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? -- ldv
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