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035c012e32
It turns out the startup code in csu/elf-init.c has a perfect pair of ROP gadgets (see Marco-Gisbert and Ripoll-Ripoll, "return-to-csu: A New Method to Bypass 64-bit Linux ASLR"). These functions are not needed in dynamically-linked binaries because DT_INIT/DT_INIT_ARRAY are already processed by the dynamic linker. However, the dynamic linker skipped the main program for some reason. For maximum backwards compatibility, this is not changed, and instead, the main map is consulted from __libc_start_main if the init function argument is a NULL pointer. For statically linked binaries, the old approach based on linker symbols is still used because there is nothing else available. A new symbol version __libc_start_main@@GLIBC_2.34 is introduced because new binaries running on an old libc would not run their ELF constructors, leading to difficult-to-debug issues.
128 lines
4.4 KiB
ArmAsm
128 lines
4.4 KiB
ArmAsm
/* Startup code compliant to the ELF x86-64 ABI.
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Copyright (C) 2001-2021 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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Contributed by Andreas Jaeger <aj@suse.de>, 2001.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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In addition to the permissions in the GNU Lesser General Public
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License, the Free Software Foundation gives you unlimited
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permission to link the compiled version of this file with other
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programs, and to distribute those programs without any restriction
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coming from the use of this file. (The GNU Lesser General Public
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License restrictions do apply in other respects; for example, they
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cover modification of the file, and distribution when not linked
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into another program.)
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Note that people who make modified versions of this file are not
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obligated to grant this special exception for their modified
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versions; it is their choice whether to do so. The GNU Lesser
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General Public License gives permission to release a modified
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version without this exception; this exception also makes it
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possible to release a modified version which carries forward this
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exception.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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/* This is the canonical entry point, usually the first thing in the text
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segment. The SVR4/i386 ABI (pages 3-31, 3-32) says that when the entry
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point runs, most registers' values are unspecified, except for:
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%rdx Contains a function pointer to be registered with `atexit'.
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This is how the dynamic linker arranges to have DT_FINI
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functions called for shared libraries that have been loaded
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before this code runs.
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%rsp The stack contains the arguments and environment:
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0(%rsp) argc
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LP_SIZE(%rsp) argv[0]
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...
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(LP_SIZE*argc)(%rsp) NULL
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(LP_SIZE*(argc+1))(%rsp) envp[0]
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...
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NULL
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*/
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#include <sysdep.h>
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ENTRY (_start)
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/* Clearing frame pointer is insufficient, use CFI. */
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cfi_undefined (rip)
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/* Clear the frame pointer. The ABI suggests this be done, to mark
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the outermost frame obviously. */
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xorl %ebp, %ebp
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/* Extract the arguments as encoded on the stack and set up
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the arguments for __libc_start_main (int (*main) (int, char **, char **),
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int argc, char *argv,
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void (*init) (void), void (*fini) (void),
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void (*rtld_fini) (void), void *stack_end).
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The arguments are passed via registers and on the stack:
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main: %rdi
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argc: %rsi
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argv: %rdx
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init: %rcx
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fini: %r8
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rtld_fini: %r9
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stack_end: stack. */
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mov %RDX_LP, %R9_LP /* Address of the shared library termination
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function. */
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#ifdef __ILP32__
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mov (%rsp), %esi /* Simulate popping 4-byte argument count. */
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add $4, %esp
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#else
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popq %rsi /* Pop the argument count. */
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#endif
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/* argv starts just at the current stack top. */
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mov %RSP_LP, %RDX_LP
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/* Align the stack to a 16 byte boundary to follow the ABI. */
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and $~15, %RSP_LP
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/* Push garbage because we push 8 more bytes. */
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pushq %rax
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/* Provide the highest stack address to the user code (for stacks
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which grow downwards). */
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pushq %rsp
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/* These used to be the addresses of .fini and .init. */
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xorl %r8d, %r8d
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xorl %ecx, %ecx
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#ifdef PIC
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mov main@GOTPCREL(%rip), %RDI_LP
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#else
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mov $main, %RDI_LP
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#endif
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/* Call the user's main function, and exit with its value.
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But let the libc call main. Since __libc_start_main in
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libc.so is called very early, lazy binding isn't relevant
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here. Use indirect branch via GOT to avoid extra branch
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to PLT slot. In case of static executable, ld in binutils
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2.26 or above can convert indirect branch into direct
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branch. */
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call *__libc_start_main@GOTPCREL(%rip)
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hlt /* Crash if somehow `exit' does return. */
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END (_start)
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/* Define a symbol for the first piece of initialized data. */
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.data
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.globl __data_start
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__data_start:
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.long 0
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.weak data_start
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data_start = __data_start
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