Re: Unaligned accesses when loading modules

[Jan Beulich <jbeulich@suse.com>]

On 15.01.2025 19:02, Mikulas Patocka wrote:
> On Tue, 14 Jan 2025, Sami Tolvanen wrote:
>> On Tue, Jan 14, 2025 at 6:07 PM Mikulas Patocka <mpatocka@redhat.com> wrote:
>>> On PA-RISC, with the kernel 6.12.9, I get unaligned pointer warnings when
>>> a module is loaded. The warnings are caused by the fact that the
>>> .gnu.linkonce.this_module section is not aligned to the appropriate
>>> boundary. If I dump the module content with "objdump -h configs.ko", I get
>>> this. Note that the .gnu.linkonce.this_module has "File off 000042d2" and
>>> "Algn 2**4".
>>>
>>> On x86-64, the same misalignment can be seen, but it doesn't cause
>>> warnings because unaligned pointers are handled in hardware.
>>>
>>> This seems to be a bug in the linker, because when I compile an old kernel
>>> with a new linker, I also get the misalignment. Do you have an idea how to
>>> work around this bug?
>>
>> Does explicitly specifying section alignment in the module linker
>> script fix this by any chance?
>>
>>> kernel-6.12.9, binutils from Debian ports:
>>> [...]
>>> kernel 6.10, older binutils:
>>
>> Which exact versions of binutils were used here? I don't see the
>> alignment issue with binutils 2.42 on either x86_64 or parisc64, so I
>> assume you're testing with something newer?
>>
>> $ hppa64-linux-gnu-ld.bfd --version
>> GNU ld (GNU Binutils for Debian) 2.42.50.20240625
>>
>> $ hppa64-linux-gnu-objdump -h configs.ko | grep -E '(format|this_module)'
>> configs.ko:     file format elf64-hppa-linux
>>  17 .gnu.linkonce.this_module 00000300  0000000000000000
>> 0000000000000000  00005c50  2**4
>>
>> Sami
> 
> Hi
> 
> I use version "GNU ld (GNU Binutils for Debian) 2.43.50.20250108".
> 
> It was broken in the commit 1f1b5e506bf0d9bffef8525eb9bee19646713eb6 in 
> the binutils-gdb git and partially fixed in the commit 
> d41df13ab36b224a622c0bdf28a96a0dee79db77 - the section is still not 
> aligned at their specified boundary (16), but at least it is aligned on 8 
> bytes, which avoids the warnings.

When you say "broken", can you please explain what it is that is _broken_?
Things have changed, yes, but the produced ELF is - afaict - still within
spec. The "partial fix" as you call it wasn't really a fix, but a band-aid
for some broken consumers of ELF. Plus modpost, being one such example,
was supposedly corrected already (Linux commit 8fe1a63d3d99). Said "partial
fix" was also confirmed to help modpost [1] - are you saying that wasn't
quite true?

Jan

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=32435

> With binutils from git, I no longer see warnings when loading modules, but 
> there are warnings in modpost when compiling the kernel:
> 
> 2025-01-15T17:09:29.408347+01:00 phoebe kernel: handle_unaligned: 2165 callbacks suppressed
> 2025-01-15T17:09:29.408433+01:00 phoebe kernel: modpost(9283): unaligned access to 0xf9096bd5 at ip 0x015d03 (iir 0xf381086)
> 2025-01-15T17:09:29.602490+01:00 phoebe kernel: modpost(9283): unaligned access to 0xf9096bd1 at ip 0x015d07 (iir 0xf301088)
> 2025-01-15T17:09:29.700610+01:00 phoebe kernel: modpost(9283): unaligned access to 0xf9096be9 at ip 0x015d0b (iir 0x4b3c0040)
> 2025-01-15T17:09:29.700653+01:00 phoebe kernel: modpost(9283): unaligned access to 0xf9096bdd at ip 0x015d0f (iir 0x4b240028)
> 2025-01-15T17:09:29.897081+01:00 phoebe kernel: modpost(9283): unaligned access to 0xf9096bd9 at ip 0x015d13 (iir 0x4b270020)
> 
> The affected instructions are in the function do_pci_entry:
>    15d00:       0f 38 10 86     ldw c(r25),r6
>    15d04:       0f 30 10 88     ldw 8(r25),r8
>    15d08:       4b 3c 00 40     ldw 20(r25),ret0
>    15d0c:       4b 24 00 28     ldw 14(r25),r4
>    15d10:       4b 27 00 20     ldw 10(r25),r7
>    15d14:       0f 28 10 89     ldw 4(r25),r9
>    15d18:       82 93 20 e0     cmpb,= r19,r20,15d90 <do_pci_entry+0xcc>
>    15d1c:       0f 20 10 8a     ldw 0(r25),r10
> they happen in the expansion of the macro DEF_FIELD.
> 
> Mikulas