Re: New ARM asm/syscall.h incompatible? (commit bf2c9f9866928df60157bc4f1ab39f93a32c754e)

[Will Drewry]

On Wed, May 23, 2012 at 1:56 PM, Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxxx> wrote:
> On Wed, May 23, 2012 at 11:01:50AM -0500, Will Drewry wrote:
>> Hi Wade and Steven,
>>
>> I don't believe the syscall_get_arguments/syscall_set_arguments
>> implementation that landed in 3.4 is correct or safe.  I didn't see it
>> get pulled in - sorry for not mailing sooner! :(
>>
>> The current implementation allows for _7_ arguments and allows the 0th
>> index to be the ARM_ORIG_r0 instead of starting with ARM_r0 == 0.  In
>> the global description of syscall_*_arguments it says:
>>
>>  * It's only valid to call this when @task is stopped for tracing on
>>  * entry to a system call, due to %TIF_SYSCALL_TRACE or %TIF_SYSCALL_AUDIT.
>>  * It's invalid to call this with @i + @n > 6; we only support system calls
>>  * taking up to 6 arguments.
>>
>> This means that the current implementation is broken when matching
>> system call arguments for ftrace (unless there is an arch specific
>> hack in there) and it breaks internal kernel API for any other
>> consumers without arch knowledge (like seccomp mode=2).  Is there a
>> reason to expose ARM_ORIG_r0 this way?  Am I misreading?
>>
>> My understanding of the arch register usage at syscall time is something like:
>> - ORIG_r0 gets the syscall number
>> - r0 becomes the first system call argument
>> - system call proceeds
>> - on return, r0 is the return value
>
> Wrong.  You're far too used to the x86 way of doing things.
>
> For ARM, on entry to a system call, r0 _and_ orig_r0 are the first
> syscall argument.  For other exceptions, orig_r0 will be -1 (but you
> can't rely upon that meaning anything, because a syscall can take -1
> as the first argument.)
>
> On exit from a system call, r0 will be the return value, and orig_r0
> will _still_ be the first system call argument.

Thanks - as usual, I can't keep them straight without the asm in front of me.

I'm still curious if it wouldn't make more sense to handle the
sys_syscall special case prior to any cross-arch (slowpath) code
involvement rather than truncating the 7th parameter making
sys_syscall a second class citizen for those cross-arch paths.
Perhaps that's not acceptable for ptrace/tracehook, but it seems like
it would make sense for ftrace and seccomp.

cheers!
will
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