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

["Dmitry V. Levin" <ldv-u2l5PoMzF/Vg9hUCZPvPmw@public.gmane.org>]

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On Tue, Dec 11, 2018 at 04:29:54PM +0100, Oleg Nesterov wrote:
> On 12/10, Dmitry V. Levin wrote:
> > On Mon, Dec 10, 2018 at 03:11:07PM +0100, Oleg Nesterov wrote:
> > > On 12/10, Dmitry V. Levin wrote:
> > > >
> > > > +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.

> > 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?


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