[PATCH v6 2/6] arm64: ptrace: allow tracer to skip a system call

AKASHI Takahiro takahiro.akashi at linaro.org
Fri Sep 5 03:08:33 PDT 2014

On 09/02/2014 06:16 PM, Russell King - ARM Linux wrote:
> On Tue, Sep 02, 2014 at 05:47:29PM +0900, AKASHI Takahiro wrote:
>> On 09/01/2014 08:47 PM, Russell King - ARM Linux wrote:
>>> On Wed, Aug 27, 2014 at 02:55:46PM +0900, AKASHI Takahiro wrote:
>>>> 1)
>>>> setting x0 to -ENOSYS is necessary because, otherwise, user-issued syscall(-1) will
>>>> return a bogus value when audit tracing is on.
>>>> Please note that, on arm,
>>>>                    not traced      traced
>>>>                    ------          ------
>>>> syscall(-1)      aborted         OOPs(BUG_ON)
>>>> syscall(-3000)   aborted         aborted
>>>> syscall(1000)    ENOSYS          ENOSYS
>>> Two points here:
>>> 1. You've found a case which causes a BUG_ON().  Where is the bug report
>>>      for this, so the problem can be investigated and resolved?
>> I think that I mentioned it could also happen on arm somewhere in a talk
>> with Will, but don't remember exactly when.
> Sorry, not good enough.  Please report this bug so it can be investigated
> and fixed.

Please review my patch as well as the commit message.

>>> 2. What do you mean by "aborted" ?
>> I mean that the process will receive SIGILL and get aborted.
>> A system call number, like -1 and -3000, won't be trapped by *switch*
>> statement in asm_syscall() and end up with being signaled.
> That is correct behaviour - because numbers greater than 0xf0000 (or
> 0x9f0000 for OABI - the 0x900000 offset on the syscalls on OABI is to
> distinguish them from syscalls used by RISC OS) are not intended to
> be Linux syscalls per-se.

I tried to make such invalid/pseudo syscalls hehave in the same way
whether or not a task is traced (by seccomp, ptrace or audit).

>>> Please, if you find a problem with 32-bit ARM, report it.  Don't hide it,
>>> because hiding it can be a security issue or in the case of BUG_ON(), it
>>> could be a denial of service issue.
>>> As you're part of Linaro, I would have thought you'd be more responsible
>>> in this regard - after all, Linaro is supposed to be about improving the
>>> ARM kernel...  Maybe I got that wrong, and Linaro is actually about
>>> ensuring that the ARM kernel is stuffed full of broken features?
>> I thought my first priority was on arm64 (and then arm), but now that
>> you and Will seem to want to see the fix first on arm, okey, I will
>> start with arm issue.
> So what you're saying there is that if you find a bug in ARM code, which
> everyone is currently using, you can ignore it until you've sorted out
> ARM64 which almost no one is using.
> This is absurd, and whoever has set your priorities is clearly on drugs.

Thank you. I learned a new English word, absurd.

-Takahiro AKASHI


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