linux-kernel - Re: [PATCH] x86, syscalls: use SYSCALL_DEFINE() macros for sys_modify

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Date:   Wed, 18 Oct 2017 18:18:36 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, luto@...nel.org,
        brgerst@...il.com
Subject: Re: [PATCH] x86, syscalls: use SYSCALL_DEFINE() macros for
 sys_modify_ldt()


* Dave Hansen <dave.hansen@...ux.intel.com> wrote:

> 
> We do not have tracepoints for sys_modify_ldt() because we define
> it directly instead of using the normal SYSCALL_DEFINEx() macros.
> 
> However, there is a reason sys_modify_ldt() does not use the macros:
> it has an 'int' return type instead of 'unsigned long'.  This is
> a bug, but it's a bug cemented in the ABI.
> 
> What does this mean?  If we return -EINVAL from a function that
> returns 'int', we have 0x00000000ffffffea in %rax.  But, if we
> return -EINVAL from a function returning 'unsigned long', we end
> up with 0xffffffffffffffea in %rax, which is wrong.
> 
> To work around this and maintain the 'int' behavior while using
> the SYSCALL_DEFINEx() macros, so we add a cast to 'unsigned int'
> in both implementations of sys_modify_ldt().
> 
> Cc: x86@...nel.org
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Brian Gerst <brgerst@...il.com>
> 
> ---
> 
>  b/arch/x86/include/asm/syscalls.h |    2 +-
>  b/arch/x86/kernel/ldt.c           |   16 +++++++++++++---
>  b/arch/x86/um/ldt.c               |    6 ++++--
>  3 files changed, 18 insertions(+), 6 deletions(-)

Fails to build on UML:

/home/mingo/tip/arch/x86/um/ldt.c:372:29: error: expected ‘)’ before ‘int’
 SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr ,
                             ^
/home/mingo/tip/arch/x86/um/ldt.c:206:13: warning: ‘do_modify_ldt_skas’ defined 
but not used [-Wunused-function]
 static long do_modify_ldt_skas(int func, void __user *ptr,

etc.

Thanks,

	Ingo