3398744a61
Adds the following: * A note that you can use attaching to debug the right lldb-server process, though there are drawbacks. * A section on debugging the remote protocol. * Reducing bugs, including reducing ptrace bugs to remove the need for LLDB. I've added a standlone ptrace program to the examples folder because: * There's no better place to put it. * Adding it to the page seems like wasting space, and would be harder to update. * I link to Eli Bendersky's classic blog on the subject, but we are safer with our own example as well. * Eli's example is for 32 bit Intel, AArch64 is more common these days. * It's easier to show the software breakpoint steps in code than explain it (though I still do that in the text). * It was living on my laptop not helping anyone so I think it's good to have it upstream for others, including future me.
107 lines
2.5 KiB
C
107 lines
2.5 KiB
C
//===-- ptrace_example.c --------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#include <asm/ptrace.h>
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#include <linux/elf.h>
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#include <stdint.h>
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#include <stdio.h>
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#include <sys/prctl.h>
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#include <sys/ptrace.h>
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#include <sys/uio.h>
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#include <sys/wait.h>
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#include <unistd.h>
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// The demo program shows how to do basic ptrace operations without lldb
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// or lldb-server. For the purposes of experimentation or reporting bugs
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// in kernels.
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//
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// It is AArch64 Linux specific, adapt as needed.
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//
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// Expected output:
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// Before breakpoint
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// After breakpoint
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void inferior() {
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if (ptrace(PTRACE_TRACEME, 0, 0, 0) < 0) {
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perror("ptrace");
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return;
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}
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printf("Before breakpoint\n");
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// Go into debugger. Instruction replaced with nop later.
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// We write 2 instuctions because POKETEXT works with
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// 64 bit values and we don't want to overwrite the
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// call to printf accidentally.
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asm volatile("BRK #0 \n nop");
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printf("After breakpoint\n");
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}
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void debugger(pid_t child) {
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int wait_status;
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// Wait until it hits the breakpoint.
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wait(&wait_status);
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while (WIFSTOPPED(wait_status)) {
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if (WIFEXITED(wait_status)) {
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printf("inferior exited normally\n");
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return;
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}
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// Read general purpose registers to find the PC value.
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struct user_pt_regs regs;
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struct iovec io;
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io.iov_base = ®s;
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io.iov_len = sizeof(regs);
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if (ptrace(PTRACE_GETREGSET, child, NT_PRSTATUS, &io) < 0) {
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printf("getregset failed\n");
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return;
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}
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// Replace brk #0 / nop with nop / nop by writing to memory
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// at the current PC.
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uint64_t replace = 0xd503201fd503201f;
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if (ptrace(PTRACE_POKETEXT, child, regs.pc, replace) < 0) {
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printf("replacing bkpt failed\n");
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return;
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}
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// Single step over where the brk was.
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if (ptrace(PTRACE_SINGLESTEP, child, 0, 0) < 0) {
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perror("ptrace");
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return;
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}
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// Wait for single step to be done.
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wait(&wait_status);
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// Run to completion.
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if (ptrace(PTRACE_CONT, child, 0, 0) < 0) {
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perror("ptrace");
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return;
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}
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// Wait to see that the inferior exited.
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wait(&wait_status);
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}
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}
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int main() {
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pid_t child = fork();
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if (child == 0)
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inferior();
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else if (child > 0)
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debugger(child);
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else
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return -1;
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return 0;
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}
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