Re: Unaligned accesses when loading modules

[Mikulas Patocka]

On Tue, 14 Jan 2025, Sami Tolvanen wrote:

> Hi Mikulas,
> 
> On Tue, Jan 14, 2025 at 6:07 PM Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
> >
> > Hi
> >
> > 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.

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