Re: [RFC][PATCH] x86, syscalls: use SYSCALL_DEFINE() macros for sys_modify_ldt()

[Andy Lutomirski]

On Fri, Oct 13, 2017 at 1:39 PM, Dave Hansen
<dave.hansen@xxxxxxxxxxxxxxx> wrote:
>
> I noticed that we don't have tracepoints for sys_modify_ldt().  I
> think that's because we define it directly instead of using the
> normal SYSCALL_DEFINEx() macros.
>
> Is there a reason for that, or were they just missed when the
> macros were created?

No, and it's a longstanding fsckup that I think you can't fix like
this because...

>
> Cc: x86@xxxxxxxxxx
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
>
> ---
>
>  b/arch/x86/include/asm/syscalls.h |    2 +-
>  b/arch/x86/kernel/ldt.c           |    5 +++--
>  b/arch/x86/um/ldt.c               |    3 ++-
>  3 files changed, 6 insertions(+), 4 deletions(-)
>
> diff -puN arch/x86/kernel/ldt.c~x86-syscall-macros-modify_ldt arch/x86/kernel/ldt.c
> --- a/arch/x86/kernel/ldt.c~x86-syscall-macros-modify_ldt       2017-10-13 13:30:12.802553391 -0700
> +++ b/arch/x86/kernel/ldt.c     2017-10-13 13:30:12.817553391 -0700
> @@ -12,6 +12,7 @@
>  #include <linux/string.h>
>  #include <linux/mm.h>
>  #include <linux/smp.h>
> +#include <linux/syscalls.h>
>  #include <linux/slab.h>
>  #include <linux/vmalloc.h>
>  #include <linux/uaccess.h>
> @@ -294,8 +295,8 @@ out:
>         return error;
>  }
>
> -asmlinkage int sys_modify_ldt(int func, void __user *ptr,
> -                             unsigned long bytecount)
> +SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr ,
> +               unsigned long , bytecount)

sys_modify_ldt() returns int, which is wrong, and it's visibly wrong
to 64-bit user code.  So I think you need to make sure that the return
value is cast to int in all cases.

--Andy