Re: [PATCH 2/2] ARM: entry-common,ptrace: do not pass scno to syscall_trace_enter

[Roman Peniaev]

On Wed, Jan 14, 2015 at 5:08 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Sun, Jan 11, 2015 at 6:32 AM, Roman Pen <r.peniaev@xxxxxxxxx> wrote:
>> In previous patch current_thread_info()->syscall is set with
>> corresponding syscall number prior to further calls, thus there
>> is no any need to pass 'scno'.
>>
>> Also, add explicit comment why do we have to reread 'scno' local
>> variable.
>>
>> Signed-off-by: Roman Pen <r.peniaev@xxxxxxxxx>
>> Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
>> Cc: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
>> Cc: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
>> Cc: Sekhar Nori <nsekhar@xxxxxx>
>> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
>> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>> Cc: Eric Paris <eparis@xxxxxxxxxx>
>> Cc: Will Deacon <will.deacon@xxxxxxx>
>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>> ---
>>  arch/arm/kernel/entry-common.S | 1 -
>>  arch/arm/kernel/ptrace.c       | 6 ++++--
>>  2 files changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
>> index 89452ff..3d12eb5 100644
>> --- a/arch/arm/kernel/entry-common.S
>> +++ b/arch/arm/kernel/entry-common.S
>> @@ -228,7 +228,6 @@ ENDPROC(vector_swi)
>>          * context switches, and waiting for our parent to respond.
>>          */
>>  __sys_trace:
>> -       mov     r1, scno
>>         add     r0, sp, #S_OFF
>>         bl      syscall_trace_enter
>>
>> diff --git a/arch/arm/kernel/ptrace.c b/arch/arm/kernel/ptrace.c
>> index ef9119f..1238787 100644
>> --- a/arch/arm/kernel/ptrace.c
>> +++ b/arch/arm/kernel/ptrace.c
>> @@ -928,9 +928,9 @@ static void tracehook_report_syscall(struct pt_regs *regs,
>>         regs->ARM_ip = ip;
>>  }
>>
>> -asmlinkage int syscall_trace_enter(struct pt_regs *regs, int scno)
>> +asmlinkage int syscall_trace_enter(struct pt_regs *regs)
>>  {
>> -       current_thread_info()->syscall = scno;
>> +       int scno = current_thread_info()->syscall;
>
> Was this assignment of current_thread_info()->syscall redundant? If
> so, this looks fine. If not, what will now be setting the thread_info?

[sorry, i have to resend this email because previously I sent it using my phone
 and arm maillist rejected it because of html inside]

Yes, it is redundant.
Because of the previous patch thread_info->syscall already contains
corresponding scnr,
so we use it instead of passing the same number from asm.

So everything should work fine without current patch, and also current
patch should not
change anything in the expected behavior.

--
Roman




>
> -Kees
>
>>
>>         /* Do the secure computing check first; failures should be fast. */
>>  #ifdef CONFIG_HAVE_ARCH_SECCOMP_FILTER
>> @@ -944,6 +944,8 @@ asmlinkage int syscall_trace_enter(struct pt_regs *regs, int scno)
>>         if (test_thread_flag(TIF_SYSCALL_TRACE))
>>                 tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
>>
>> +       /* Syscall can be aborted (-1 can be set) or even changed
>> +        * by the tracer and subsequent PTRACE_SET_SYSCALL request */
>>         scno = current_thread_info()->syscall;
>>
>>         if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
>> --
>> 2.1.3
>>
>
>
>
> --
> Kees Cook
> Chrome OS Security
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