On Mon, 4 Nov 2024 04:36:15 -0600
Segher Boessenkool [off-list ref] wrote:

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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..  

But what is there?

Function start address is what kallsyms returns. That is:

  grep function /proc/kallsyms

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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..  

Hrm.  So you allow essentially random entry points on other ABIs to
work?

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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?  

I have no idea what "offset" means here.

The offset is the number of bytes from the address that is returned by
kallsyms.

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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.  

An entry point has no instructions at all.  Oh, you mean the code at
the GEP.

The LEP can already be all the allowed distances after the GEP.  And
the .localentry GAS directive already supports all those distances
always.  Not a lot of code written in assembler does use that, and
certainly GCC does not use a lot of the freedom it has here, but it
could (and so could assembler programmers).  Typically people will want
to make the code here as short as possible, and there are restrictions
on what is *allowed* to be done here anyway (ld, the link editor, can
change this code after all!), so it is not too likely you will ever see
big code at the GEP often, but times change, etc.

This is all determined by the kernel. It's considered a function entry by
the function:

   arch_kprobe_on_func_entry()

Which on PowerPC has:

static bool arch_kprobe_on_func_entry(unsigned long offset)
{
#ifdef CONFIG_PPC64_ELF_ABI_V2
#ifdef CONFIG_KPROBES_ON_FTRACE
        return offset <= 16;
#else
        return offset <= 8;
#endif
#else
        return !offset;
#endif  
}

So, being greater than 16 on powerpc with config CONFIG_PPC64_ELF_ABI_V2
set, would work. If that function changes, then the test needs to change.

-- Steve