From f1da8108620a8560436a815f52a82439b16b6ee4 Mon Sep 17 00:00:00 2001
From: Egor Tensin <Egor.Tensin@gmail.com>
Date: Mon, 7 Nov 2022 15:28:03 +0100
Subject: add "Fun with ptrace" posts

---
 _posts/2022-11-07-ptrace-sigtraps.md | 72 ++++++++++++++++++++++++++++++++++++
 1 file changed, 72 insertions(+)
 create mode 100644 _posts/2022-11-07-ptrace-sigtraps.md

(limited to '_posts/2022-11-07-ptrace-sigtraps.md')

diff --git a/_posts/2022-11-07-ptrace-sigtraps.md b/_posts/2022-11-07-ptrace-sigtraps.md
new file mode 100644
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+++ b/_posts/2022-11-07-ptrace-sigtraps.md
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+---
+title: 'Fun with ptrace: SIGTRAPs galore'
+date: 2022-11-07 13:00 +0100
+---
+When using `PTRACE_ATTACH` the `ptrace` mechanism reuses SIGTRAP for a number
+of things by default.
+This makes it unnecessarily hard to distinguish regular traps possibly caused
+by breakpoints we might place from other events.
+
+1. After `ptrace(PTRACE_SYSCALL)`, syscall-stops will be reported as SIGTRAPs.
+
+   ```c
+   int status;
+
+   ptrace(PTRACE_SYSCALL, pid, 0, 0);
+   waitpid(pid, &status, 0);
+
+   if (WIFSTOPPED(status) && WSTOPSIG(status) == SIGTRAP) {
+       /* We don't know if the tracee has just entered/exited a syscall or
+        * received a regular SIGTRAP (could be caused by a breakpoint we
+        * placed). */
+   }
+   ```
+
+   This is fixed by using the `PTRACE_O_TRACESYSGOOD` option.
+
+   ```c
+   int status;
+
+   ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_TRACESYSGOOD);
+   ptrace(PTRACE_SYSCALL, pid, 0, 0);
+   waitpid(pid, &status, 0);
+
+   if (WIFSTOPPED(status) && WSTOPSIG(status) == (SIGTRAP | 0x80)) {
+       /* We know for sure that the tracee has just entered/exited a
+        * syscall. */
+   }
+   ```
+
+2. Every `execve` call will be reported as a SIGTRAP.
+
+   ```c
+   int status;
+
+   ptrace(PTRACE_CONT, pid, 0, 0);
+   waitpid(pid, &status, 0);
+
+   if (WIFSTOPPED(status) && WSTOPSIG(status) == SIGTRAP) {
+       /* We don't know if the tracee just called execve() or received a
+        * regular SIGTRAP (could be caused by a breakpoint we placed). */
+   }
+   ```
+
+   This is fixed by using the `PTRACE_O_TRACEEXEC` option.
+
+   ```c
+   int status;
+
+   ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_TRACEEXEC);
+   ptrace(PTRACE_CONT, pid, 0, 0);
+   waitpid(pid, &status, 0);
+
+   if (WIFSTOPPED(status) && status >> 8 == (SIGTRAP | PTRACE_EVENT_EXEC << 8)) {
+       /* We know for sure that the tracee has just called execve(). */
+   }
+   ```
+
+   This point doesn't apply to tracees attached using `PTRACE_SEIZE`.
+   {: .alert .alert-info }
+
+As you can see, you should always use at least the `PTRACE_O_TRACESYSGOOD` and
+`PTRACE_O_TRACEEXEC` options to be able to distinguish between SIGTRAPs.
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