Re: [PATCH v5 24/25] ptrace: add PTRACE_GET_SYSCALL_INFO request

On 12/10, Dmitry V. Levin wrote:

On Mon, Dec 10, 2018 at 03:11:07PM +0100, Oleg Nesterov wrote:

quoted

On 12/10, Dmitry V. Levin wrote:

quoted

+struct ptrace_syscall_info {
+	__u8 op;	/* PTRACE_SYSCALL_INFO_* */
+	__u8 __pad0[3];
+	__u32 arch;
+	__u64 instruction_pointer;
+	__u64 stack_pointer;
+	__u64 frame_pointer;
+	union {
+		struct {
+			__u64 nr;
+			__u64 args[6];
+		} entry;
+		struct {
+			__s64 rval;
+			__u8 is_error;
+			__u8 __pad1[7];
+		} exit;
+		struct {
+			__u64 nr;
+			__u64 args[6];
+			__u32 ret_data;
+			__u8 __pad2[4];
+		} seccomp;
+	};
+};

Could you explain why ptrace_syscall_info needs __pad{0,1,2} ? I simply can't
understand why...

I suppose the idea behind the use of these pads was to make the structure
arch-independent.

Still can't understand... are you saying that without (say) __pad2[4]
sizeof(ptrace_syscall_info) or offsetofend(ptrace_syscall_info, seccomp)
will depend on arch? Or what? I am just curious.

I don't think we really need to keep it exactly the same on all
architectures - the only practical requirement is to avoid any compat
issues, but I don't mind keeping the structure arch-independent.

OK, but may be you can add a short comment to explain these pads.

Oleg.