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

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Message-ID: <1916cb5c-cb3d-427c-bcf0-2c1b905fd6d1@linux.ibm.com>
Date: Mon, 4 Nov 2024 14:51:57 +0530
From: Hari Bathini <hbathini@...ux.ibm.com>
To: Segher Boessenkool <segher@...nel.crashing.org>
Cc: Shuah Khan <shuah@...nel.org>, linux-kselftest@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu
 <mhiramat@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Madhavan Srinivasan <maddy@...ux.ibm.com>,
        "Naveen N. Rao"
 <naveen@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        linux-trace-kernel@...r.kernel.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] selftests/ftrace: update kprobe syntax error test for
 ppc64le



On 02/11/24 2:29 am, Segher Boessenkool wrote:
> Hi!
> 
> 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?

Thanks
Hari