, nNumOfElements = 10, nNextFreeElement = 0, pInternalPointer = 0x83edc98, pListHead = 0x83edc98,
pListTail = 0x837a3fc, arBuckets = 0x83705a8, pDestructor = 0x81923b0 <_zval_ptr_dtor>, persistent = 0 '
bApplyProtection = 1 '(gdb) p *ht.pListHead
$2 = {h = 2572561225, nKeyLength = 8, pData = 0x83edca4, pDataPtr = 0x83edc7c, pListNext = 0x8378c4c, pListLast = 0x0, pNext = 0x0, pLast = 0x0,
arKey = "G"}
(gdb) p *ht.pListHead.pListNext
$3 = {h = 253399445, nKeyLength = 5, pData = 0x8378c58, pDataPtr = 0x8378b60, pListNext = 0x8378c7c, pListLast = 0x83edc98, pNext = 0x0, pLast = 0x0,
arKey = "a"}
(gdb) p *ht.pListHead.pListNext.pListNext
$4 = {h = 253398818, nKeyLength = 5, pData = 0x8378c88, pDataPtr = 0x8378c30, pListNext = 0x8378d20, pListLast = 0x8378c4c, pNext = 0x0, pLast = 0x0,
arKey = "a"}
(gdb) p *ht.pListHead.pListNext.pListNext.pListNext
$5 = {h = 3947724458, nKeyLength = 6, pData = 0x8378d2c, pDataPtr = 0x8378cac, pListNext = 0x8378d54, pListLast = 0x8378c7c, pNext = 0x0, pLast = 0x0,
arKey = "_"}
(gdb) p *ht.pListHead.pListNext.pListNext.pListNext.pListNext
$6 = {h = 249444164, nKeyLength = 5, pData = 0x8378d60, pDataPtr = 0x83edd1c, pListNext = 0x8378d84, pListLast = 0x8378d20, pNext = 0x0, pLast = 0x0,
arKey = "_"}
(gdb) p *ht.pListHead.pListNext.pListNext.pListNext.pListNext.pListNext
$7 = {h = 195471710, nKeyLength = 8, pData = 0x8378d90, pDataPtr = 0x83edd38, pListNext = 0x8378e2c, pListLast = 0x8378d54, pNext = 0x0, pLast = 0x0,
arKey = "_"}
(gdb) p *ht.pListHead.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext
$8 = {h = 1027153623, nKeyLength = 7, pData = 0x8378e38, pDataPtr = 0x8378db8, pListNext = 0x8379e8c, pListLast = 0x8378d84, pNext = 0x0, pLast = 0x0,
arKey = "_"}
(gdb) p *ht.pListHead.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext
$9 = {h = 3291685243, nKeyLength = 8, pData = 0x8379e98, pDataPtr = 0x8378e88, pListNext = 0x837a3cc, pListLast = 0x8378e2c, pNext = 0x0, pLast = 0x0,
arKey = "_"}
(gdb) p *ht.pListHead.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext
$10 = {h = 2090069483, nKeyLength = 4, pData = 0x837a3d8, pDataPtr = 0x8379ef8, pListNext = 0x837a3fc, pListLast = 0x8379e8c, pNext = 0x0, pLast = 0x0,
arKey = "a"}
(gdb) p *ht.pListHead.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext
$11 = {h = 2090180660, nKeyLength = 4, pData = 0x837a408, pDataPtr = 0x8379edc, pListNext = 0x0, pListLast = 0x837a3cc, pNext = 0x0, pLast = 0x0,
arKey = "d"}
有点乱，这里第一条就是现实了EG这个hash表里面有nNumOfElements =10个元素
这里的
(gdb) p *ht.pListHead.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext
$10 = {h = 2090069483, nKeyLength = 4, pData = 0x837a3d8, pDataPtr = 0x8379ef8, pListNext = 0x837a3fc, pListLast = 0x8379e8c, pNext = 0x0, pLast = 0x0,
arKey = "a"}
(gdb) p *ht.pListHead.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext
$11 = {h = 2090180660, nKeyLength = 4, pData = 0x837a408, pDataPtr = 0x8379edc, pListNext = 0x0, pListLast = 0x837a3cc, pNext = 0x0, pLast = 0x0,
arKey = "d"}
就是测试php代码里面的
$abc = 'string';
$def = 'string2';
这两个变量名称的具体hash的bucket了
(gdb) p *ht.pListHead.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext
$10 = {h = 2090069483, nKeyLength = 4, pData = 0x837a3d8, pDataPtr = 0x8379ef8, pListNext = 0x837a3fc, pListLast = 0x8379e8c, pNext = 0x0, pLast = 0x0,
arKey = "a"}
第一个字符arKey为a，有nKeyLength = 4四个字符长度
(gdb) p ht.pListHead.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.arKey[0]
$10 = 97 'a'
(gdb) p ht.pListHead.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.arKey[1]
$11 = 98 'b'
(gdb) p ht.pListHead.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.arKey[2]
$12 = 99 'c'
(gdb) p ht.pListHead.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.pListNext.arKey[3]
$13 = 0 '