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

On Tue, Dec 11, 2018 at 04:29:54PM +0100, Oleg Nesterov wrote:

On 12/10, Dmitry V. Levin wrote:

quoted

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.

Yes, without padding these sizes will depend on architecture:

$ cat t.c
#include <linux/types.h>
int main() {
	struct s {
		__u64 nr;
		__u64 args[6];
		__u32 ret_data;
	};
	return sizeof(struct s);
}

$ gcc -m64 -Wall -O2 t.c && ./a.out; echo $?
64
$ gcc -m32 -Wall -O2 t.c && ./a.out; echo $?
60

This happens because __u64 has 32-bit alignment on some 32-bit
architectures like x86.

There is also m68k where __u32 has 16-bit alignment.

quoted

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.

Alternatively, we could use __attribute__((aligned(N))), e.g.

struct ptrace_syscall_info {
	__u8 op;	/* PTRACE_SYSCALL_INFO_* */
	__u32 arch __attribute__((aligned(4)));
	__u64 instruction_pointer;
	__u64 stack_pointer;
	union {
		struct {
			__u64 nr __attribute__((aligned(8)));
			__u64 args[6];
		} entry;
		struct {
			__s64 rval __attribute__((aligned(8)));
			__u8 is_error;
		} exit;
		struct {
			__u64 nr __attribute__((aligned(8)));
			__u64 args[6];
			__u32 ret_data;
		} seccomp;
	};
};

Do you prefer __attribute__((aligned(N))) to padding?


-- 
ldv