Fix syscall_info

According to the man page:

> The addr argument contains the size of the buffer pointed to by the
> data argument (i.e., sizeof(struct ptrace_syscall_info)).

We were setting the data argument, so this syscall was returning
garbage. This is easily reproduced and verified by looking at the `op`
value, which should range from 0 to 3. The PR associated with this
commit contains a sample program to demonstrate this.

The fix is done in the `ptrace_get_data` helper to avoid duplicating its
implementation just for `syscall_info`.

Of all the other callers, all but one are documented as ignoring `addr`.
The other one (`PTRACE_GETREGS`) is documented as ignoring `addr`
_except on SPARC systems_, on which `addr` and `data` are reversed.
However, this is already not respected by `nix`, so this changes is not
disruptive in this regard.

There should be no performance concerns as we are replacing one constant
with another.

Author: cbarrete

changelog/2653.fixed.md

@@ -0,0 +1 @@
+Fixed `nix::sys::ptrace::syscall_info`, which was not setting the `data` argument properly, causing garbage values to be returned.

examples/ptrace.rs

@@ -0,0 +1,36 @@
+//! Traces a child process using `ptrace`.
+//!
+//! The child issues a single `write` syscall, which is printed upon entry and exit.
+
+#[cfg(target_os = "linux")]
+fn main() {
+    let pid = unsafe { nix::unistd::fork().unwrap() };
+
+    match pid {
+        nix::unistd::ForkResult::Child => {
+            nix::sys::ptrace::traceme().unwrap();
+            nix::sys::signal::raise(nix::sys::signal::Signal::SIGCONT).unwrap();
+            println!("I'm issuing a syscall!");
+        }
+        nix::unistd::ForkResult::Parent { child } => {
+            nix::sys::wait::waitpid(Some(child), None).unwrap();
+            nix::sys::ptrace::setoptions(
+                child,
+                nix::sys::ptrace::Options::PTRACE_O_TRACESYSGOOD,
+            )
+            .unwrap();
+
+            nix::sys::ptrace::syscall(child, None).unwrap();
+            nix::sys::wait::waitpid(Some(child), None).unwrap();
+            let syscall_info = nix::sys::ptrace::syscall_info(child).unwrap();
+            println!("{syscall_info:?}");
+            assert!(syscall_info.op == libc::PTRACE_SYSCALL_INFO_ENTRY);
+
+            nix::sys::ptrace::syscall(child, None).unwrap();
+            nix::sys::wait::waitpid(Some(child), None).unwrap();
+            let syscall_info = nix::sys::ptrace::syscall_info(child).unwrap();
+            println!("{syscall_info:?}");
+            assert!(syscall_info.op == libc::PTRACE_SYSCALL_INFO_EXIT);
+        }
+    }
+}

src/sys/ptrace/linux.rs

@@ -504,7 +504,7 @@ fn ptrace_get_data<T>(request: Request, pid: Pid) -> Result<T> {
         libc::ptrace(
             request as RequestType,
             libc::pid_t::from(pid),
-            ptr::null_mut::<T>(),
+            std::mem::size_of::<T>(),
             data.as_mut_ptr(),
         )
     };