Add ptrace::read_user and ptrace::write_user

CHANGELOG.md

@@ -64,6 +64,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 - Added `Ipv6DontFrag` for android, iOS, linux and macOS.
 - Added `IpDontFrag` for iOS, macOS.
   (#[1692](https://github.com/nix-rust/nix/pull/1692))
+- Added `ptrace::read_user` and `ptrace::write_user` for Linux.
 
 ### Changed
 

src/sys/ptrace/linux.rs

@@ -481,3 +481,24 @@ pub unsafe fn write(
 {
     ptrace_other(Request::PTRACE_POKEDATA, pid, addr, data).map(drop)
 }
+
+/// Reads a word from a user area at `offset`.
+/// The user struct definition can be found in `/usr/include/sys/user.h`.
+pub fn read_user(pid: Pid, offset: AddressType) -> Result<c_long> {
+    ptrace_peek(Request::PTRACE_PEEKUSER, pid, offset, ptr::null_mut())
+}
+
+/// Writes a word to a user area at `offset`.
+/// The user struct definition can be found in `/usr/include/sys/user.h`.
+///
+/// # Safety
+///
+/// The `data` argument is passed directly to `ptrace(2)`.  Read that man page
+/// for guidance.
+pub unsafe fn write_user(
+    pid: Pid,
+    offset: AddressType,
+    data: *mut c_void) -> Result<()>
+{
+    ptrace_other(Request::PTRACE_POKEUSER, pid, offset, data).map(drop)
+}

test/sys/test_ptrace.rs

@@ -197,15 +197,36 @@ fn test_ptrace_syscall() {
             #[cfg(target_arch = "x86")]
             let get_syscall_id = || ptrace::getregs(child).unwrap().orig_eax as libc::c_long;
 
+            // this duplicates `get_syscall_id` for the purpose of testing `ptrace::read_user`.
+            #[cfg(target_arch = "x86_64")]
+            let get_rax_offset = |user_struct_ptr: *const libc::user| {
+                unsafe { &(*user_struct_ptr).regs.orig_rax as *const _ }
+            };
+            #[cfg(target_arch = "x86")]
+            let get_rax_offset = |user_struct_ptr: *const libc::user| {
+                unsafe { &(*user_struct_ptr).regs.orig_eax as *const _ }
+            };
+
+            let get_syscall_from_user_area = || {
+                // Find the offset of `user.regs.rax` (or `eax` for x86)
+                let user_struct = std::mem::MaybeUninit::<libc::user>::uninit();
+                let user_struct_ptr = user_struct.as_ptr();
+                let rax_offset = get_rax_offset(user_struct_ptr) as usize - user_struct_ptr as usize;
+
+                ptrace::read_user(child, rax_offset as _).unwrap() as libc::c_long
+            };
+
             // kill entry
             ptrace::syscall(child, None).unwrap();
             assert_eq!(waitpid(child, None), Ok(WaitStatus::PtraceSyscall(child)));
             assert_eq!(get_syscall_id(), ::libc::SYS_kill);
+            assert_eq!(get_syscall_from_user_area(), ::libc::SYS_kill);
 
             // kill exit
             ptrace::syscall(child, None).unwrap();
             assert_eq!(waitpid(child, None), Ok(WaitStatus::PtraceSyscall(child)));
             assert_eq!(get_syscall_id(), ::libc::SYS_kill);
+            assert_eq!(get_syscall_from_user_area(), ::libc::SYS_kill);
 
             // receive signal
             ptrace::syscall(child, None).unwrap();