Questions about ld h->pointer_equality_needed on x86-64

[Jens Remus]

On 28.03.2025 08:04, Jan Beulich wrote:
> On 27.03.2025 18:20, Michael Matz wrote:
>> On Thu, 27 Mar 2025, Jens Remus wrote:

Thank you both for your very helpful replies!

>>> Wouldn't lea with R_X86_64_PC32 instead of movl with R_X86_64_32S also
>>> be valid?
>>> Then restricting R_X86_64_PC32 to non-SEC_CODE would be wrong.
>>
>> The pointer_equality_needed flag is basically set optimistically to false
>> on x86-64 (in the sense that it's sometimes not set, even though it would
>> be correct to do so).  Only in situations where we know that a use
>> certainly must be an address-taken, do we set it.  PC32 in non-CODE
>> sections fits that, it's unrealistic to have that situation and then not
>> somehow using the address such that equality might matter.  For PC32 in
>> CODE sections, again see above.  It could be a non-address-use, so we
>> don't record it as address-use.
>>
>> PC32S is similar, just that it cannot occur (realistically) in non-CODE
>> sections anyway, so all occurrences are regarded as
>> might-be-non-address-use.
> 
> Why only non-CODE? Especially in the kernel memory model data (pointers)
> can be expressed this way as all. Recall that even a special .slong
> directive was added to gas at some point, to be able to control the
> involved relocation.
> 
> (For context: I'm sure you mean 32S here.)
IIUC the pointer_equality_needed flag is only relevant for non-PIC code
linked as PDE (Position- Dependent Executable; non-PIE) that references
a non-local function in a DSO.  The linker then needs to resolve that
symbol using a PLT entry to ensure that the symbol is reachable within
the code model limits.  For pointer equality the DSO now also needs to
resolve any references to the "canonical PLT".  For that purpose the
dynamic symbol value in the PDE is set to the address of the canonical
PLT and the dynamic linker then needs to to use that symbol value
instead of the DSO local function address.
That is also why R_X86_64_64 does not need pointer_equality_needed set
when in a non-SEC_READONLY section, as the loader or dynamic linker
then can resolve the symbol to the function.  A PLT is required in a
SEC_READONLY section, so that the linker can resolve the relocation at
link time.

Possibly this is not a thing in the Kernel, except for the Kernel module
loader perhaps?

In the s390x PR ld/29655 case I need to prevent the linker from setting
the dynamic symbol value to the "canonical PLT" address, if that exists
due to a separate function call, in PIC code that is using GOT in
address taken context when linked as PDE.  Currently ld on s390x is
always setting the dynamic symbol value to the "canoncial PLT" address,
if a PLT exists for the symbol.

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@de.ibm.com

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