Re: [PATCH] selftests/ftrace: update kprobe syntax error test for ppc64le

[Hari Bathini <hbathini@linux.ibm.com>]

On 04/11/24 3:14 pm, Segher Boessenkool wrote:
> Hi!
> 
> On Mon, Nov 04, 2024 at 02:51:57PM +0530, Hari Bathini wrote:
>> On 02/11/24 2:29 am, Segher Boessenkool wrote:
>>> On Sat, Nov 02, 2024 at 12:49:25AM +0530, Hari Bathini wrote:
>>>> For ppc64le, depending on the kernel configuration used, offset 16
>>> >from function start address can also be considered function entry.
>>>> Update the test case to accommodate such configurations.
>>>
>>> (This is true for all ELfv2, not just LE.  For the kernel that is about
>>> the same).
>>>
>>> The LEP and GEP can differ by zero, one, two, four, eight, or sixteen
>>> insns (where an insn is four bytes).  Four insns is common, yes, but
>>> maybe you can support all?  See the function symbol's st_other field
>>> to see what the offset is:
>>> 0, 1: zero insns, zero bytes
>>> N = 2..6: 1 << (N-2) insns, i.e. 1<<N bytes
>>> 7: reserved
>>>
>>> (This is the top 3 bits of st_other, the other bits have other meanings).
>>>
>>> Four insns is common, yes, but by no means the only possibility.
>>
>> Hi Segher,
>>
>> Querying for function arguments is supported on kprobes only at function
>> entry. This is a negative test case where the offset is intentionally
>> set beyond function entry while querying for function arguments.
>> I guess, simply setting the offset to 20 (vfs_read is anyway
>> going to be beyond 5 instructions) instead of 8 for powerpc would
>> make all platforms and ABI variants happy?
> 
> I have no idea.  What is this "offset" anyway?

offset (in bytes) from function start address..

> 
> This is just the ELFv2 ABI.  No platform can make up its own thing at
> all (well, none decided to be gratuitously incompatible, so far).  And
> there are no "ABI variants"!

The test case applies for ABIv1 & ABIv2. All ppc32 & ppc64 platforms..

> 
> You're just making assumptions here that are based on nothing else but
> observations of what is done most of the time.  That might work for a
> while -- maybe a long while even! -- but it can easily break down.

Hmmm.. I understand that you want the test case to read st_other field
but would you rather suggest an offset of 64?
Is a GEP of 8/16 instructions going to be true anytime soon or is it
true already for some cases? The reason I ask that is some kprobe/ftrace
code in the kernel might need a bit of re-look if that is the case.

Thanks
Hari