Gdb method traverses EG (symbol_table) hash table key_PHP tutorial

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 '

(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 '