Re: [PATCH] parisc: Fix ptrace: syscall number and return value modification

[Helge Deller <deller@gmx.de>]

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On 19.01.2016 20:47, Mike Frysinger wrote:
> On 19 Jan 2016 16:08, Helge Deller wrote:
>> Mike Frysinger reported that his ptrace testcase showed strange
>> behaviour on parisc: It was not possible to avoid a syscall and the
>> return value of a syscall couldn't be changed.
>>
>> To modify a syscall number, we were missing to save the new syscall
>> number to gr20 which is then picked up later in assembly again.
>>
>> The effect that the return value couldn't be changed is a side-effect of
>> another bug in the assembly code. When a process is ptraced, userspace
>> expects each syscall to report entrance and exit of a syscall.  If a
>> syscall number was given which doesn't exist, we jumped to the normal
>> syscall exit code instead of informing userspace that the (non-existant)
>> syscall exits. This unexpected behaviour confuses userspace and thus the
>> bug was misinterpreted as if we can't change the return value.
>>
>> This patch fixes both problems and was tested on 64bit kernel with
>> 32bit userspace.
> 
> thanks, i'll give it a spin on my box too
> 
> on a related note, can you check if arg reloading works too ?
> i.e. i should also be able to mung syscall args on the fly.
> e.g. if the tracee does open("/foo", O_RDWR), the tracer should
> be able to munge it to do open("/foo", O_RDONLY).

I didn't tested it, but from looking at the assembly the user args get
reloaded from the task struct after having called do_syscall_trace_enter().
So, in theory it should work.

Helge
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